LTC1283ACN

LTC1283 3V Single Chip 10-Bit Data Acquisition System FEATURES ■ ■ DESCRIPTIO ■ ■ ■ Single Supply 3.3V or ± 3.3V Operation Software Programmable Features: Unipolar/Bipolar Conversions 4 Differential/8 Single-Ended Inputs MSB- or LSB-First Data Sequence Variable Data Word Length Built-In Sample-and-Hold Direct 4-Wire Interface to Most MPU Serial Ports and all MPU Parallel Ports 15kHz Maximum Throughput Rate The LTC1283 is a 3V data acquisition component which contains a serial I/O successive approximation A/D converter. It uses LTCMOSTM switched capacitor technology to perform either 10-bit unipolar, or 9-bit plus sign bipolar A/D conversions. The 8-channel input multiplexer can be configured for either single-ended or differential inputs (or combinations thereof). An on-chip sample-and-hold is included for all single-ended input channels. The serial I/O is designed to be compatible with industrystandard full-duplex serial interfaces. It allows either MSBor LSB-first data and automatically provides 2’s complement output coding in the bipolar mode. The output data word can be programmed for a length of 8-, 10-, 12-, or 16-bit. This allows easy interface to shift registers and a variety of processors. Both the LTC1283A and LTC1283 are specified with offset and linearity errors less than ± 0.5LSB. The LTC1283A has a gain error limit of ± 1LSB. The 1283 is specified with a gain error limit of ± 2LSB for applications where gain is adjustable or less critcial. KEY SPECIFICATIO S ■ ■ ■ ■ ■ ■ ■ Minimum Guaranteed Supply Voltage: 3V Resolution: 10 Bits Offset Error: ± 0.5LSB Max Linearity Error: ± 0.5LSB Max Gain Error (LTC1283A): ± 1LSB Max Conversion Time: 44μs Supply Current: 350μA Max, 150μA Typ , LT, LTC and LTM are registered trademarks of Linear Technology Corporation. LTCMOS is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. TYPICAL APPLICATI 4.7μF 3V LTC1283 DIFFERENTIAL INPUT 3V BIPOLAR INPUT –3V 3V DOUT DIN SCLK T UNIPOLAR INPUTS CS FOR 83CL410 CODE SEE APPLICATIONS INFORMATION SECTION MPU (e.g., 83CL410) 1.0 0.5 ERROR (LSBs) 0 P1.1 P1.2 P1.3 P1.4 SERIAL DATA LINK –0.5 – 1.0 0 (+) (–) –UNIPOLAR INPUT LTC1283 • TA01 –3V U Linearity Plot 512 OUTPUT CODE LTC1283 • TA02 UO U 1024 1283fb 1 LTC1283 ABSOLUTE (Notes 1 and 2) AXI U RATI GS PACKAGE/ORDER I FOR ATIO TOP VIEW CH0 CH1 CH2 CH3 CH4 CH5 CH6 CH7 COM 1 2 3 4 5 6 7 8 9 20 VCC 19 ACLK 18 SCLK 17 DIN 16 DOUT 15 CS 14 REF + 13 REF – 12 V – 11 AGND Supply Voltage (VCC) to GND or V – ......................... 12V Voltage Analog and Reference Inputs ................................. (V –) –0.3V to VCC + 0.3V Digital Inputs ......................................... –0.3V to 12V Digital Outputs ........................... –0.3V to VCC + 0.3V Negative Supply Voltage (V–) ..................... –6V to GND Power Dissipation .............................................. 500mW Operating Temperature LTC1283AC, LTC1283C ......................... 0°C to 70°C Storage Temperature Range ................. – 65°C to 150°C Lead Temperature (Soldering, 10 sec)................. 300°C ORDER PART NUMBER LTC1283ACN LTC1283CN DGND 10 N PACKAGE 20-LEAD PLASTIC DIP TJ MAX = 150°C, θJA = 100°C/W Order Options Tape and Reel: Add #TR Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF Lead Free Part Marking: http://www.linear.com/leadfree/ RECO SYMBOL VCC V– fSCLK fACLK E DED OPERATI G CO DITIO S CONDITIONS V – = 0V VCC = 3.3V VCC = 3V VCC = 3V TA ≤ 25°C TA ≤ 70°C See Operating Sequence VCC = 3V VCC = 3V VCC = 3V VCC = 3V VCC = 3V VCC = 3V VCC = 3V LTC1283/LTC1283A MIN TYP MAX 3.0 – 3.6 0 0.01 0.05 10 SCLK + 48 ACLK 0 200 2 ACLK Cycles + 1μs 400 250 400 44 ns ns ns ACLK Cycles 3.6 0 500 1.00 1.00 UNITS V V kHz MHz MHz Cycles ns ns PARAMETER Positive Supply Voltage Negative Supply Voltage Shift Clock Frequency A/D Clock Frequency tCYC thCS thDI tsuCS tsuDI tWHACLK tWLACLK tWHCS Total Cycle Time Hold Time, CS Low After Last SCLK↓ Hold Time, DIN After SCLK↑ Setup Time CS↓ Before Clocking in First Address Bit (Note 8) Setup Time, DIN Stable Before SCLK↑ ACLK High Time ACLK Low Time CS High Time During Conversion 1283fb 2 U W U U U U U WW W U WW LTC1283 CO VERTER A D PARAMETER Offset Error Linearity Error Gain Error Minimum Resolution for Which No Missing Codes are Guaranteed Reference Input Resistance Analog and REF Input Range On Channel Leakage Current (Note 7) (Note 6) On Channel = 3V Off Channel = 0V On Channel = 0V Off Channel = 3V Off Channel Leakage Current (Note 7) On Channel = 3V Off Channel = 0V On Channel = 0V Off Channel = 3V ● ● ● ● AC CHARACTERISTICS (Note 3) SYMBOL tACC tSMPL tCONV tdDO tdis ten thDO tf tr CIN PARAMETER Delay Time From CS↓ to DOUT Data Valid Analog Input Sample Time Conversion Time Delay Time, SCLK↓ to DOUT Data Valid Delay Time, CS↑ to DOUT Hi-Z Delay Time, 2nd CLK↓ to DOUT Enabled Time Output Data Remains Valid After SCLK↓ DOUT Fall Time DOUT Rise Time Input Capacitance See Test Circuits See Test Circuits Analog Inputs On Channel Off Channel Digital Inputs ● ● DIGITAL A D DC ELECTRICAL CHARACTERISTICS (Note 3) SYMBOL VIH VIL IIH IIL VOH VOL PARAMETER High Level Input Voltage Low Level Input Voltage High Level Input Current Low Level