LTC2435

LTC2435/LTC2435-1 20-Bit No Latency ∆ΣTM ADCs with Differential Input and Differential Reference DESCRIPTIO The LTC®2435/2435-1 are 2.7V to 5.5V micropower 20-bit differential ∆Σ analog to digital converters with integrated oscillator, 3ppm INL and 0.8ppm RMS noise. They use delta-sigma technology and provide single cycle settling time for multiplexed applications. Through a single pin, the LTC2435 can be configured for better than 110dB input differential mode rejection at 50Hz or 60Hz ± 2%, or it can be driven by an external oscillator for a user defined rejection frequency. The LTC2435-1 can be configured for better than 87dB input differential mode rejection over the range of 49Hz to 61.2Hz (50Hz and 60Hz ±2% simultaneously). The internal oscillator requires no external frequency setting components. The converters accept any external differential reference voltage from 0.1V to VCC for flexible ratiometric and remote sensing measurement configurations. The fullscale differential input range is from – 0.5VREF to 0.5VREF. The reference common mode voltage, VREFCM, and the input common mode voltage, VINCM, may be independently set anywhere within the GND to VCC range of the LTC2435/LTC2435-1. The DC common mode input rejection is better than 120dB. The LTC2435/LTC2435-1 communicate through a flexible 3-wire digital interface which is compatible with SPI and MICROWIRETM protocols. , LTC and LT are registered trademarks of Linear Technology Corporation. No Latency ∆Σ is a trademark of Linear Technology Corporation. MICROWIRE is a trademark of National Semiconductor Corporation. Protected by U.S. Patents including 6140950, 6169506. FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ 2× Speed Up Version of the LTC2430: 15Hz Output Rate, 60Hz Notch—LTC2435; 13.75Hz Output Rate, Simultaneous 50Hz/60Hz Notch—LTC2435-1 Differential Input and Differential Reference with GND to VCC Common Mode Range 3ppm INL, No Missing Codes 10ppm Gain Error 0.8ppm Noise Single Conversion Settling Time for Multiplexed Applications Internal Oscillator—No External Components Required Single Supply 2.7V to 5.5V Operation Low Supply Current (200µA,4µA in Auto Sleep) 20-Bit ADC in Narrow SSOP-16 Package (SO-8 Footprint) APPLICATIO S ■ ■ ■ ■ ■ ■ ■ ■ ■ Direct Sensor Digitizer Weight Scales Direct Temperature Measurement Gas Analyzers Strain Gage Transducers Instrumentation Data Acquisition Industrial Process Control 6-Digit DVMs TYPICAL APPLICATIO S 2.7V TO 5.5V 2 14 INL (ppm OF VREF) 10 8 6 VCC 1µF VCC FO LTC2435/ LTC2435-1 REFERENCE VOLTAGE 0.1V TO VCC ANALOG INPUT RANGE –0.5VREF TO 0.5VREF 3 4 5 6 REF + REF IN + IN – GND – = INTERNAL OSC/50Hz REJECTION (LTC2435) = EXTERNAL CLOCK SOURCE = INTERNAL OSC/60Hz REJECTION (LTC2435) = INTERNAL 50Hz/60Hz REJECTION (LTC2435-1) SCK 13 3-WIRE SPI INTERFACE –2 –4 –6 –8 FO = GND VCC = 5V VREF = 5V VINCM = VINCM = 2.5V –1.5 SDO CS 12 11 1, 7, 8, 9, 10, 15, 16 –10 –2.5 2435 TA01 U 4 2 0 U U Integral Nonlinearity vs Input TA = 25°C TA = 85°C TA = – 45°C –0.5 0.5 1.5 INPUT VOLTAGE (V) 2.5 2435 G04 24351fa 1 LTC2435/LTC2435-1 ABSOLUTE (Notes 1, 2) AXI U RATI GS PACKAGE/ORDER I FOR ATIO TOP VIEW GND VCC REF + REF – IN + IN – GND GND 1 2 3 4 5 6 7 8 16 GND 15 GND 14 FO 13 SCK 12 SDO 11 CS 10 GND 9 GND Supply Voltage (VCC) to GND .......................– 0.3V to 7V Analog Input Pins Voltage to GND .................................... – 0.3V to (VCC + 0.3V) Reference Input Pins Voltage to GND .................................... – 0.3V to (VCC + 0.3V) Digital Input Voltage to GND ........ – 0.3V to (VCC + 0.3V) Digital Output Voltage to GND ..... – 0.3V to (VCC + 0.3V) Operating Temperature Range LTC2435C/LTC2435-1C ........................... 0°C to 70°C LTC2435I/LTC2435-1I ........................ – 40°C to 85°C Storage Temperature Range ................. – 65°C to 150°C Lead Temperature (Soldering, 10 sec).................. 300°C ORDER PART NUMBER LTC2435CGN LTC2435IGN LTC2435-1CGN LTC2435-1IGN GN PART MARKING 2435 2435I 24351 24351I GN PACKAGE 16-LEAD PLASTIC SSOP TJMAX = 125°C, θJA = 95°C/W Consult LTC Marketing for parts specified with wider operating temperature ranges. The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Notes 3, 4) PARAMETER Integral Nonlinearity CONDITIONS ● ● ● ELECTRICAL CHARACTERISTICS MIN 20 TYP 2 3 10 2 100 MAX UNITS Bits ppm of VREF ppm of VREF ppm of VREF mV nV/°C Resolution (No Missing Codes) 0.1V ≤ VREF ≤ VCC, –0.5 • VREF ≤ VIN ≤ 0.5 • VREF, (Note 5) 5V ≤ VCC ≤ 5.5V, REF+ = 2.5V, REF– = GND, VINCM = 1.25V, (Note 6) 5V ≤ VCC ≤ 5.5V, REF+ = 5V, REF– = GND, VINCM = 2.5V, (Note 6) 2.7V ≤ VCC ≤ 5.5V, REF+ = 2.5V, REF– = GND, VINCM = 1.25V, (Note 6) 2.5V ≤ REF+ ≤ VCC, REF– = GND, GND ≤ IN+ = IN– ≤ VCC, (Note 14) 2.5V ≤ REF+ ≤ VCC, REF– = GND, GND ≤ IN+ = IN– ≤ VCC 2.5V ≤ REF+ ≤ VCC, REF– = GND, IN+ = 0.75REF+, IN– = 0.25 • REF+ 2.5V ≤ REF+ ≤ VCC, REF– = GND, IN+ = 0.75REF+, IN– = 0.25 • REF+ 2.5V ≤ REF+ ≤ VCC, REF– = GND, IN+ = 0.25 • REF+, IN– = 0.75 • REF+ 2.5V ≤ REF+ ≤ VCC, REF– = GND, IN+ = 0.25 • REF+, IN– = 0.75 • REF+ 5V ≤ VCC ≤ 5.5V, REF+ = 5V, REF – = GND, GND ≤ IN– = IN+ ≤ VCC, (Note 13) 20 5 Offset Error Offset Error Drift Positive Gain Error Positive Gain Error Drift Negative Gain Error Negative Gain Error Drift Output Noise ● 10 0.1 25 ppm of VREF ppm of VREF/°C ● 10 0.1 4 25 ppm of VREF ppm of VREF/°C µVRMS CO VERTER CHARACTERISTICS PARAMETER Input Common Mode Rejection DC CONDITIONS 2.5V GND ≤ REF+ ≤ The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Notes 3, 4) MIN ● TYP 120 MAX UNITS dB VCC, REF– = GND, ≤ IN– = IN+ ≤ VCC (Note 5) 110 24351fa 2 U W U U WW W U LTC2435/LTC2435-1 CO VERTER CHARACTERISTICS PARAMETER Input Common Mode Rejection 60Hz ± 2% (LTC2435) Input Common Mode Rejection 50Hz ± 2% (LTC2435) Input Normal Mode Rejection 60Hz ± 2% (LTC2435) Input Normal Mode Rejection 50Hz ± 2% (LTC2435) Input Common Mode Rejection 49Hz to 61.2Hz (LTC2435-1) Input Normal Mode Rejection 49Hz to 61.2Hz (LTC2435-1) Input Normal Mode Rejection External Clock fEOSC/2560 ±14% Input Normal Mode Rejection External Clock fEOSC/2560 ±4% Reference Common Mode Rejection DC Power Supply Rejection, DC Power Supply Rejection, 50Hz ± 2% CONDITIONS – 2.5V CC, REF = GND, – = IN+ ≤ V , (Notes 5, 7) GND ≤ IN CC 2.5V ≤ REF+ ≤ VCC, REF– = GND, GND ≤ IN – = IN+ ≤ VCC, (Notes 5, 8) The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Notes 3, 4) MIN ● ● ● ● ● ● ● ● ● Power Supply Rejection, 60Hz ± 2% REF+ = 2.5V, REF– = GND, IN– = IN+ = GND, (Note 7) REF+ = 2.5V, REF– = GND, IN– = IN+ = GND, (Note 8) A ALOG I PUT A D REFERE CE The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 3) SYMBOL IN+ IN– VIN REF+ REF– VREF CS (IN+) CS CS (IN–) (REF–) (IN+) (REF+) (REF–) CS (REF+) IDC_LEAK IDC_LEAK IDC_LEAK PARAMETER Absolute/Common Mode IN+ Voltage Absolute/Common Mode IN– Voltage Input Differential Voltage Range (IN+ – IN–) Absolute/Common Mode REF+ Voltage Absolute/Common Mode REF– Voltage Reference Differential Voltage Range (REF+ – REF–) IN+ Sampling Capacitance IN– Sampling Capacitance REF+ Sampling Capacitance REF– Sampling Capacitance IN+ DC Leakage Current IN– DC Leakage Current REF+ DC Leakage Current REF– DC Leakage Current CS = VCC, IN+ = GND CS = VCC, IN– = VCC CS = VCC, REF+ = VCC CS = VCC, REF– = GND ● ● ● ● IDC_LEAK (IN–) U U U U TYP MAX UNITS dB dB ≤ REF+ ≤ V 140 140 110 110 120 87 87 110 130 120 140 100 120 120 120 120 (Notes 5, 7) (Notes 5, 8) 2.5V ≤ REF+ ≤ VCC, REF– = GND, GND ≤ IN– = IN+ ≤ VCC, (Notes 5, 7) FO = GND (Note 5) External Oscillator (Note 5) External Oscillator (Note 5) 2.5V ≤ REF+ ≤ VCC, GND ≤ REF– ≤ 2.5V, VREF = 2.5V, IN– = IN+ = GND (Note 5) REF+ = VCC, REF– = GND, IN– = IN+ = GND dB dB dB dB dB dB dB dB dB dB U CONDITIONS ● ● ● ● ● ● MIN GND – 0.3V GND – 0.3V –VREF/2 