Input Current High Level Output Voltage Low Level Output Voltage CONDITIONS VCC = 3.6V VCC = 3V VIN = VCC VIN = 0V VCC = 3V, IO = – 20μA IO = – 200μA VCC = 3V, IO = 20μA IO = 400μA ● ● ● ● ● ● WU U U ULTIPLEXER CHARACTERISTICS (Note 3) CONDITIONS (Note 4) (Notes 4 and 5) (Note 4) ● ● ● ● MIN LTC1283A TYP MAX ± 0.5 ± 0.5 ±1.0 10 10 (V –) – 0.05V to V 1 –1 –1 1 MIN LTC1283 TYP MAX ± 0.5 ± 0.5 ±2.0 10 10 UNITS LSB LSB LSB Bits kΩ V CC + 0.05V 1 –1 –1 1 μA μA μA μA CONDITIONS (Note 8) See Operating Sequence See Operating Sequence See Test Circuts See Test Circuits See Test Circuits ● ● ● LTC1283/LTC1283A MIN TYP MAX 2 5 44 400 240 300 75 90 80 65 5 5 300 300 900 500 800 UNITS ACLK Cycles SCLK Cycles ACLK Cycles ns ns ns ns ns ns pF pF pF LTC1283/LTC1283A MIN TYP MAX 1.7 0.45 2.5 – 2.5 2.6 2.0 2.8 0.05 0.10 UNITS V V μA μA V V 0.30 V V 1283fb 3 LTC1283 The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 3) SYMBOL IOZ ISOURCE ISINK ICC IREF I– PARAMETER Hi-Z Output Leakage Output Source Current Output Sink Current Positive Supply Current Reference Current Negative Supply Current CONDITIONS VOUT = VCC, CS High VOUT = 0V, CS High VOUT = 0V VOUT = VCC CS High, REF + Open VREF = 2.5V CS High, V – = – 3V ● ● ● ● ● DIGITAL A D DC ELECTRICAL CHARACTERISTICS Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: All voltage values are with respect to ground with DGND, AGND and REF – wired together (unless otherwise noted). Note 3: VCC = 3V, VREF+ = 2.5V, VREF– = 0V, V – = 0V for unipolar mode and – 3V for bipolar mode, ACLK = 1MHz, SCLK = 0.25MHz unless otherwise specified. Note 4: These specifications apply for both unipolar and bipolar modes. In bipolar mode, one LSB is equal to the bipolar input span (2VREF) divided by 1024. For example, when VREF = 2.5V, 1LSB (bipolar) = 2(2.5V)/1024 = 4.88mV. Note 5: Linearity error is the deviation from ideal of the slope between the two end points of the transfer curve. TYPICAL PERFOR A CE CHARACTERISTICS Supply Current vs Temperature 250 REF + OPEN ACLK = 500kHz VCC = CS = 3V MAGNITUDE OF OFFSET CHANGE, ⎪Δ OFFSET⎪ (LSB) SUPPLY CURRENT, ICC (μA) 200 IREF (μA) 150 100 50 0 50 25 0 75 100 –50 –25 AMBIENT TEMPERATURE (°C) 4 UW U LTC1283/LTC1283A MIN TYP MAX 3 –3 – 4.5 4.5 150 250 –1 350 500 – 50 UNITS μA μA mA mA μA μA μA Note 6: Two on-chip diodes are tied to each reference and analog input which will conduct for reference or analog input voltages one diode drop below V – or one diode drop above VCC. Be careful during testing at low VCC levels, as high level reference or analog inputs can cause this input diode to conduct, especially at elevated temperatures, and cause errors for inputs near full scale. This spec allows 50mV forward bias of either diode. This means that as long as the reference or analog input does not exceed the supply voltage by more than 50mV, the output code will be correct. Note 7: Channel leakage current is measured after the channel selection. Note 8: To minimize errors caused by noise at the chip select input, the internal circuitry waits for two ACLK falling edges after a chip select falling edge is detected before responding to control input signals. Therefore, no attempt should be made to clock an address in or data out until the minimum chip select setup time has elapsed. Reference Current vs Temperature 500 VCC = 3V VREF = 2.5V 400 0.5 Unadjusted Offset Error vs Reference Voltage VCC = 3V VREF = 2.5V ACLK = 500kHz 0.4 300 0.3 200 0.2 100 0.1 125 0 –50 –25 50 25 0 75 TEMPERATURE (°C) 100 125 0 –50 50 25 0 75 100 –25 AMBIENT TEMPERATURE (°C) 125 LTC1283 • G01 LTC1283 • G02 LTC1283 • G06 1283fb LTC1283 TYPICAL PERFOR A CE CHARACTERISTICS MAGNITUDE OF OFFSET CHANGE, ⎪Δ OFFSET⎪ (LSB) Change in Full-Scale Error vs Reference Voltage 2.0 1 × VREF) 1024 MAXIMUM ACLK FREQUENCY* (MHz) 1.75 1.5 1.25 1.0 0.75 0.5 0.25 0 –50 –25 VCC = 3V VREF = 2.5V LINEARITY ERROR (LSBs = 75 100 50 25 AMBIENT TEMPERATURE (˚C) 0 MAGNITUDE OF LINEARITY CHANGE, ⎪Δ LINEARITY⎪ (LSB) Change in Linearity Error vs Temperature MAGNITUDE OF GAIN CHANGE, ⎪Δ GAIN⎪ (LSB) 0.5 VCC = 3V VREF = 2.5V ACLK = 500kHz MAXIMUM ACLK FREQUENCY* (MHz) 0.4 0.3 0.2 0.1 0 50 25 0 75 100 –50 –25 AMBIENT TEMPERATURE (°C) Maximum Conversion Clock Rate vs Reference Voltage 1500 VCC = 3V 1500 MAXIMUM ACLK FREQUENCY* (kHz) 1250 MAXIMUM ACLK FREQUENCY* (kHz) 1250 1000 750 500 250 0 0 0.5 1.0 1.5 2.0 REFERENCE VOLTAGE (V) 2.5 MAXIMUM RFILTER** (Ω) *Maximum ACLK frequency represents the ACLK frequency at which a 0.1LSB shift in the error at any code transition from its 100kHz value is first detected. UW LTC1283 • G09 Linearity Error vs Reference Voltage 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 0.5 1.5 2.0 1.0 REFERENCE VOLTAGE (V) 2.5 VCC = 3V ACLK = 500kHz Change in Offset Error vs Temperature 0.5 VCC = 3V VREF = 2.5V ACLK = 500kHz 0.4 0.3 0.2 0.1 125 0 50 25 –50 –25 0 75 100 AMBIENT TEMPERATURE (°C) 125 LTC1283 • G05 LTC1283 • G06 Change in Gain Error vs Temperature 0.5 VCC = 3V VREF = 2.5V ACLK = 500kHz 2.0 1.75 1.5 1.25 1.0 0.75 0.5 0.25 Maximum Conversion Clock Rate vs Temperature VCC = 3V VREF = 2.5V 0.4 0.3 0.2 0.1 125 0 –50 –25 50 25 0 75 TEMPERATURE (°C) 100 125 0 –50 –25 75 100 0 50 25 AMBIENT TEMPERATURE (˚C) 125 LTC1283 • G07 LTC1283 • G08 LTC1283 • G09 Maximum Conversion Clock Rate vs Source Resistance VCC = 3V 100k Maximum Filter Resistor vs Cycle Time RFILTER VIN CFILTER ≥ 1μF + VCC = 3V 10k 1000 750 VIN + INPUT – 1k 500 – INPUT 100 250 0 1 RSOURCE– 10 RSOURCE (kΩ) 100 LTC1283 • G11 10 10 100 1000 10000 LTC1283 • G12 LTC1283 • G10 CYCLE TIME (μs) **Maximum RFILTER represents the filter resistor value at which a 0.1LSB change in full-scale error from its value at RFILTER = 0 is first detected. 1283fb 5 LTC1283 TYPICAL PERFOR A CE CHARACTERISTICS Sample-and-Hold Acquisition Time vs Source Resistance INPUT CHANNEL LEAKAGE CURRENT (nA) S&H ACQUISITION TIME TO 0.1% (μs) PEAK-TO-PEAK NOISE ERROR (LSB ) 10 9 8 7 6 5 VCC = 3V VREF = 2.5V TA = 25°C 0V TO 2.5V INPUT STEP + RSOURCE + 4V IN 3 – 2 1 0.1 1 RSOURCE (kΩ) PI FU CTIO S # 1-8 9 10 11 12 13, 14 15 16 17 18 19 20 PIN CH0-CH7 COM DGND AGND V– REF –, REF + CS DOUT DIN SCLK ACLK VCC FUNCTION Analog Inputs Common Digital Ground Analog Ground Negative Supply Reference Inputs Chip Select Input Digital Data Output Data Input Shift Clock A/D Conversion Clock Positive Supply DESCRIPTION The analog inputs must be free of noise with respect to AGND. The common pin defines the zero reference point for all single-ended inputs. It must be free of noise and is usually tied to the analog ground plane. This is the ground for the internal logic. Tie to the ground plane. AGND should be tied directly to the analog ground plane. Tie V – to most negative potential in the circuit. (Ground in single supply applications.) The reference inputs must be kept free of noise with respect to AGND. A logic low on this input enables data transfer. The A/D conversion result is shifted out of this output. The A/D configuration word is shifted into this input. This clock synchronizes the serial data transfer. This clock controls the A/D conversion process. This supply must be kept free of noise and ripple by bypassing directly to the analog ground plane. BLOCK DIAGRA VCC DIN 20 17 CH0 CH1 CH2 CH3 CH4 CH5 CH6 CH7 COM 1 2 3 4 5 6 7 8 9 CONTROL AND TIMING 10-BIT CAPACITIVE DAC 19 15 ACLK CS 1283fb ANALOG INPUT MUX 10 DGND 6 UW 10 LTC1283 • G13 Input Channel Leakage Current vs Temperature 1000 VCC = 3V VREF = 2.5V 100 GUARANTEED Noise Error vs Reference Voltage 1.2 VCC = 3V ACLK = 500kHz 1.0 0.8 LTC1283 NOISE = 200μVP-P 0.6 0.4 0.2 0 125 10 ON CHANNEL OFF CHANNEL 1 0.1 0.01 0 25 50 –50 –25 75 100 AMBIENT TEMPERATURE (°C) 0 0.5 1.0 1.5 2.0 REFERENCE VOLTAGE (V) 2.5 LTC1283 • G14 LTC1283 • G15 W U U U 18 INPUT SHIFT REGISTER OUTPUT SHIFT REGISTER SCLK 16 DOUT SAMPLEANDHOLD COMP 10-BIT SAR 11 AGND 12 V– 13 REF – 14 REF+ LTC1283 BD LTC1283 TEST CIRCUITS On and Off Channel Leakage Current 3V ION A IOFF A OFF CHANNELS DOUT 0.6V LTC1283 TC02 Voltage Waveforms for DOUT Delay Time, tdDO SCLK 0.6V tdDO 2.0V ON CHANNEL POLARITY LTC1283 TC01 Load Circuit for tdDO, tr, tf and ten 1.5V Voltage Waveform for DOUT Rise and Fall Times, tr and tf 2.0V DOUT 0.6V 3k DOUT 100pF LTC1283 TC06 TEST POINT tr tf LTC1283 TC03 Load Circuit for tdis TEST POINT 3k DOUT 100pF 3V WAVEFORM 2 WAVEFORM 1 LTC1283 TC05 Voltage Waveforms for ten and tdis 1 ACLK 2 CS 2.0V DOUT WAVEFORM 1 (SEE NOTE 1) ten DOUT WAVEFORM 2 (SEE NOTE 2) 2.0V tdis 0.6V NOTE 1: WAVEFORM 1 IS FOR AN OUTPUT WITH INTERNAL CONDITIONS SUCH THAT THE OUTPUT IS HIGH UNLESS DISABLED BY THE OUTPUT CONTROL. NOTE 2: WAVEFORM 2 IS FOR AN OUTPUT WITH INTERNAL CONDITIONS SUCH THAT THE OUTPUT IS LOW UNLESS DISABLED BY THE OUTPUT CONTROL. 90% 10% LTC1283 TC04 1283fb 7 LTC1283 APPLICATI S I FOR ATIO The LTC1283 is a 3V data acquisition component which contains the following functional blocks: 1. 10-bit successive approximation capacitive A/D converter 2. Analog multiplexer (MUX) 3. Sample-and-hold (S&H) 4. Synchronous, full duplex serial interface 5. Control and timing logic DIGITAL CONSIDERATIONS 1. Serial Interface The LTC1283 communicates with microprocessors and other external circuitry via a synchronous, full duplex, 4wire serial interface (see Operating Sequence). The shift clock (SCLK) synchronizes the data transfer with each bit being transmitted on the falling SCLK edge and captured on the rising SCLK edge in both transmitting and receiving systems. The data is transmitted and received simultaneously (full duplex). Data transfer is initiated by a falling chip select (CS) signal. After the falling CS is recognized, an 8-bit input word is shifted into the DIN input which configures the LTC1283 for the next conversion. Simultaneously, the result of the Operating Sequence (Example: Differential Inputs (CH3-CH2), Bipolar, MSB-First and 10-Bit Word Length) tCYC 1 SCLK tSMPL CS ODD/ SEL SGL/ SIGN SEL 0 WL0 MSBF WL1 UNI DIFF 1 2 3 4 5 6 7 8 9 10 DON’T CARE tCONV DIN DOUT SHIFT CONFIGURATION WORD IN 8 U previous conversion is output on the DOUT line. At the end of the data exchange the requested conversion begins and CS should be brought high. After tCONV, the conversion is complete and the results will be available on the next data transfer cycle. As shown below, the result of a conversion is delayed by one CS cycle from the input word requesting