0.1 GND 0.1 TYP MAX VCC + 0.3V VCC + 0.3V VREF/2 VCC VCC – 0.1V VCC UNITS V V V V V V pF pF pF pF 1.5 1.5 1.5 1.5 –10 –10 –10 –10 1 1 1 1 10 10 10 10 nA nA nA nA 24351fa 3 LTC2435/LTC2435-1 DIGITAL I PUTS A D DIGITAL OUTPUTS SYMBOL VIH VIL VIH VIL IIN IIN CIN CIN VOH VOL VOH VOL IOZ PARAMETER High Level Input Voltage CS, FO Low Level Input Voltage CS, FO High Level Input Voltage SCK Low Level Input Voltage SCK Digital Input Current CS, FO Digital Input Current SCK Digital Input Capacitance CS, FO Digital Input Capacitance SCK High Level Output Voltage SDO Low Level Output Voltage SDO High Level Output Voltage SCK Low Level Output Voltage SCK Hi-Z Output Leakage SDO (Note 9) IO = –800µA IO = 1.6mA IO = –800µA (Note 10) IO = 1.6mA (Note 10) ● ● ● ● ● The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 3) CONDITIONS 2.7V ≤ VCC ≤ 5.5V 2.7V ≤ VCC ≤ 3.3V 4.5V ≤ VCC ≤ 5.5V 2.7V ≤ VCC ≤ 5.5V 2.7V ≤ VCC ≤ 5.5V (Note 9) 2.7V ≤ VCC ≤ 3.3V (Note 9) 4.5V ≤ VCC ≤ 5.5V (Note 9) 2.7V ≤ VCC ≤ 5.5V (Note 9) 0V ≤ VIN ≤ VCC 0V ≤ VIN ≤ VCC (Note 9) ● ● ● ● ● ● POWER REQUIRE E TS SYMBOL VCC ICC PARAMETER Supply Voltage Supply Current Conversion Mode Sleep Mode Sleep Mode The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 3) CONDITIONS ● 4 UW U U MIN 2.5 2.0 TYP MAX UNITS V V 0.8 0.6 2.5 2.0 0.8 0.6 –10 –10 10 10 VCC – 0.5 0.4 VCC – 0.5 0.4 –10 10 10 10 V V V V V V µA µA pF pF V V V V µA MIN 2.7 TYP MAX 5.5 UNITS V µA µA µA CS = 0V (Note 12) CS = VCC (Note 12) CS = VCC, 2.7V ≤ VCC ≤ 3.3V (Note 12) ● ● ● 200 4 2 300 10 24351fa LTC2435/LTC2435-1 TI I G CHARACTERISTICS SYMBOL fEOSC tHEO tLEO tCONV PARAMETER External Oscillator Frequency Range External Oscillator High Period External Oscillator Low Period Conversion Time (LTC2435) The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 3) CONDITIONS ● ● ● fISCK DISCK fESCK tLESCK tHESCK tDOUT_ISCK tDOUT_ESCK t1 t2 t3 t4 tKQMAX tKQMIN t5 t6 Note 1: Absolute Maximum Ratings are those values beyond which the life of the device may be impaired. Note 2: All voltage values are with respect to GND. Note 3: VCC = 2.7 to 5.5V unless otherwise specified. VREF = REF + – REF –, VREFCM = (REF + + REF –)/2; VIN = IN + – IN –, VINCM = (IN + + IN –)/2. Note 4: FO pin tied to GND or to VCC or to external conversion clock source with fEOSC = 153600Hz unless otherwise specified. Note 5: Guaranteed by design, not subject to test. Note 6: Integral nonlinearity is defined as the deviation of a code from a straight line passing through the actual endpoints of the transfer curve. The deviation is measured from the center of the quantization band. Note 7: FO = 0V (internal oscillator) or fEOSC = 153600Hz ± 2% (external oscillator) for the LTC2435 or fEOSC = 139800Hz ±2% for the LTC2435-1. UW MIN 5 0.25 0.25 65.6 78.7 72 TYP MAX 2000 200 200 UNITS kHz µs µs ms ms ms ms ms kHz kHz kHz F O = 0V FO = VCC External Oscillator (Note 11) F O = 0V External Oscillator (Note 11) Internal Oscillator (Note 10), LTC2435 Internal Oscillator (Note 10), LTC2435-1 External Oscillator (Notes 10, 11) (Note 10) (Note 9) (Note 9) (Note 9) ● ● ● ● ● 68.3 81.9 10278/fEOSC (in kHz) 73.5 75 10278/fEOSC (in kHz) 19.2 17.5 fEOSC/8 66.9 80.3 Conversion Time (LTC2435-1) Internal SCK Frequency Internal SCK Duty Cycle External SCK Frequency Range External SCK Low Period External SCK High Period Internal SCK 24-Bit Data Output Time ● ● ● ● 45 250 250 1.22 1.34 55 2000 % kHz ns ns Internal Oscillator (Notes 10, 12), LTC2435 ● Internal Oscillator (Notes 10, 12), LTC2435-1 ● ● External Oscillator (Notes 10, 11) ● ● ● 1.25 1.28 1.37 1.40 192/fEOSC (in kHz) 24/fESCK (in kHz) 200 200 200 220 ms ms ms ms ns ns ns ns ns ns ns External SCK 24-Bit Data Output Time (Note 9) CS ↓ to SDO Low Z CS ↑ to SDO High Z CS ↓ to SCK ↓ CS ↓ to SCK ↑ SCK ↓ to SDO Valid SDO Hold After SCK ↓ SCK Set-Up Before CS ↓ SCK Hold After CS ↓ (Note 5) (Note 10) (Note 9) 0 0 0 50 15 50 ● ● ● ● ● ● 50 ns Note 8: FO = VCC (internal oscillator) or fEOSC = 128000Hz ± 2% (external oscillator). Note 9: The converter is in external SCK mode of operation such that the SCK pin is used as digital input. The frequency of the clock signal driving SCK during the data output is fESCK and is expressed in kHz. Note 10: The converter is in internal SCK mode of operation such that the SCK pin is used as digital output. In this mode of operation the SCK pin has a total equivalent load capacitance CLOAD = 20pF. Note 11: The external oscillator is connected to the FO pin. The external oscillator frequency, fEOSC, is expressed in kHz. Note 12: The converter uses the internal oscillator. FO = 0V or FO = VCC. Note 13: The output noise includes the contribution of the internal calibration operations. Note 14: Refer to Offset Accuracy and Drift in the Applications Information section. 24351fa 5 LTC2435/LTC2435-1 TYPICAL PERFOR A CE CHARACTERISTICS Total Unadjusted Error (VCC = 5V, VREF = 5V) –340 –680 FO = GND VCC = 5V VREF = 5V –345 VINCM = VINCM = 2.5V TA = – 45°C TUE (ppm OF VREF) TUE (ppm OF VREF) –690 TA = 25°C –695 –700 –705 TA = 85°C TUE (ppm OF VREF) –350 TA = 25°C –355 TA = 85°C –360 –2.5 –1.5 0.5 1.5 –0.5 INPUT VOLTAGE (V) Integral Nonlinearity (VCC = 5V, VREF = 5V) 10 8 6 INL (ppm OF VREF) INL (ppm OF VREF) 2 0 –2 –4 –6 –8 FO = GND VCC = 5V VREF = 5V VINCM = VINCM = 2.5V –1.5 TA = 25°C TA = 85°C TA = – 45°C 1 0 –1 TA = 85°C INL (ppm of VREF) 4 –10 –2.5 –0.5 0.5 1.5 INPUT VOLTAGE (V) Noise Histogram (Output Rate = 15Hz, VCC = 5V, VREF = 5V) 30 10,000 CONSECUTIVE READINGS VCC = 5V 25 VREF = 5V GAUSSIAN VIN = 0V DISTRIBUTION VINCM = 2.5V m = –325.4ppm 20 FO = GND σ = 0.79ppm TA = 25°C 15 10 5 0 –330 –329 –328 –327 –326 –325 –324 –323 –322 –321 OUTPUT CODE(ppm OF VREF) 2435 G07 NUMBER OF READINGS (%) NUMBER OF READINGS (%) 6 UW 2435 G01 Total Unadjusted Error (VCC = 5V, VREF = 2.5V) FO = GND VCC = 5V –685 VREF = 2.5V VINCM = VINCM = 1.25V Total Unadjusted Error (VCC = 2.7V, VREF = 2.5V) –320 FO = GND VCC = 2.7V VREF = 2.5V –330 VINCM = VINCM = 1.25V TA = – 45°C –340 TA = 25°C TA = 85°C –350 TA = – 45°C 2.5 –710 –1.25 –0.75 –0.25 0.25 0.75 INPUT VOLTAGE (V) 1.25 2435 G02 –360 –1.25 –0.75 –0.25 0.25 0.75 INPUT VOLTAGE (V) 1.25 2435 G03 Integral Nonlinearity (VCC = 5V, VREF = 2.5V) 3 2 TA = – 45°C 10 Integral Nonlinearity (VCC = 2.7V, VREF = 2.5V) FO = GND 8 VCC = 2.7V VREF = 2.5V 6V INCM = VINCM = 1.25V 4 2 0 –2 –4 –6 –8 TA = 85°C TA = 25°C TA = – 45°C 2.5 2435 G04 TA = 25°C FO = GND –2 VCC = 5V VREF = 2.5V VINCM = VINCM = 1.25V –3 –1.25 –0.75 –0.25 0.25 0.75 INPUT VOLTAGE (V) 1.25 2435 G05 –10 –1.25 –0.75 –0.25 0.25 0.75 INPUT VOLTAGE (V) 1.25 2435 G06 Noise Histogram (Output Rate = 15Hz, VCC = 2.7V, VREF = 2.5V) 14 10,000 CONSECUTIVE READINGS VCC = 2.7V GAUSSIAN 12 V DISTRIBUTION REF = 2.5V VIN = 0V m = –365ppm 10 VINCM = 2.5V σ = 1.55ppm FO = GND 8 TA = 25°C 6 4 2 0 –372 –370 –368 –366 –364 –362 –360 –358 OUTPUT CODE (ppm OF VREF) 2435 G08 24351fa LTC2435/LTC2435-1 TYPICAL PERFOR A CE CHARACTERISTICS RMS Noise vs Input Differential Voltage 1.5 VCC = 5V 1.4 VREF = 5V VINCM = 2.5V 1.3 F = GND O 1.2 TA = 25°C 1.1 1.0 0.9 0.8 0.7 0.6 0.5 –2.5 –2 –1.5 –1 – 0.5 0 0.5 1 1.5 2 INPUT DIFFERENTIAL VOLTAGE (V) 2.5 5.0 RMS NOISE (ppm OF VREF) RMS NOISE (µV) RMS NOISE (µV) RMS Noise vs VCC = VREF 5.0 FO = GND 4.8 REF+ = VCC REF– = GND 4.6 T = 25°C A 4.4 VIN = 0V VINCM = GND 4.2 4.0 3.8 3.6 3.4 3.2 3.0 2.7 3.1 3.5 3.9 4.3 VCC (V) 4.7 5.1 5.5 5.0 OFFSET ERROR (ppm OF VREF) RMS NOISE (µV) RMS NOISE (µV) Offset Error vs Temperature –320 –322 VCC = 5V VREF = 5V VIN = 0V VINCM = GND FO = GND –320 OFFSET ERROR (ppm OF VREF) –324 –326 –328 –330 –332 –334 –336 –338 –340 – 45 –30 –15 OFFSET ERROR (ppm OF VREF) OFFSET ERROR (mV) 0 15 30 45 60 TEMPERATURE (°C) UW 2435 G10 2435 G13 RMS Noise vs VINCM FO = GND 4.8 REF+ = 5V REF – = GND 4.6 T = 25°C A 4.4 VCC = 5V VIN = 0V 4.2 VINCM = GND 4.0 3.8 3.6 3.4 3.2 3.0 –1 0 1 3 2 VINCM (V) 4 5 6 2435 G11 RMS Noise vs Temperature (TA) 5.0 FO = GND 4.8 VCC = 5V = 5V V 4.6 VREF 0V IN = 4.4 VINCM = GND 4.2 4.0 3.8 3.6 3.4 3.2 3.0 –50 –25 50 25 0 TEMPERATURE (°C) 75 100 2435 G12 RMS Noise vs VREF FO = GND 4.8 REF– = GND T = 25°C 4.6 VA = 5V CC 4.4 VIN = 0V VINCM = GND 4.2 4.0 3.8 3.6 3.4 3.2 3.0 0 1 3 2 VREF (V) 4 5 2435 G14 Offset Error vs VINCM –320 –322 –324 –326 –328 –330 –332 VCC = 5V –334 REF+ = 5V REF – = GND –336 VIN = 0V –338 FO = GND TA = 25°C –340 1 –1 0 3 2 VINCM (V) 4 5 6 2435 G15 Offset Error vs VCC = VREF REF+ = VCC –322 REF – = GND V = 0V –324 IN VINCM = GND –326 FO = GND TA = 25°C –328 –330 –332 –334 –336 –338 75 90 –340 2.7 3.1 3.5 3.9 4.3 VCC (V) 4.7 5.1 5.5 –1.60 –1.61 –1.62 –1.63 –1.64 –1.65 –1.66 –1.67 –1.68 –1.69 –1.70 Offset Error vs VREF FO = GND REF– = GND TA = 25°C VCC = 5V VIN = 0V VINCM = GND 0 1 3 2 VREF (V) 4 5 2435 G18 2435 G16 2435 G17 24351fa 7 LTC2435/LTC2435-1 TYPICAL PERFOR A CE CHARACTERISTICS Full-Scale Error vs Temperature –330 FULL-SCALE ERROR (ppm OF VREF) FULL-SCALE ERROR(ppm OF VREF) –340 +FS ERROR –350 –FS ERROR –360 –500 –600 –700 –800 –900 2.7 –FS ERROR +FS GAIN ERROR (ppm OF VREF) FO = GND VCC = 5V VREF = 5V VINCM = 2.5V –370 –60 –40 –20 20 40 60 0 TEMPERATURE (°C) PSRR vs Frequency at VCC (LTC2435-1) VCC = 4.1VDC ±1.4V REF+ = 2.5V –20 REF – = GND IN+ = GND – –40 IN = GND FO = GND T = 25°C –60 A –80 –100 –120 –140 0 20 40 60 80 100 120 140 160 180 200 220 FREQUENCY AT VCC (Hz) 2435 G22 0 REJECTION (dB) REJECTION (dB) –80 –100 –120 –140 1 10 100 1000 10000 100000 1000000 FREQUENCY AT VCC (Hz) 2435 G23 REJECTION (dB) PSRR vs Frequency at VCC (LTC2435) VCC = 4.1VDC ±1.4V REF+ = 2.5V –20 REF – = GND IN+ = GND –40 IN– = GND FO = GND –60 TA = 25°C –80 –100 –120 –140 0 40 80 120 160 200 FREQUENCY AT VCC (Hz) 240 2435 G25 0 REJECTION (dB) REJECTION (dB) –80 –100 –120 –140 1 10 100 1000 10000 100000 1000000 FREQUENCY AT VCC (Hz) 2435 G26 REJECTION (dB) 8 UW 80 2435 G19 Full-Scale Error vs VCC –300 +FS ERROR –400 VREF = 2.5V REF – = GND VINCM = 0.5VREF FO = GND TA = 25°C 20 +Full-Scale Gain Error vs VCC VREF = 2.5V REF – = GND = 0.5VREF V 15 INCM FO = GND TA = 25°C 10 5 0 100 3.1 3.5 3.9 4.3 VCC (V) 4.7 5.1 5.5 –5 2.7 3.1 3.5 3.9 4.3 VCC (V) 4.7 5.1 5.5 2435 G20 2435 G21 PSRR vs Frequency at VCC (LTC2435-1) VCC = 4.1VDC REF+ = 2.5V –20 REF – = GND IN+ = GND –40 IN– = GND FO = GND –60 TA = 25°C 0 PSRR vs Frequency at VCC (LTC2435-1) VCC = 4.1VDC ± 0.7V REF+ = 2.5V –20 REF – = GND IN+ = GND –40 IN– = GND FO = GND –60 TA = 25°C –80 –100 –120 –140 13800 0 13850 13900 13950 FREQUENCY AT VCC (Hz) 14000 2435 G24 PSRR vs Frequency at VCC (LTC2435) VCC = 4.1VDC REF+ = 2.5V –20 REF – = GND IN+ = GND –40 IN– = GND FO = GND –60 TA = 25°C 0 0 PSRR vs Frequency at VCC (LTC2435) VCC = 4.1VDC ± 0.7V REF+ = 2.5V –20 REF – = GND IN+ = GND –40 IN– = GND FO = GND –60 TA = 25°C –80 –100 –120 –140 15250 15300 15350 15400 FREQUENCY AT VCC (Hz) 15450 2435 G27 24351fa LTC2435/LTC2435-1 TYPICAL PERFOR A CE CHARACTERISTICS Conversion Current vs Temperature 240 VCC = 5.5V 230 CONVERSION CURRENT (µA) SUPPLY CURRENT (µA) 220 210 FO = GND CS = GND 200 SCK = NC SDO = NC 190 180 170 160 –45 –30 –15 0 15 30 45 VCC = 5V 800 700 600 500 400 300 200 VCC = 5V SLEEP-MODE CURRENT (µA) TEMPERATURE (°C) 2435 G28 Offset Change* vs Output Data Rate 50 OFFSET CHANGE* (ppm OF VREF) RESOLUTION (BITS) 10 0 –10 –20 –30 VCC = VREF = 5V RESOLUTION (BITS) VINCM = VREFCM 40 VIN = 0V REF – = GND 30 F = EXT OSC O 20 TA = 25°C VCC = 2.7V VREF = 2.5V –40 * RELATIVE TO OFFSET AT NORMAL OUTPUT RATE –50 0 20 40 60 80 100 120 140 160 180 200 OUTPUT DATA RATE (READINGS/SEC) 2435 G31 UW VCC = 3V VCC = 2.7V 60 75 Conversion Current vs Output Data Rate 1000 900 VREF = VCC IN+ = GND IN– = GND SCK = NC SDO = NC SDI = GND CS = GND FO = EXT OSC TA = 25°C 6 5 4 3 2 1 Sleep-Mode Current vs Temperature FO = GND CS = VCC SCK = NC SDO = NC VCC = 5.5V VCC = 5V VCC = 3V VCC = 3V VCC = 2.7V 90 100 0 10 20 30 40 50 60 70 80 90 100 OUTPUT DATA RATE (READINGS/SEC) 2435 G29 0 –45 –30 –15 0 15 30 45 60 75 90 TEMPERATURE (°C) 2435 G30 Resolution (NoiseRMS ≤ 1LSB) vs Output Data Rate 22 21 20 19 18 VINCM = VREFCM 17 VIN = 0V REF – = GND FO = EXT OSC 16 TA = 25°C RES = LOG2 (VREF/NOISERMS) 15 0 20 40 60 80 100 120 140 160 180 200 OUTPUT DATA RATE (READINGS/SEC) 2435 G32 Resolution (INLMAX ≤ 1LSB) vs Output Data Rate 21 VCC = VREF = 5V 20 VCC = VREF = 5V 19 18 17 VINCM = VREFCM 16 VIN = 0V REF – = GND FO = EXT OSC 15 TA = 25°C RES = LOG2 (VREF/INLMAX) 14 0 20 40 60 80 100 120 140 160 180 200 OUTPUT DATA RATE (READINGS/SEC) 2435 G33 VCC = 2.7V VREF = 2.5V VCC = 2.7V VREF = 2.5V 24351fa 9 LTC2435/LTC2435-1 PI FU CTIO S GND (Pins 1, 7, 8, 9, 10, 15, 16): Ground. Multiple ground pins internally connected for optimum ground current flow and VCC decoupling. Connect each one of these pins to a ground plane through a low impedance connection. All seven pins must be connected to ground for proper operation. VCC (Pin 2): Positive Supply Voltage. Bypass to GND (Pin 1) with a 10µF tantalum capacitor in parallel with 0.1µF ceramic capacitor as close to the part as possible. REF + (Pin 3), REF – (Pin 4): Differential Reference Input. The voltage on these pins can have any value between GND and VCC as long as the reference positive input, REF +, is maintained more positive than the reference negative input, REF –, by at least 0.1V. IN + (Pin 5), IN– (Pin 6): Differential Analog Input. The voltage on these pins can have any value between GND – 0.3V and VCC + 0.3V. Within these limits the converter bipolar input range (VIN = IN+ – IN–) extends from – 0.5 • (VREF ) to 0.5 • (VREF ). Outside this input range the converter produces unique overrange and underrange output codes. CS (Pin 11): Active LOW Digital Input. A LOW on this pin enables the SDO digital output and wakes up the ADC. Following each conversion, the ADC automatically enters the Sleep mode and remains in this low power state as long as CS is HIGH. A LOW-to-HIGH transition on CS during the Data Output transfer aborts the data transfer and starts a new conversion. SDO (Pin 12): Three-State Digital Output. During the Data Output period, this pin is used as serial data output. When the chip select CS is HIGH (CS = VCC) the SDO pin is in a high impedance state. During the Conversion and Sleep periods, this pin is used as the conversion status output. The conversion status can be observed by pulling CS LOW. SCK (Pin 13): Bidirectional Digital Clock Pin. In Internal Serial Clock Operation mode, SCK is used as digital output for the internal serial interface clock during the Data Output period. In External Serial Clock Operation mode, SCK is used as digital input for the external serial interface clock during the Data Output period. A weak internal pullup is automatically activated in Internal Serial Clock Operation mode. The Serial Clock Operation mode is determined by the logic level applied to the SCK pin at power up or during the most recent falling edge of CS. FO (Pin 14): Frequency Control Pin. Digital input that controls the ADC’s notch frequencies and conversion time. When the FO pin is connected to VCC (LTC2435 only), the converter uses its internal oscillator and the digital filter first null is located at 50Hz. When the FO pin is connected to GND (FO = OV), the converter uses its internal oscillator and the digital filter first null is located at 60Hz (LTC2435) or simultaneous 50Hz/60Hz (LTC2435-1). When FO is driven by an external clock signal with a frequency fEOSC, the converter uses this signal as its system clock and the digital filter first null is located at a frequency fEOSC/2560. 10 U U U 24351fa LTC2435/LTC2435-1 FU CTIO AL BLOCK DIAGRA VCC GND AUTOCALIBRATION AND CONTROL IN + IN – + –∫ ∫ ∫ ∑ ADC SERIAL INTERFACE DECIMATING FIR REF + REF – –+ DAC Figure 1. Functional Block Diagram TEST CIRCUITS SDO 1.69k CLOAD = 20pF Hi-Z TO VOH VOL TO VOH VOH TO Hi-Z 2435 TA03 W INTERNAL OSCILLATOR FO (INT/EXT) SDO SCK CS 2435 F01 U U VCC 1.69k SDO CLOAD = 20pF Hi-Z TO VOL VOH TO VOL VOL TO Hi-Z 2435 TA04 24351fa 11 LTC2435/LTC2435-1 APPLICATIO S I FOR ATIO CONVERTER OPERATION Converter Operation Cycle The LTC2435/LTC2435-1 are low power, delta-sigma analog-to-digital converters with an easy to use 3-wire serial interface (see Figure 1). Their operation is made up of three states. The converter operating cycle begins with the conversion, followed by the sleep state and ends with the data output (see Figure 2). The 3-wire interface consists of serial data output (SDO), serial clock (SCK) and chip select (CS). CONVERT SLEEP FALSE CS = LOW AND SCK TRUE DATA OUTPUT 2435 F02 Figure 2. LTC2435 State Transition Diagram Initially, the LTC2435/LTC2435-1 perform a conversion. Once the conversion is complete, the device enters the sleep state. While in this sleep state, power consumption is reduced by an order of magnitude if CS is HIGH. The part remains in the sleep state as long as CS is HIGH. The conversion result is held indefinitely in a static shift register while the converter is in the sleep state. Once CS is pulled LOW, the device exits the low power sleep mode and enters the data output state. If CS is pulled HIGH before the first rising edge of SCK, the device returns to the sleep mode and the conversion result is still held in the internal static shift register. If CS remains LOW after the first rising edge of SCK, the device begins outputting the conversion result. Taking CS HIGH at this point will terminate the data output state and start a new conversion. 12 U There is no latency in the conversion result. The data output corresponds to the conversion just performed. This result is shifted out on the serial data out pin (SDO) under the control of the serial clock (SCK). Data is updated on the falling edge of SCK allowing the user to reliably latch data on the rising edge of SCK (see Figure 3). The data output state is concluded once 24 bits are read out of the ADC or when CS is brought HIGH. The device automatically initiates a new conversion and the cycle repeats. Through timing control of the CS and SCK pins, the LTC2435/LTC2435-1 offer several flexible modes of operation (internal or external SCK and free-running conversion modes). These various modes do not require programming configuration registers; moreover, they do not disturb the cyclic operation described above. These modes of operation are described in detail in the Serial Interface Timing Modes section. Conversion Clock A major advantage the delta-sigma converter offers over conventional type converters is an on-chip digital filter (commonly implemented as a Sinc or Comb filter). For high resolution, low frequency applications, this filter is typically designed to reject line frequencies of 50Hz or 60Hz plus their harmonics. The filter rejection performance is directly related to the accuracy of the converter system clock. The LTC2435/LTC2435-1 incorporate a highly accurate on-chip oscillator. This eliminates the need for external frequency setting components such as crystals or oscillators. Clocked by the on-chip oscillator, the LTC2435 achieves a minimum of 110dB rejection at the line frequency (50Hz or 60Hz ± 2%), while the LTC2435-1 achieves a minimum of 87db rejection at 50Hz ±2% and 60Hz ±2% simultaneously. Ease of Use The LTC2435/LTC2435-1 data output has no latency, filter settling delay or redundant data associated with the conversion cycle. There is a one-to-one correspondence between the conversion and the output data. Therefore, multiplexing multiple analog voltages is easy. 24351fa W U U LTC2435/LTC2435-1 APPLICATIO S I FOR ATIO The LTC2435/LTC2435-1 perform a full-scale calibration every conversion cycle. This calibration is transparent to the user and has no effect on the cyclic operation described above. The advantage of continuous calibration is extreme stability of full-scale readings with respect to time, supply voltage change and temperature drift. Unlike the LTC2430, the LTC2435 and LTC2435-1 do not perform an offset calibration every conversion cycle. This enables the LTC2435/LTC2435-1 to double their output rate while maintaining line frequency rejection. The initial offset of the LTC2435/LTC2435-1 is within 5mV independent of VREF. Based on the LTC2435/LTC2435-1 new modulator architecture, the temperature drift of the offset is less than 100nV/ ° C. More information on the LTC2435/ LTC2435-1 offset is described in the Offset Accuracy and Drift section of this data sheet. Power-Up Sequence The LTC2435/LTC2435-1 automatically enter an internal reset state when the power supply voltage VCC drops below approximately 2.2V. This feature guarantees the integrity of the conversion result and of the serial interface mode selection. (See the 2-wire I/O sections in the Serial Interface Timing Modes section.) When the VCC voltage rises above this critical threshold, the converter creates an internal power-on-reset (POR) signal with a duration of approximately 1ms. The POR signal clears all internal registers. Following the POR signal, the LTC2435/LTC2435-1 start a normal conversion cycle and follow the succession of states described above. The first conversion result following POR is accurate within the specifications of the device if the power supply voltage is restored within the operating range (2.7V to 5.5V) before the end of the POR time interval. Reference Voltage Range These converters accept a truly differential external reference voltage. The absolute/common mode voltage specification for the REF + and REF – pins covers the entire range from GND to VCC. For correct converter operation, the REF + pin must always be more positive than the REF – pin. U The LTC2435/LTC2435-1 can accept a differential reference voltage from 0.1V to VCC. The converter output noise is determined by the thermal noise of the front-end circuits, and as such, its value is nearly constant with reference voltage. A decrease in reference voltage will not significantly improve the converter’s effective resolution. On the other hand, a reduced reference voltage will improve the converter’s overall INL performance. A reduced reference voltage will also improve the converter performance when operated with an external conversion clock (external FO signal) at substantially higher output data rates (see the Output Data Rate section). Input Voltage Range The analog input is truly differential with an absolute/ common mode range for the IN+ and IN– input pins extending from GND – 0.3V to VCC + 0.3V. Outside these limits, the ESD protection devices begin to turn on and the errors due to input leakage current increase rapidly. Within these limits, the LTC2435/LTC2435-1 convert the bipolar differential input signal, VIN = IN+ – IN–, from – FS = – 0.5 • VREF to +FS = 0.5 • VREF where VREF = REF+ – REF–. Outside this range, the converters indicate the overrange or the underrange condition using distinct output codes. Input signals applied to IN+ and IN– pins may extend by 300mV below ground and above VCC. In order to limit any fault current, resistors of up to 5k may be added in series with the IN+ and IN– pins without affecting the performance of the device. In the physical layout, it is important to maintain the parasitic capacitance of the connection between these series resistors and the corresponding pins as low as possible; therefore, the resistors should be located as close as practical to the pins. The effect of the series resistance on the converter accuracy can be evaluated from the curves presented in the Input Current/ Reference Current sections. In addition, series resistors will introduce a temperature dependent offset error due to the input leakage current. A 1nA input leakage current will develop a 1ppm offset error on a 5k resistor if VREF = 5V. This error has a very strong temperature dependency. 