it. DIN DOUT DIN WORD 1 DOUT WORD 0 DATA TRANSFER tCONV A/D CONVERSION DIN WORD 2 DOUT WORD 1 DATA TRANSFER tCONV A/D CONVERSION LTC1283 • AI02 W U UO DIN WORD 3 DOUT WORD 2 2. Input Data Word The LTC1283 8-bit input data word is clocked into the DIN input on the first eight rising SCLK edges after chip select is recognized. Further inputs on the DIN pin are then ignored until the next CS cycle. The eight bits of the input word are defined as follows: DATA INPUT (DIN) WORD: SGL/ DIFF ODD/ SIGN SELECT 1 SELECT 0 UNIPOLAR/ BIPOLAR WORD LENGTH UNI MSBF WL1 WL0 MUX ADDRESS MSB-FIRST/ LSB-FIRST LTC1283 • AI03 DON’T CARE B9 (SB) B8 B7 B6 B5 B4 B3 B2 B1 B0 SHIFT A/D RESULT OUT AND NEW CONFIGURATION WORD IN LTC1283 • AI01 1283fb LTC1283 APPLICATI S I FOR ATIO Multiplexer (MUX) Address The first four bits of the input word assign the MUX configuration for the requested conversion. For a given channel selection, the converter will measure the voltage between the two channels indicted by the + and – signs in the selected row of Table 1. Note that in differential mode Table 1. Multiplexer Channel Selection SGL/ DIFF 0 0 0 0 0 0 0 0 MUX ADDRESS SELECT 0DD/ 0 SIGN 1 0 0 0 1 0 0 0 0 1 1 0 1 0 1 0 1 1 0 0 1 1 1 1 1 DIFFERENTIAL CHANNEL SELECTION 0 + 1 – 2 + 3 – + – + – + – + – + – + – 4 5 6 7 4 Differential CHANNEL 0,1 { { { { 2,3 + ( –) – ( +) + ( –) – ( +) + ( –) – ( +) + ( –) – ( +) 4,5 CHANNEL 0 1 2 3 4 5 6 7 6,7 Changing the MUX Assignment “On the Fly” 4,5 { { 6,7 + – + – COM (UNUSED) 1ST CONVERSION Figure 1. Examples of Multiplexer Options on the LTC1283 U (SGL/DIFF = 0) measurements are limited to four adjacent input pairs with either polarity. In single-ended mode, all input channels are measured with respect to COM. Figure 1 shows some examples of multiplexer assignments. SGL/ DIFF 1 1 1 1 1 1 1 1 MUX ADDRESS 0DD/ SELECT SIGN 1 0 0 0 0 0 1 0 0 0 1 0 1 1 1 0 0 1 1 0 1 0 1 1 1 1 SINGLE-ENDED CHANNEL SELECTION 0 + 1 2 + + + + + + 3 4 5 6 7 COM – – – – – – – – + W U UO 8 Single-Ended Combinations of Differential and Single-Ended CHANNEL 0,1 + + + + + + + + COM (–) { { 4 5 6 7 + – – + + + + + COM (–) 2,3 5,4 6 7 { { – + + + COM (–) 2ND CONVERSION LTC1283 • F01 1283fb 9 LTC1283 APPLICATI S I FOR ATIO Unipolar/Bipolar (UNI) The fifth input bit (UNI) determines whether the conversion will be unipolar or bipolar. When UNI is a logical one, a unipolar conversion will be performed on the selected Unipolar Transfer Curve (UNI = 1) 1111111111 1111111110 OUTPUT CODE 1111111111 1111111110 • • • 0000000001 0000000000 INPUT VOLTAGE VREF – 1LSB VREF – 2LSB • • • 1LSB 0V • • • 0000000001 0000000000 0V 1LSB VREF – 2LSB VREF LTC1283 AI04 VREF – 1LSB Bipolar Transfer Curve (UNI = 0) 0111111111 0111111110 1LSB –VREF + 1LSB –VREF 0000000001 0000000000 VIN 1111111111 1111111110 –1LSB –2LSB 1000000001 1000000000 VREF – 1LSB VREF – 2LSB VREF Bipolar Output Code (UNI = 0) OUTPUT CODE 0111111111 0111111110 • • • 0000000001 0000000000 1111111111 1111111110 • • • 1000000001 1000000000 INPUT VOLTAGE VREF – 1LSB VREF – 2LSB • • • 1LSB 0V –1LSB –2LSB • • • –(VREF) + 1LSB – (VREF) INPUT VOLTAGE (VREF = 2.5V) 2.4951V 2.4902V • • • 0.0049V 0V – 0.0049V – 0.0098V • • • –2.4951V –2.5000V LTC1283 AI07 10 U input voltage. When UNI is a logical zero, a bipolar conversion will result. The input span and code assignment for each conversion type are shown in the figures below. Unipolar Output Code (UNI = 1) INPUT VOLTAGE (VREF = 2.5V) 2.4976V 2.4951V • • • 0.0024V 0V LTC1283 AI06 W U UO VIN LTC1283 AI05 1283fb LTC1283 APPLICATI S I FOR ATIO MSB-First/LSB-First Format (MSBF) The output data of the LTC1283 is programmed for MSBfirst or LSB-first sequence using the MSBF bit. For MSBfirst output data the input word clocked to the LTC1283 should always contain a logical one in the sixth bit location (MSBF bit). Likewise for LSB-first output data, the input word clocked to the LTC1283 should always contain a zero in the MSBF bit location. The MSBF bit in a given DIN word will control the order of the next DOUT word. The MSBF bit affects only the order of the output data word. The order of the input word is unaffected by this bit. MSBF 0 1 OUTPUT FORMAT LSB-First MSB-First LTC1283 AI08 Word Length (WL1, WL0) The last two bits of the input word (WL1 and WL0) program the output data word length of the LTC1283. Word lengths of 8-, 10-, 12- or 16-bit can be selected according to the following table. The WL1 and WL0 bits in a given DIN word control the length of the present, not the next, DOUT word. WL1 and WL2 are never “don’t cares” and must be set for the correct DOUT word length even when a “dummy” DIN word is sent. On any transfer cycle, the word length should be made equal to the number of SCLK cycles sent by the MPU. WL1 0 0 1 1 WL0 0 1 0 1 OUTPUT WORD LENGTH 8 Bits 10 Bits 12 Bits 16 Bits LTC1283 • AI09 Figure 2 shows how the data output (DOUT) timing can be controlled with word length selection and MSB/LSB-first format selection. 