24351fa W U U 13 LTC2435/LTC2435-1 APPLICATIO S I FOR ATIO Output Data Format The LTC2435/LTC2435-1 serial output data stream is 24 bits long. The first 3 bits represent status information indicating the sign and conversion state. The next 21 bits are the conversion result, MSB first. The third and fourth bit together are also used to indicate an underrange condition (the differential input voltage is below –FS) or an overrange condition (the differential input voltage is above +FS). Bit 23 (first output bit) is the end of conversion (EOC) indicator. This bit is available at the SDO pin during the conversion and sleep states whenever the CS pin is LOW. This bit is HIGH during the conversion and goes LOW when the conversion is complete. Bit 22 (second output bit) is a dummy bit (DMY) and is always LOW. Bit 21 (third output bit) is the conversion result sign indicator (SIG). If VIN is >0, this bit is HIGH. If VIN is 0.01µF) may be required as reference filters in certain configurations. Such capacitors will average the reference sampling charge and the external source resistance will see a quasi constant reference differential impedance. For the LTC2435, when FO = LOW (internal oscillator and 60Hz notch), the typical differential reference resistance is 15.6MΩ which will generate a +FS gain error of approximately 0.032ppm for each ohm of source resistance driving REF+ or REF–. When FO = HIGH (internal oscillator and 50Hz notch), the typical differential reference resistance is 18.7MΩ which will generate a +FS gain error of approximately 0.027ppm for each ohm of source resistance driving REF+ or REF–. For the LTC2435-1, the typical differential reference resistance is 17.1MΩ which will generate a +FS gain error of approximately 0.029ppm for each ohm of source resistance driving REF + or REF –. When FO is driven by an external oscillator with a frequency fEOSC (external conversion clock operation), the typical differential reference resistance is 2.4 • 1012/fEOSCΩ and each ohm of source resistance driving REF + o r REF – w ill result in 0.21 • 10–6 • fEOSCppm +FS gain error. The effect of the source resistance on the two reference pins is additive with respect to this gain error. The typical +FS and –FS errors for various combinations of source resistance seen by the REF+ and REF– pins and external capacitance CREF connected to these pins are shown in Figures 21, 22, 23 and 24. 10 0 –10 –20 –30 –40 –50 –60 –70 –80 –90 –100 1 VCC = 5V VREF+ = 5V VREF– = GND VIN+ = 1.25V VIN– = 3.75V FO = GND TA = 25°C 10 CIN = 0.001µF CIN = 100pF CIN = 0.01µF CIN = 0pF 1000 100 RSOURCE (Ω) 10000 100000 2435 F22 W U U Figure 22. –FS Error vs RSOURCE at REF+ or REF– (Small CIN) 24351fa LTC2435/LTC2435-1 APPLICATIO S I FOR ATIO 100 +FS ERROR VARIATION (ppm) –FS ERROR VARIATION (ppm) VCC = 5V 90 VREF+ = 5V V – = GND 80 VREF = 3.75V IN+ 70 VIN– = 1.25V FO = GND 60 TA = 25°C 50 40 30 20 10 0 0 400 800 CIN = 1µF, 10µF CIN = 0.1µF CIN = 0.01µF 1200 RSOURCE (Ω) 1600 2000 2435 F23 Figure 23. +FS Error vs RSOURCE at REF+ and REF– (Large CREF) In addition to this gain error, the converter INL performance is degraded by the reference source impedance. When FO = LOW (internal oscillator and 60Hz notch), every 100Ω of source resistance driving REF+ or REF– translates into about 0.11ppm additional INL error. For the LTC2435, when FO = HIGH (internal oscillator and 50Hz notch), every 100Ω of source resistance driving REF+ or REF– translates into about 0.092ppm additional INL error; and for the LTC2435-1 operating with simultaneous 50Hz/60Hz rejection, every 100Ω of source resistance leads to an additional 0.10ppm of additional INL error. When FO is driven by an external oscillator with a frequency fEOSC, every 100Ω of source resistance driving REF+ or REF– translates into about 0.73 • 10–6 • fEOSCppm additional INL error. Figure 25 shows the typical INL error due to the source resistance driving the REF+ or REF– pins when large CREF values are used. The effect of the source resistance on the two reference pins is additive with respect to this INL error. In general, matching of source impedance for the REF+ and REF– pins does not help the gain or the INL error. The user is thus advised to minimize the combined source impedance driving the REF+ and REF– pins rather than to try to match it. The magnitude of the dynamic reference current depends upon the size of the very stable internal sampling capacitors and upon the accuracy of the converter sampling clock. The accuracy of the internal clock over the entire temperature and power supply range is typical better than 0.5%. Such a specification can also be easily achieved by INL (ppm OF VREF) U 0 –10 –20 –30 –40 –50 –60 VCC = 5V VREF+ = 5V –70 VREF– = GND V + = 1.25V –80 VIN– = 3.75V IN –90 FO = GND TA = 25°C –100 0 400 CIN = 1µF, 10µF CIN = 0.1µF CIN = 0.01µF 800 1200 RSOURCE (Ω) 1600 2000 2435 F24 W U U Figure 24. –FS Error vs RSOURCE at REF+ and REF– (Large CREF) an external clock. When relatively stable resistors (50ppm/°C) are used for the external source impedance seen by REF+ and REF–, the expected drift of the dynamic current gain error will be insignificant (about 1% of its value over the entire temperature and voltage range). Even for the most stringent applications a one-time calibration operation may be sufficient. In addition to the reference sampling charge, the reference pins ESD protection diodes have a temperature dependent leakage current. This leakage current, nominally 1nA (±10nA max), results in a small gain error. A 100Ω source resistance will create a 0.05µV typical and 0.5µV maximum full-scale error. 15 VINCM = 0.5 • (IN + + IN –) = 2.5V 12 VCC = 5V REF+ = 5V 9 REF– = GND 6 FO = GND CREF = 10µF 3 TA = 25°C 0 –3 –6 –9 RSOURCE = 10k –15 –0.5 –0.4 –0.3 –0.2 –0.1 0 0.1 0.2 0.3 0.4 0.5 VINDIF /VREFDIF (V) 2435 F25 RSOURCE = 1k RSOURCE = 5k –12 Figure 25. INL vs Differential Input Voltage (VIN = IN+ – IN–) and Reference Source Resistance (RSOURCE at REF+ and REF– for Large CREF Values (CREF ≥ 1µF) 24351fa 29 LTC2435/LTC2435-1 APPLICATIO S I FOR ATIO Output Data Rate When using its internal oscillator, the LTC2435 can produce up to 15 readings per second with a notch frequency of 60Hz (FO = LOW) and 12.5 readings per second with a notch frequency of 50Hz (FO = HIGH) and the LTC2435-1 can produce up to 13.6 readings per second with FO = LOW. The actual output data rate will depend upon the length of the sleep and data output phases which are controlled by the user and which can be made insignificantly short. When operated with an external conversion clock (FO connected to an external oscillator), the LTC2435/ LTC2435-1 output data rate can be increased as desired. The duration of the conversion phase is 10278/fEOSC. If fEOSC = 153600Hz, the converter behaves as if the internal oscillator is used and the notch is set at 60Hz. There is no significant difference in the LTC2435/LTC2435-1 performance between these two operation modes. An increase in fEOSC over the nominal 153600Hz will translate into a proportional increase in the maximum output data rate. This substantial advantage is nevertheless accompanied by three potential effects, which must be carefully considered. First, a change in fEOSC will result in a proportional change in the internal notch position and in a reduction of the converter differential mode