3. Deglitcher A deglitching circuit has been added to the chip select input of the LTC1283 to minimize the effects of errors caused by noise on that input. This circuit ignores changes in state on the CS input that are shorter in duration than U 1 ACLK cycle. After a change of state on the CS input, the LTC1283 waits for two falling edges of the ACLK before recognizing a valid chip select. One indication of CS low recognition is the DOUT line becoming active (leaving the Hi-Z state). Note that the deglitching applies to both the rising and falling CS edges. ACLK CS DOUT HI-Z VALID OUTPUT LOW CS RECOGINZED INTERNALLY ACLK CS DOUT VALID OUTPUT HI-Z HIGH CS RECOGNIZED INTERNALLY LTC1283 • AI10 W U UO 4. CS Low During Conversion In the normal mode of operation, CS is brought high during the conversion time (see Figure 3). The serial port ignores any SCLK activity while CS is high. The LTC1283 will also operate with CS low during the conversion. In this mode, SCLK must remain low during the conversion as shown in Figure 4. After the conversion is complete, the DOUT line will become active with the first output bit. Then the data transfer can begin as normal. 5. Microprocessor Interfaces The LTC1283 can interface directly (without external hardware) to most popular microprocessor (MPU) synchronous serial formats (see Table 2). If an MPU without a serial interface is used, then four of the MPU’s parallel port lines can be programmed to form the serial link to the LTC1283. Included here are three serial interface examples and one example showing a parallel port programmed to form the serial interface. 1283fb 11 LTC1283 APPLICATI S I FOR ATIO 8-Bit Word Length CS SCLK DOUT MSB-FIRST DOUT LSB-FIRST 1 (SB) B9 B8 B7 B6 B5 B4 B3 B0 B1 B2 B3 B4 10-Bit Word Length tSMPL CS tCONV SCLK DOUT MSB-FIRST DOUT LSB-FIRST 1 (SB) B9 B8 B7 B6 B5 B4 B3 B2 B1 B0 B1 B2 B3 B4 12-Bit Word Length tSMPL CS tCONV SCLK 1 (SB) DOUT MSB-FIRST DOUT LSB-FIRST B9 B8 B7 B6 B5 B0 B1 B2 B3 B4 16-Bit Word Length tSMPL CS tCONV SCLK 1 (SB) DOUT MSB-FIRST DOUT LSB-FIRST B9 B8 B7 B6 B5 B0 B1 B2 B3 B4 * IN UNIPOLAR MODE, THESE BITS ARE FILLED WITH ZEROES. IN BIPOLAR MODE, THE SIGN BIT IS EXTENDED INTO THESE LOCATIONS. Figure 2. Data Output (DOUT) Timing with Different Word Lengths 1283fb 12 U tSMPL tCONV 8 B2 THE LAST TWO BITS ARE TRUNCATED B5 B6 B7 10 B0 (SB) B5 B6 B7 B8 B9 10 12 B4 B3 B2 B1 B0 (SB) B5 B6 B7 B8 B9 * * FILL ZEROES 10 16 B4 B3 B2 B1 B0 (SB) B5 B6 B7 B8 B9 * * * * * * FILL ZEROES LTC1283 • F02 W U UO LTC1283 APPLICATI CS S I FOR ATIO tSMPL SAMPLE ANALOG INPUT SHIFT MUX ADDRESS IN SCLK DIN DOUT SGL/ SEL SEL DIFF ODD/ 1 0 UNI MSBF WL1 WL0 SIGN B9 B8 B7 B6 B5 B4 B3 B2 B1 B0 DON’T CARE SGL/ SEL DIFF ODD/ 1 SEL UNI MSBF WL1 WL0 0 SIGN B9 B8 B7 B6 B5 B4 B3 B2 B1 B0 LTC1283 • F03 Figure 3. CS High During Conversion SHIFT MUX ADDRESS IN CS tSMPL SAMPLE ANALOG INPUT SCLK DIN DOUT SGL/ SEL SEL DIFF ODD/ 1 0 UNI MSBF WL1 WL0 SIGN B9 B8 B7 B6 B5 B4 B3 B2 B1 B0 Figure 4. CS Low During Conversion Table 2. 3V Microprocessor with Hardware Serial Interfaces Compatible with the LTC1283* PART NUMBER Motorola MC68HC11 MC68HC05 RCA CDP68HC05 National Semiconductor COP800 Family HPC16000 Family Texas Instruments TMS70C02 TMS70C42 TYPE OF INTERFACE SPI SPI SPI MICROWIRE/PLUS† MICROWIRE/PLUS† Serial Port Serial Port *Contact factory for interface information for processors not on this list † MICROWIRE/PLUS is a trademark of National Semiconductor Corp. U 40 TO 44 ACLK CYCLES SHIFT RESULT OUT AND NEW ADDRESS IN 40 TO 44 ACLK CYCLES SHIFT RESULT OUT AND NEW ADDRESS IN SCLK MUST REMAIN LOW SEL SGL/ DIFF ODD/ 1 SEL UNI MSBF WL1 WL0 0 SIGN B9 B8 B7 B6 B5 B4 B3 B2 B1 B0 LTC1283 • F04 W U UO DON’T CARE Serial Port Microprocessors Most synchronous serial formats contain a shift clock (SCLK) and two data lines, one for transmitting and one for receiving. In most cases data bits are transmitted on the falling edge of the clock (SCLK) and captured on the rising edge. However, serial port formats vary among MPU manufacturers as to the smallest number of bits that can be sent in one group (e.g., 4-bit, 8-bit or 16-bit transfers). They also vary as to the order in which the bits are transmitted (LSB- or MSB-first). The following examples show how the LTC1283 accommodates these differences. National MICROWIRE (COP820C) The COP820C transfers data MSB-first and in 8-bit increments. This is easily accommodated by setting the LTC1283 to MSB-first format and 10-bit word length. The data output word is then received by the COP820C in one 8-bit block and one 2-bit block. 1283fb 13 LTC1283 APPLICATI S I FOR ATIO COP820C G1 SK SO SI LTC1283 • AI11 Hardware and Software Interface to National Semiconductor COP820C Processor LTC1283 CS ANALOG INPUTS • • • • SCLK DIN DOUT MNEMONIC LD (F0)←0D LD (D5)←32 LD (EE)←8 LD (B)←D4 LD (A)←(F0) RBIT 1 X (A)←→(E9) LD (B)←EF SBIT 2 ↑ NOP ↓ COMMENTS LOAD 0D INTO F0 (DIN) CONFIGURE PORT G CONFIGURE CONTROL REG. PORT G DATA REG. INTO B LOAD DIN INTO ACC G1 RESET (CS GOES LOW) LOAD DIN INTO SHIFT REG. LOAD PSW REG. ADDR IN B TRANSFER BEGINS 15 NOPs FOR TIMING Motorola SPI (MC68HC05C4, MC68HC11) The MC68HC05C4 and MC68HC11 transfer data MSBfirst and in 8-bit increments. Programming the LTC1283 for MSB-first format and 16-bit word length allows the 10Hardware and Software Interface to Motorola MC68HC05C4 and MC68HC11 Processors LTC1283 CS ANALOG INPUTS • • • • SCLK DIN DOUT MC68HC05C4 MC68HC11 C0 SCK MOSI MISO LTC1283 • AI13 MNEMONIC BCLR n LDA STA ↑ NOP ↓ LDA LDA STA COMMENTS C0 IS CLEARED (CS GOES LOW) LOAD DIN FOR LTC1283 INTO ACC LOAD DIN FROM ACC TO SPI DATA REG. START SCK 8 NOPs FOR TIMING LOAD CONTENTS OF SPI STATUS REG. INTO ACC LOAD LTC1283 DOUT FROM SPI DATA REG. INTO ACC (BYTE 1) LOAD LTC1283 DOUT INTO RAM (LOCATION A) 14 U DOUT from LTC1283 stored in COP820C RAM MSB* 9 8 LSB 1 0 X X X X X X BYTE 2 LTC1283 • AI12 W U UO 7 6 5 4 3 2 BYTE 1 *B9 is MSB in unipolar or sign bit in bipolar MNEMONIC X(A)←→(E9) SBIT 2 X (A)←→(F3) RBIT 2 LD (B)←D4 SBIT 1 X (A)←→(E9) RC RRCA RRCA RRCA X (A)←→(F4) COMMENTS LOAD DOUT INTO ACC TRANSFER CONTINUES LOAD DOUT IN ADDR F3 STOP TRANSFER PUT PORT G ADDR IN B G1 SET (CS GOES HIGH) LOAD DOUT INTO ACC CLEAR CARRY SHIFT RIGHT THRU CARRY SHIFT RIGHT THRU CARRY SHIFT RIGHT THRU CARRY LOAD DOUT IN ADDR F4 bit data output to be received by the MPU as two 8-bit bytes with the final 6 unused bits filled with zeroes by the LTC1283. DOUT from LTC1283 stored in MC68HC05C4 or MC68HC11 RAM MSB* LOCATION A B9 B8 LSB LOCATION A + 1 B1 B0 0 0 0 0 0 0 BYTE 2 LTC1283 • AI14 B7 B6 B5 B4 B3 B2 BYTE 1 *B9 is MSB in unipolar or sign bit in bipolar MNEMONIC STA ↑ NOP ↓ BSET n LDA LDA STA COMMENTS START NEXT SPI CYCLE 6 NOPs FOR TIMING CO IS SET (CS GOES HIGH) LOAD CONTENTS OF SPI STATUS REG. INTO ACC LOAD LTC1283 DOUT FROM SPI DATA REG. INT ACC (BYTE 2) LOAD LTC1283 INTO RAM (LOCATION A +1) 1283fb LTC1283 APPLICATI S I FOR ATIO Texas Instruments TMS70C42 The TMS70C42 transfers serial data in 8-bit increments, LSB-first. To accommodate this, the LTC1283 is programmed for 16-bit word length and LSB-first format. The Hardware and Software Interface to TI TMS70C42 Processor LTC1283 CS ANALOG INPUTS • • • • SCLK DIN DOUT TMS70C42 AO SCLK TXD RXD LTC1283 • AI15 LABEL START MNEMONIC DINT MOVP MOVP MOV LDSP MOVP MOVP MOVP MOVP MOVP MOVP MOVP MOVP MOV CALL MOV MOV ANDP MOVP % > 2A, P0 % > 02, P16 % > 60, B % > DF, P5 % > 08, P6 % > 40, P21 % > 0C, P20 % > 00, P24 % > 00, P21 % > 00, P23 % > C0, P24 % > DF, A SXTNBIT B, R5 A, R6 % > FE, P4 A, P26 DESCRIPTION DISABLES ALL INTERRUPTS DISABLE INTERRUPT FLAGS DISABLE INTERRUPT FLAGS ADDRESS OF STACK PUT ADDRESS INTO POINTER CONFIGURE PORT A ENABLE Tx BY SETTING B3 = 1 RESET THE SERIAL PORT CONFIGURE THE SERIAL PORT TURN START BIT OFF ENABLE THE SERIAL PORT SET SCLK RATE (TIMER 3) START TIMER LOAD DIN WORD IN A ROUTINE THAT SHIFTS DATA PUT FIRST 8 LSBs IN R5 PUT MSBs IN R6 A0 CLEARED (CS GOES LOW) PUT DIN INTO TXBUF LOOP SXTNBIT Parallel Port Microprocessors When interfacing the LTC1283 to an MPU which has a parallel port, the serial signals are created on the port with software. Three MPU port lines are programmed to create the CS, SCLK and DIN signals for the LTC1283. A fourth port line reads the DOUT line. An example is made of the Signetics 83CL410. Hardware and Software Interface to Signetics 83CL410 Processor LTC1283 DOUT ANALOG INPUTS • • • • • • • • DIN SCLK CS ACLK 83CL410 P1.1 P1.2 P1.3 P1.4 ALE LTC1283 • AI17 U 10-bit output data is received by the processor as two 8-bit bytes, LSB-first. The LTC1283 fills the final 6 unused bits (after the MSB) with zeroes. DOUT from LTC1283 stored in TMS70C42 RAM LSB 7 6 5 4 3 2 1 MSB 0 0 9 8 BYTE 2 0 BYTE 1 FILL WITH ZEROES 0 0 0 0 LTC1283 • AI16 W U UO LABEL MNEMONIC MOVP MOVP MOVP MOVP MOV DJNZ NOP MOVP MOVP MOVP MOVP MOVP MOVP MOV DJNZ NOP MOVP ORP RETS % > 40, P24 % > 17, P21 % > C0, P24 % > 16, P21 % > 02, A A, WAIT1 P25, B A, P26 % > 40, P24 % > 17, P21 % > C0, P24 % > 16, P21 % > 02, A A, WAIT2 P25, A % > 01, P4 DESCRIPTION SCLK OFF (TIMER 3 DISABLED) ENABLE SERIAL PORT SCLK ON (TRANSFER BEGINS) TXEN GOES LOW LOAD COUNTER LOOP WHILE SHIFT OCCURS DELAY PUT DOUT IN B LOAD TXBUF SCLK OFF (TIMER 3 DISABLE) ENABLE SERIAL PORT SCLK ON (TRANSFER BEGINS) TXEN GOES LOW LOAD COUNTER LOOP WHILE SHIFT OCCURS DELAY PUT DOUT IN A A0 SET (CS GOES HIGH) RETURN TO MAIN PROGRAM WAIT1 WAIT2 Signetics 83CL410 To interface to the 83CL410, (a 3V version of the 80C51) the LTC1283 is programmed for MSB-first format and 10bit word length. The 83CL410 generates CS, SCLK and DIN on three port lines and reads DOUT on the fourth. DOUT from LTC1283 stored in 83CL410 RAM MSB* R2 B9 B8 LSB R3 B1 B2 0 0 0 0 0 0 B7 B6 B5 B4 B3 B2 *B9 IS MSB IN UNIPOLAR OR SIGN BIT IN BIPOLAR LTC1283 • AI18 1283fb 15 LTC1283 APPLICATI MNEMONIC S I FOR ATIO DESCRIPTION MOV P1, #02H CLR P1.3 SETB P1.4 CONTINUE: MOV A, #0DH CLR P1.4 MOV R4, #08 NOP MOV C, P1.1 RLC A MOV P1.2, C SETB P1.3 CLR P1.3 DJNZ R4, LOOP MOV R2, A LOOP: INITIALIZE PORT 1 (BIT 1 IS MADE AN INPUT) SCLK GOES LOW CS GOES LOW DIN WORD FOR THE LTC1283 IS PLACED IN ACC CS GOES LOW LOAD COUNTER DELAY FOR DEGLITCHER READ DATA BIT INTO CARRY ROTATE DATA BIT INTO ACC OUTPUT DIN BIT TO LTC1283 SCLK GOES HIGH SCLK GOES LOW NEXT BIT STORE MSBs IN R2 6. Sharing the Serial Interface The LTC1283 can share the same 3-wire serial interface with other peripheral components or other LTC1283s (see Figure 5). In this case, the CS signals decide which LTC1283 is being addressed by the MPU. ANALOG CONSIDERATIONS 1. Grounding The LTC1283 should be used with an