rejection at the power line frequency. In many applications, the subsequent performance degradation can be substantially reduced by relying upon the LTC2435/LTC2435-1’s exceptional common mode rejection and by carefully eliminating common mode to differential mode conversion sources in the input circuit. The user should avoid single-ended input filters and should maintain a very high degree of matching and symmetry in the circuits driving the IN+ and IN– pins. Second, the increase in clock frequency will increase proportionally the amount of sampling charge transferred through the input and the reference pins. If large external input and/or reference capacitors (CIN, CREF) are used, the previous section provides formulae for evaluating the effect of the source resistance upon the converter performance for any value of fEOSC. If small external input and/ or reference capacitors (CIN, CREF) are used, the effect of the external source resistance upon the LTC2435/ LTC2435-1 typical performance can be inferred from Figures 16, 17, 21 and 22 in which the horizontal axis is scaled by 153600/fEOSC. OFFSET ERROR (ppm OF VREF) +FS ERROR (ppm OF VREF) 30 U Third, the internal analog circuits are optimized for normal operation; therefore an increase in the frequency of the external oscillator will start to decrease the effectiveness of the internal analog circuits. This will result in a progressive degradation in the converter accuracy and linearity. Typical measured performance curves for output data rates up to 200 readings per second are shown in Figures 26 to 33. The degradation becomes more obvious above output data rate of 150Hz, which corresponds to an external oscillator of 1.536MHz. In order to obtain the highest possible level of accuracy from this converter at output data rates above 150 readings per second, the user is advised to maximize the power supply voltage used and to limit the maximum ambient operating temperature. In certain circumstances, a reduction of the differential reference voltage may be beneficial. –300 VINCM = VREFCM VCC = VREF = 5V VIN = 0V FO = EXT OSC TA = 25°C –310 –320 –330 TA = 85°C –340 –350 0 20 40 60 80 100 120 140 160 180 200 OUTPUT DATA RATE (READINGS/SEC) 2435 F26 W U U Figure 26. Offset Error vs Output Data Rate and Temperature –300 –320 –340 –360 –380 –400 –420 –440 –460 VINCM = VREFCM –480 VCC = VREF = 5V FO = EXT OSC –500 0 20 40 60 80 100 120 140 160 180 200 OUTPUT DATA RATE (READINGS/SEC) 2435 F27 TA = 25°C TA = 85°C Figure 27. + FS Error vs Output Data Rate and Temperature 24351fa LTC2435/LTC2435-1 APPLICATIO S I FOR ATIO –200 –FS ERROR (ppm OF VREF) VINCM = VREFCM –220 VCC = VREF = 5V FO = EXT OSC –240 RESOLUTION (BITS) –260 –280 –300 –320 –340 –360 –380 –400 0 TA = 25°C TA = 85°C 20 40 60 80 100 120 140 160 180 200 OUTPUT DATA RATE (READINGS/SEC) 2435 F28 Figure 28. – FS Error vs Output Data Rate and Temperature 21 OFFSET CHANGE* (ppm OF VREF) 20 RESOLUTION (BITS) 19 TA = 25°C 18 17 16 VCC = VREF = 5V VINCM = VREFCM REF – = GND 15 FO = EXT OSC RES = LOG2 (VREF/INLMAX) 14 0 20 40 60 80 100 120 140 160 180 200 OUTPUT DATA RATE (READINGS/SEC) 2435 F30 TA = 85°C Figure 30. Resolution (INLRMS ≤ 1LSB) vs Output Data Rate and Temperature 22 21 RESOLUTION (BITS) 20 19 18 17 16 15 0 VINCM = VREFCM VIN = 0V REF – = GND FO = EXT OSC TA = 25°C RES = LOG2(VREF/NOISERMS) 20 40 60 80 100 120 140 160 180 200 OUTPUT DATA RATE (READINGS/SEC) 2435 F32 VCC = VREF = 5V VCC = 2.7V VREF = 2.5V RESOLUTION (BITS) Figure 32. Resolution (NoiseRMS ≤ 1LSB) vs Output Data Rate and Reference Voltage U 22 21 20 19 18 17 16 15 0 VCC = VREF = 5V VINCM = VREFCM VIN = 0V REF – = GND FO = EXT OSC RES = LOG2(VREF/NOISERMS) 20 40 60 80 100 120 140 160 180 200 OUTPUT DATA RATE (READINGS/SEC) 2435 F29 W U U TA = 25°C TA = 85°C Figure 29. Resolution (NoiseRMS ≤ 1LSB) vs Output Data Rate and Temperature 50 40 30 20 10 0 –10 –20 –30 –40 * RELATIVE TO OFFSET AT NORMAL OUTPUT RATE –50 0 20 40 60 80 100 120 140 160 180 200 OUTPUT DATA RATE (READINGS/SEC) 2435 F31 VINCM = VREFCM VIN = 0V REF – = GND FO = EXT OSC TA = 25°C VCC = VREF = 5V VCC = 2.7V VREF = 2.5V Figure 31. Offset Change* vs Output Data Rate and Reference Voltage 21 20 VCC = VREF = 5V 19 18 17 VINCM = VREFCM 16 VIN = 0V REF – = GND F = EXT OSC 15 O TA = 25°C RES = LOG2 (VREF/INLMAX) 14 0 20 40 60 80 100 120 140 160 180 200 OUTPUT DATA RATE (READINGS/SEC) 2435 F33 VCC = 2.7V VREF = 2.5V Figure 33. Resolution (INLMAX ≤ 1LSB) vs Output Data Rate and Reference Voltage 24351fa 31 LTC2435/LTC2435-1 APPLICATIO S I FOR ATIO Normal Mode Rejection and Antialiasing One of the advantages delta-sigma ADCs offer over conventional ADCs is on-chip digital filtering. Combined with a large oversampling ratio, the LTC2435/LTC2435-1 significantly simplifies antialiasing filter requirements. The sinc4 digital filter provides greater than 120dB normal mode rejection at all frequencies except DC and integer multiples of the modulator sampling frequency (fS). Independent of the operating mode, fS = 256 • fN = 1024 • fOUTMAX where fN is the notch frequency and fOUTMAX is the maximum output data rate. In the internal oscillator mode, for the LTC2435, FS = 12800Hz with a 50Hz notch setting and fS = 15360Hz with a 60Hz notch setting. For the LTC2435-1, fS = 13980Hz (FO = LOW). In the external oscillator mode, fS = fEOSC/10. The normal mode rejection performance is shown in Figure 34. The regions of low rejection occurring at integer multiples of fS have a very narrow bandwidth. Magnified details of the normal mode rejection curves are shown in Figure 35 (rejection near DC) and Figure 36 (rejection at fS = 256fN) where fN represents the notch frequency. For the LTC2435, the bandwidth is 13.6Hz (FO = GND) and 11.4Hz (FO = VCC). The Bandwidth is 12.4Hz for the LTC2435-1 (FO = GND). 0 INPUT NORMAL MODE REJECTION (dB) –10 –20 –30 –50 –60 –70 –80 –90 –100 –110 –120 FO = HIGH REJECTION (dB) 0 fS 2fS 3fS 4fS 5fS 6fS 7fS 8fS 9fS 10fS11fS12fS DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz) 2435 F34a –40 Figure 34a. Input Normal Mode Rejection, Internal Oscillator and 50Hz Notch (LTC2435) 32 U Through FO connection, the LTC2435 provides better than 110dB input differential mode rejection at 50Hz or 60Hz ±2%. While for the LTC2435-1, it has a notch frequency of about 55Hz with better than 70db rejection over 48Hz to 62.4Hz, which covers both 50Hz ±2% and 60Hz ±2%. In order to achieve better rejection over the range of 48Hz to 62.4Hz, a running average can be performed. By averaging two consecutive LTC2435-1 readings, a sinc1 notch is combined with the sinc4 digital filter, yielding the frequency response shown in Figure 37. The averaging operation still keeps the output rate with the following algorithm: Result 1 = average (sample 0, sample 1) Result 2 = average (sample 1, sample 2) Result n = average (sample n-1, sample n) The user can expect to achieve in practice this level of performance using the internal oscillator as it is demonstrated by Figures 38 to 40. Typical measured values of the normal mode rejection of the LTC2435-1 operating with an internal oscillator and a 54.6Hz notch setting are shown in Figure 38 and 39 superimposed over the theoretical calculated curve. The same normal mode rejection performance is obtained for the LTC2435 with the frequency scaled to have the notch frequency at 60Hz (FO = GND) or 50Hz (FO = VCC). 