analog ground plane and single point grounding techniques. Pin 11 (AGND) should be tied directly to this ground plane. Pin 10 (DGND) can also be tied directly to this ground plane because minimal digital noise is generated within the chip itself. Pin 20 (VCC) should be bypassed to the ground plane with a 4.7μF tantalum with leads as short as possible. Pin 12 2 1 0 OUTPUT PORT SERIAL DATA MPU 3 3 CS LTC1283 8 CHANNELS 3 CS LTC1283 8 CHANNELS 3 CS LTC1283 8 CHANNELS 3-WIRE SERIAL INTERFACE TO OTHER PERIPHERALS OR LTC1283s Figure 5. Several LTC1283s Sharing One 3-Wire Serial Interface 1283fb 16 U 83CL410 Code MNEMONIC MOV C, P1.1 CLR A RLC A SETB P1.3 CLR P1.3 MOV C, P1.1 RRC A RRC A MOV R3, A SETB P1.3 CLR P1.3 SETB P1.4 MOV R5, #07H DJNZ R5, DELAY AJMP CONTINUE DESCRIPTION READ DATA BIT INTO CARRY CLEAR ACC ROTATE DATA BIT INTO ACC SCLK GOES HIGH SCLK GOES LOW READ DATA BIT IN CARRY ROTATE RIGHT INTO ACC ROTATE RIGHT INTO ACC STORE LSBs IN R3 SCLK GOES HIGH SCLK GOES LOW CS GOES HIGH LOAD COUNTER DELAY FOR LTC1283 TO PERFORM CONVERSION REPEAT PROGRAM DELAY: W U UO (V –) should be bypassed with a 0.1μF ceramic disk. For single supply applications, V – can be tied to the ground plane. It is also recommended that pin 13 (REF –) and pin 9 (COM) be tied directly to the ground plane. All analog inputs should be referenced directly to the single point ground. Digital inputs and outputs should be shielded from and/or routed away from the reference and analog circuitry. Figure 6 shows an example of an ideal ground plane design for a two-sided board. Of course this much ground plane will not always be possible, but users should strive to get as close to this ideal as possible. 2. Bypassing For good performance, VCC must be free of noise and ripple. Any changes in the VCC voltage with respect to analog ground during a conversion cycle can induce errors LTC1283 • F05 LTC1283 APPLICATI S I FOR ATIO VCC 4.7μF TANTALUM 1 2 3 4 5 6 7 8 9 10 ANALOG GROUND PLANE 20 19 18 17 16 15 14 13 12 11 V– 0.1µF CERAMIC DISK VERTICAL: 0.5mV/DIV Figure 6. Example Ground Plane for the LTC1283 or noise in the output code. VCC noise and ripple can be kept below 1mV by bypassing the VCC pin directly to the analog ground plane with a 4.7μF tantalum with leads as short as possible. Figures 7 and 8 show the effects of good and poor VCC bypassing. 3. Analog Inputs Because of the capacitive redistribution A/D conversion techniques used, the analog inputs of the LTC1283 have capacitive switching input current spikes. These current spikes settle quickly and do not cause a problem. However, if large source resistances are used or if slow settling op amps drive the inputs, care must be taken to insure that the transients caused by the current spikes settle completely before the conversion begins. Source Resistance VERTICAL: 0.5mV/DIV The analog inputs of the LTC1283 look like 65pF capacitor (CIN) in series with a 500Ω resistor (RON) as shown in Figure 9. CIN gets switched between the selected “+” and “–” inputs once during each conversion cycle. Large external source resistors and capacitances will slow the settling of the inputs. It is important that the overall RC time constants be short enough to allow the analog inputs to completely settle within the allowed time. U HORIZONTAL: 10μs/DIV W U UO Figure 7. Poor VCC Bypassing. Noise and Ripple can Cause A/D Errors LTC1283 • F06 HORIZONTAL: 10μs/DIV Figure 8. Good VCC Bypassing Keeps Noise and Ripple on VCC Below 1mV VIN + RSOURCE + “+” INPUT LTC1283 C1 4TH SCLK RON = 500Ω LAST SCLK CIN = 65pF VIN – RSOURCE – “–” INPUT C2 LTC1283 • F09 Figure 9. Analog Input Equivalent Circuit 1283fb 17 LTC1283 APPLICATI S I FOR ATIO SAMPLE MUX ADDRESS SHIFTED IN “+” INPUT MUST SETTLE DURING THIS TIME tSMPL ••• CS SCLK 1 2 3 4 ••• ACLK ••• “+” INPUT “–” INPUT LTC1283 • F10 Figure 10. “+” and “–” Input Settling Windows “+” Input Settling This input capacitor is switched onto the “+” input during the sample phase (tSMPL, see Figure 10). The sample phase starts at the 4th SCLK cycle and lasts until the falling edge of the last SCLK (the 8th, 10th, 12th or 16th SCLK cycle depending on the selected word length). The voltage on the “+” input must settle completely within this sample time. Minimizing RSOURCE+ and C1 will improve the input settling time. If large “+” input source resistance must be used, the sample time can be increased by using a slower SCLK frequency or selecting a longer word length. With the minimum possible sample time of 8μs, RSOURCE+ < 2k and C1 < 20pF will provide adequate settling. “–” Input Settling At the end of the sample phase the input capacitor switches to the “–” input and the conversion starts (see Figure 10). During the conversion, the “+” input voltage is effectively “held” by the sample-and-hold and will not affect the conversion result. However, it is critical that the “–” input voltage be free of noise and settle completely during the first four ACLK cycles of the conversion time. Minimizing RSOURCE– and C2 will improve settling time. If large “–” input source resistance must be used, the time allowed for settling can be extended by using a slower ACLK frequency. At the maximum