0 –20 –40 –60 –80 –100 –120 –140 0 fS/2 INPUT FREQUENCY 2435 F34b W U U fS Figure 34b. Input Normal Mode Rejection, Internal Oscillator and FO = Low or External Oscillator 24351fa LTC2435/LTC2435-1 APPLICATIO S I FOR ATIO As a result of these remarkable normal mode specifications, minimal (if any) antialias filtering is required in front of the LTC2435/LTC2435-1. If passive RC components are placed in front of the LTC2435/LTC2435-1, the input dynamic current should be considered (see Input Current section). In cases where large effective RC time constants are used, an external buffer amplifier may be required to minimize the effects of dynamic input current. Traditional high order delta-sigma modulators, while providing very good linearity and resolution, suffer from potential instabilities at large input signal levels. The proprietary architecture used for the LTC2435/LTC2435-1 third order modulator resolves this problem and guarantees a predictable stable behavior at input signal levels of up to 150% of full scale. In many industrial applications, it is not uncommon to have to measure microvolt level signals superimposed over volt level perturbations and LTC2435/LTC2435-1 are eminently suited for such tasks. When the perturbation is differential, the specification of interest is the normal mode rejection for large input signal levels. With a reference voltage VREF = 5V, the LTC2435/ LTC2435-1 have a full-scale differential input range of 5V peak-to-peak. Figure 40 shows measurement results for the LTC2435-1 normal mode rejection ratio with a 7.5V peak-to-peak (150% of full scale) input signal superim- INPUT NORMAL REJECTION (dB) 0 INPUT NORMAL REJECTION (dB) NORMAL MODE REJECTION (dB) –20 –40 –60 –80 –100 –120 0 fN 2fN 3fN 4fN 5fN 6fN 7fN INPUT SIGNAL FREQUENCY (fN) 8fN 2435 F35 Figure 35. Input Normal Mode Rejection U posed over the more traditional normal mode rejection ratio results obtained with a 5V peak-to-peak (full scale) input signal. The same performance is obtained for the LTC2435 with the frequency scaled to have the notch frequency at 60Hz (FO = GND) or 50Hz (FO = VCC). It is clear that the LTC2435/LTC2435-1 rejection performance is maintained with no compromises in this extreme situation. When operating with large input signal levels, the user must observe that such signals do not violate the device absolute maximum ratings. 0 –20 –40 –60 –80 –100 –120 248 250 252 254 256 258 260 262 264 INPUT SIGNAL FREQUENCY (fN) 2435 F36 W U U Figure 36. Input Normal Mode Rejection –70 –80 NO AVERAGE –90 –100 –110 –120 –130 –140 60 62 54 56 58 48 50 52 DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz) 2435 F37 WITH RUNNING AVERAGE Figure 37. LTC2435-1 Input Normal Mode Rejection 24351fa 33 LTC2435/LTC2435-1 APPLICATIO S I FOR ATIO 0 NORMAL MODE REJECTION (dB) –40 –60 –80 NORMAL MODE REJECTION (dB) –20 MEASURED DATA VCC = 5V CALCULATED DATA VREF = 5V REF– = GND VINCM = 2.5V VIN(P-P) = 5V FO = GND TA = 25°C –100 –120 0 25 50 75 100 125 150 175 200 225 INPUT FREQUENCY (Hz) 2435 F38 Figure 38. Input Normal Mode Rejection vs Input Frequency (LTC2435-1) 0 NORMAL MODE REJECTION (dB) –20 –40 –60 –80 –100 –120 0 Figure 40. Measured Input Normal Mode Rejection vs Input Frequency (fN = 54.6Hz) 34 U 0 –20 –40 –60 –80 MEASURED DATA VCC = 5V CALCULATED DATA VREF = 5V REF– = GND VINCM = 2.5V VIN(P-P) = 5V FO = GND TA = 25°C –100 –120 0 25 50 75 100 125 150 175 200 225 INPUT FREQUENCY (Hz) 2435 F39 W U U Figure 39. Input Normal Mode Rejection vs Input Frequency with Running Average VIN(P-P) = 5V VCC = 5V VIN(P-P) = 7.5V VREF = 5V (150% OF FULL SCALE) REF– = GND VINCM = 2.5V FO = GND TA = 25°C 25 50 75 100 125 150 175 200 225 INPUT FREQUENCY (Hz) 2435 F40 24351fa LTC2435/LTC2435-1 APPLICATIO S I FOR ATIO U The listing in Figure 43 is a data collection program for the LTC2435/LTC2435-1 using the PIC16F73 microcontroller. The microcontroller is configured to transfer data through the SPI serial interface. Figure 42 shows the connection. The LT1180A is a dual RS232 driver/receiver pair with integral charge pump that generates RS232 voltage levels from a single 5V supply. The program begins by declaring variables and allocating memory locations to store the 24-bit conversion result. The main sequence starts with pulling CS LOW. It then waits for SDO to go LOW to start reading data. Three bytes are read to the MCU and the LTC2435/LTC2435-1 will automatically start a new conversion. CS is also raised to HIGH to ensure that a new conversion is started. The collected data are sent out through the serial port at 57600 baud. This can be captured with a terminal program and analyzed with a spreadsheet using the HEX2DEC function. 5V 16 12 14 15 11 1 5 TO OTHER DEVICES 2 4 8 9 10 2435 F41 Sample Driver for LTC2435/LTC2435-1 SPI Interface Figure 41 shows the use of an LTC2435/LTC2435-1 with a differential multiplexer. This is an inexpensive multiplexer that will contribute some error due to leakage if used directly with the output from the bridge, or if resistors are inserted as a protection mechanism from overvoltage. Although the bridge output may be within the input range of the A/D and multiplexer in normal operation, some thought should be given to fault conditions that could result in full excitation voltage at the inputs to the multiplexer or ADC. The use of amplification prior to the multiplexer will largely eliminate errors associated with channel leakage developing error voltages in the source impedance. The LTC2435/LTC2435-1 have a very simple serial interface that makes interfacing to microprocessors and microcontrollers very easy. 5V Figure 41. Use a Differential Multiplexer to Expand Channel Capability VCC 20 SCK LTC2435/ SDO LTC2435-1 CS 13 12 11 PIC16F73 13 17 RC2 RC6 14 RC3 18 15 RC7 RC4 8 19 VCC LT1180A 12 11 13 10 18 2 C1 T1IN T2IN R1OUT R2OUT SHDN C1+ T1OUT T2OUT R1IN R2IN VCC V+ V– 15 8 14 9 17 C3 3 7 C4 GND 16 2435 F42 Figure 42. Connecting the LTC2435/LTC2435-1 to a PIC16F73 MCU Using the SPI Serial Interface W U U + 47µF 3 4 REF + REF – 2 VCC 74HC4052 13 3 6 5 6 LTC2435/ LTC2435-1 IN + IN – GND 1, 7, 8, 9, 10, 15, 16 A0 A1 X1 1 2 3 4 5 VCC 6 7 8 9 4 C1– 5 C2+ 6 C2– C5 C2 24351fa 35 LTC2435/LTC2435-1 APPLICATIO S I FOR ATIO // Basic data collection program for the LTC2435 using the // PIC16F73 microcontroller. Collects data as fast as possible // and sends it out the serial port at 57600 baud as six // hexadecimal characters, followed by a carriage return. // This can be captured with a terminal program and analyzed // with a spreadsheet using the HEX2DEC function (in Excel.) // // Written for the CCS compiler, version 3.049. //////////////////////////////////////////////////////////////////// #include #byte SSPCON = 0x14 #byte SSPSTAT = 0x94 #bit CKE = SSPSTAT.6 #bit CKP = SSPCON.4 #bit SSPEN = SSPCON.5 #fuses HS,NOWDT,PUT #use delay(clock=10000000) // Synchronous serial port control // registers. // For baud rate calculation. #use rs232(baud=57600,parity=N,xmit=PIN_C6,rcv=PIN_C7) // Serial data is sent on pin C6. #define CS_ PIN_C2 // Chip select connected to pin C2 #define CLOCK PIN_C // Clock connected to pin C3 #define SDO PIN_C4 // SDO on the LTC2435 connected to pin C4 // (this is SDI on the PIC; // Master In, Slave Out (MISO) is less ambiguous) void main() { // Basic configuration, no bearing on operation of LTC2435 setup_adc_ports(NO_ANALOGS); setup_adc(ADC_CLOCK_DIV_2); setup_counters(RTCC_INTERNAL,RTCC_DIV_2); setup_timer_1(T1_DISABLED); setup_timer_2(T2_DISABLED,0,1); setup_ccp1(CCP_OFF); setup_ccp2(CCP_OFF); // LTC2435 is connected to the processor’s hardware SPI port. // This sets the port such that data is shifted on clock falling edges and // valid on rising edges. For a 10 MHz master clock, the SPI clock frequency // wil be 2.5 MHz. setup_spi(SPI_MASTER|SPI_L_TO_H|SPI_CLK_DIV_4|SPI_SS_DISABLED); CKP = 0; // Set up clock edges - clock idles low, data changes on CKE = 1; // falling edges, valid on rising edges. 36 U 24351fa W U U LTC2435/LTC2435-1 APPLICATIO S I FOR ATIO U 24351fa while(1) { output_low(CS_); // Enable LTC2435 while(input(SDO)) { /* Wait for SDO to fall, indicating end of conversion.*/ } printf(“%2X”,spi_read(0)); // Read first byte, send 2 hex characters. printf(“%2X”,spi_read(0)); // Read second byte, send 2 hex characters. printf(“%2X”,spi_read(0)); / Read third byte, send 2 hex characters. printf(“\r”); // Send carriage return. output_high(CS_); // Conversion actually started after last data byte was read, // but raising CS_ ensures the loop will never lock up waiting for // a low on SDO if a clock pulse is missed for some reason. } } Figure 43. A Sample Program for Data Collection from the LTC2435/LTC2435-1 Using the PIC16F73 Microcontroller. W U U 37 LTC2435/LTC2435-1 APPLICATIO S I FOR ATIO Correlated Double Sampling with the LTC2435/LTC2435-1 The Typical Application on the back page of this data sheet shows the LTC2435/LTC2435-1 in a correlated double sampling circuit that achieves a noise floor of under 100nV. In this scheme, the polarity of the bridge is alternated every other sample and the result is the average of a pair of samples of opposite sign. This technique has the benefit of canceling any fixed DC error components in the bridge, amplifiers and the converter, as these will alternate in polarity relative to the signal. Offset voltages and currents, thermocouple voltages at junctions of dissimilar metals and the lower frequency components of 1/f noise are virtually eliminated. The LTC2435/LTC2435-1 have the virtue of being able to digitize an input voltage that is outside the range defined by the reference, thereby providing a simple means to implement a ratiometric example of correlated double sampling. This circuit uses a bipolar amplifier (LT1219—U1 and U2) that has neither the lowest noise nor the highest gain. It does, however, have an output stage that can effectively suppress the conversion spikes from the LTC2435/ LTC2435-1. The LT1219 is a C-LoadTM stable amplifier that, by design, needs at least 0.1µF output capacitance to remain stable. The 0.1µF ceramic capacitors at the outputs (C1 and C2) should be placed and routed to minimize lead inductance or their effectiveness in preventing envelope detection in the input stage will be reduced. Alternatively, several smaller capacitors could be placed so that lead inductance is further reduced. This is a consideration because the frequency content of the conversion spikes extends to 50MHz or more. The output impedance of most op amps increases dramatically with frequency but the effective output impedance of the LT1219 remains 38 U low, determined by the ESR and inductance of the capacitors above 10MHz. The conversion spikes that remain at the output of other bipolar amplifiers pass through the feedback network and often overdrive the input of the amplifier, producing envelope detection. RFI may also be present on the signal lines from the bridge; C3 and C4 provide RFI suppression at the signal input, as well as suppressing transient voltages during bridge commutation. The wideband noise density of the LT1219 is 33nV/ √Hz, seemingly much noisier than the lowest noise amplifiers. However, in the region just below the 1/f corner that is not well suppressed by the correlated double sampling, the average noise density is similar to the noise density of many low noise amplifiers. If the amplifier is rolled off below about 1500Hz, the total noise bandwidth is determined by the converter’s Sinc4 filter at about 12Hz. The use of correlated double sampling involves averaging even numbers of samples; hence, in this situation, two samples would be averaged to give an input-referred noise level of about 100nVRMS. Level shift transistors Q4 and Q5 are included to allow excitation voltages up to the maximum recommended for the bridge. In the case shown, if a 10V supply is used, the excitation voltage to the bridge is 8.5V and the outputs of the bridge are above the supply rail of the ADC. U1 and U2 are also used to produce a level shift to bring the outputs within the input range of the converter. This instrumentation amplifier topology does not require well-matched resistors in order to produce good CMRR. However, the use of R2 requires that R3 and R6 match well, as the common mode gain is approximately –12dB. If the bridge is composed of four equal 350Ω resistors, the differential component associated with mismatch of R3 and R6 is nearly constant with either polarity of excitation and, as with offset, its contribution is canceled. C-Load is a trademark of Linear Technology Corporation. 24351fa W U U LTC2435/LTC2435-1 PACKAGE DESCRIPTIO U GN Package 16-Lead Plastic SSOP (Narrow .150 Inch) (Reference LTC DWG # 05-08-1641) .189 – .196* (4.801 – 4.978) 16 15 14 13 12 11 10 9 .045 ± .005 .009 (0.229) REF .150 – .165 .254 MIN .229 – .244 (5.817 – 6.198) .150 – .157** (3.810 – 3.988) .0165 ± .0015 .0250 BSC RECOMMENDED SOLDER PAD LAYOUT 1 23 4 56 7 8 .004 – .0098 (0.102 – 0.249) .015 ± .004 × 45° (0.38 ± 0.10) .007 – .0098 (0.178 – 0.249) 0° – 8° TYP .0532 – .0688 (1.35 – 1.75) .016 – .050 (0.406 – 1.270) NOTE: 1. CONTROLLING DIMENSION: INCHES INCHES 2. DIMENSIONS ARE IN (MILLIMETERS) 3. DRAWING NOT TO SCALE *DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE .008 – .012 (0.203 – 0.305) TYP .0250 (0.635) BSC GN16 (SSOP) 0204 24351fa Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 39 LTC2435/LTC2435-1 TYPICAL APPLICATIO ELIMINATE FOR 5V OPERATION (CONNECT 2.7k RESISTORS TO 100Ω RESISTORS) 100Ω 1.5k Q2 100Ω 22Ω 5V Q4 5V Q5 22Ω 2.7k 2.7k C3 2.2nF C4 2.2nF R5 499Ω 1000pF R6 10k 10V 0.1µf 1k 5 IN+ POL 22Ω R1 61.9Ω 0.1% 22Ω 33Ω 100Ω Q1: SILICONIX Si9802DY Q2, Q3: MMBD2907 Q4, Q5: MMBD3904 (800) 554-5565 RELATED PARTS PART NUMBER LT1019 LT1025 LTC1043 LTC1050 LT1236A-5 LT1460 LTC2400 DESCRIPTION Precision Bandgap Reference, 2.5V, 5V Micropower Thermocouple Cold Junction Compensator Dual Precision Instrumentation Switched Capacitor Building Block Precision Chopper Stabilized Op Amp Precision Bandgap Reference, 5V Micropower Series Reference 24-Bit, No Latency ∆Σ ADC in SO-8 COMMENTS 3ppm/°C Drift, 0.05% Max Initial Accuracy 80µA Supply Current, 0.5°C Initial Accuracy Precise Charge, Balanced Switching, Low Power No External Components 5µV Offset, 1.6µVP-P Noise 0.05% Max Initial Accuracy, 5ppm/°C Drift 0.075% Max Initial Accuracy, 10ppm/°C Max Drift, 0.3ppm Noise, 4ppm INL, 10ppm Total Unadjusted Error, 200µA 0.6ppm Noise, 4ppm INL, 10ppm Total Unadjusted Error, 200µA 800nVRMS Noise, Pin Compatible with LTC2435 1.45µVRMS Noise, 4ppm INL, Simultaneous 50Hz/60Hz Rejection (LTC2411-1) Simultaneous 50Hz/60Hz Rejection, 800nVRMS Noise Pin Compatible with the LTC2435/LTC2435-1 0.2ppm Noise, 2ppm INL, 3ppm Total Unadjusted Errors 200µA 1.2ppm Noise, 8ppm INL, Pin Compatible with LTC2400 2.8µV Noise, SSOP-16/MSOP Package 24351fa LT/TP 0804 1K REV A • PRINTED IN USA LTC2401/LTC2402 1-/2-Channel, 24-Bit, No Latency ∆Σ ADCs in MSOP LTC2410 LTC2411/ LTC2411-1 LTC2413 LTC2415/ LTC2415-1 LTC2420 24-Bit, No Latency ∆Σ ADC with Differential Inputs 24-Bit, No Latency ∆Σ ADC with Differential Inputs in MSOP 24-Bit, No Latency ∆Σ ADC with Differential Inputs 24-Bit, No Latency ∆Σ ADC with 15Hz Output Rate LTC2404/LTC2408 4-/8-Channel, 24-Bit, No Latency ∆Σ ADCs with Differential Inputs 0.3ppm Noise, 4ppm INL, 10ppm Total Unadjusted Error, 200µA LTC2414/LTC2418 8-/16-Channel 24-Bit, No Latency ∆Σ ADC 20-Bit, No Latency ∆Σ ADC in SO-8 LTC2430/LTC2431 20-Bit, No Latency ∆Σ ADC with Differential Inputs 40 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com + Q1 3 – 74HC04 U Correlated Double Sampling Resolves 100nV 10V 1.5k DIFFERENCE AMP Q3 3 R2 27k 1k 1000pF R4 499Ω 10V 0.1µf + – 7 U1 LT1219 5 4 SHDN 6 5k 2 C1 0.1µF 5V R3 10k 350Ω ×4 6 IN– LTC2435/ LTC2435-1 3 REF+ 4 2 7 U2 LT1219 5 4 SHDN 6 5k REF– GND C2 0.1µF 30pF 30pF 2435 F46 © LINEAR TECHNOLOGY CORPORATION 2001