ACLK rate of 1MHz, RSOURCE– < 1k and C2 < 20pF will provide adequate settling. VERTICAL: 5mV/DIV 18 U HOLD LAST SCLK (8TH, 10TH, 12TH OR 16TH DEPENDING ON WORD LENGTH) 1 2 3 4 ••• 1ST BIT TEST “–” INPUT MUST SETTLE DURING THIS TIME W U UO Input Op Amps When driving the analog inputs with an op amp it is important that the op amp settle within the allowed time (see Figure 10). Again, the “+” and “–” input sampling times can be extended as described above to accommodate slower op amps. Most op amps including the LT1006 and LT1013 single supply op amps can be made to settle well even with the minimum settling windows of 8μs (“+” input) and 4μs (“–” input) which occur at the maximum clock rates (ACLK = 1MHz and SCLK = 0.5MHz). Figures 11 and 12 show examples of adequate and poor op amp settling. HORIZONTAL: 1μs/DIV Figure 11. Adequate Settling of Op Amp Driving Analog Input 1283fb LTC1283 APPLICATI S I FOR ATIO VERTICAL: 5mV/DIV HORIZONTAL: 1μs/DIV Figure 12. Poor Op Amp Settling Can Cause A/D Errors RC Input Filtering It is possible to filter the inputs with an RC network as shown in Figure 13. For large values of CF (e.g., 1μF), the capacitive input switching currents are averaged into a net DC current. Therefore, a filter should be chosen with small resistor and large capacitor to prevent DC drops across the resistor. The magnitude of the DC current is approximately IDC = 65pF × VIN /tCYC and is roughly proportional to VIN. When running at the minimum cycle time of 68μs, the input current equals 2.5μA at VIN = 2.5V. In this case, a filter resistor of 100Ω will cause 0.1LSB of full-scale error. If a larger filter resistor must be used, errors can be eliminated by increasing the cycle time as shown in the typical curve Maximum Filter Resistor vs Cycle Time. RFILTER VIN CFILTER IDC “+” LTC1283 “–” LTC1283 • F13 Figure 13. RC Input Filtering Input Leakage Current Input leakage currents can also create errors if the source resistance gets too large. For instance, the maximum input U leakage specification of 1μA (at 125°C) flowing through a source resistance of 1k will cause a voltage drop of 1mV or 0.4LSB. This error will be much reduced at lower temperatures because leakage drops rapidly (see typical curve of Input Channel Leakage Current vs Temperature). Noise Coupling into Inputs W U UO High source resistance input signals (>500Ω) are more sensitive to coupling from external sources. It is preferable to use channels near the center of the package (i.e., CH2-CH7) for signals which have the highest output resistance because they are essentially shielded by the pins of the package ends (DGND and CH0). Grounding any unused inputs (especially the end pin, CH0) will also reduce outside coupling into high source resistances. 4. Sample-and-Hold Single-Ended Inputs The LTC1283 provides a built-in sample-and-hold (S&H) function for all signals acquired in the single-ended mode (COM pin grounded). This sample-and-hold allows the LTC1283 to convert rapidly varying signals (see typical curve of S&H Acquisition Time vs Source Resistance). The input voltage is sampled during the tSMPL time as shown in Figure 10. The sampling interval begins after the fourth MUX address bit is shifted in and continues during the remainder of the data transfer. On the falling edge of the final SCLK, the S&H goes into hold mode and the conversion begins. The voltage will be held on either the 8th, 10th, 12th or 16th falling edge of the SCLK depending on the word length selected. Differential Inputs With differential inputs, or when the COM pin is not tied to ground, the A/D no longer converts just a single voltage but rather the difference between two voltages. In these cases, the voltage on the selected “+” input is still sampled and held and therefore may be rapidly time varying just as in single-ended mode. However, the voltage on the selected “–” input must remain constant and be free of noise and ripple throughout the conversion time. Otherwise, the 1283fb 19 LTC1283 APPLICATI S I FOR ATIO differencing operation may not be performed accurately. The conversion time is 44 ACLK cycles. Therefore, a change in the “–” input voltage during this interval can cause conversion errors. For a sinusoidal voltage on the “–” input this error would be: VERROR (MAX) = VPEAK × 2 × π × f(“–”) × 44/fACLK Where f(“–”) is the frequency of the “–” input voltage, VPEAK is its peak amplitude and fACLK is the frequency of the ACLK. In most cases VERROR will not be significant. For a 60Hz signal on the “–” input to generate a 1/4LSB error (0.61mV) with the converter running at ACLK = 1MHz, its peak value would have to be 38mV. 5. Reference Inputs The voltage between the reference inputs of the LTC1283 defines the voltage span of the A/D converter. The reference inputs look primarily like a 10k resistor but will have transient capacitive switching currents due to the switchedcapacitor conversion technique (see Figure 14). During each bit test of the conversion (every 4 ACLK cycles), a capacitive current spike will be generated on the reference pins by the A/D. These current spikes settle quickly and do not cause a problem. However, if slow settling circuitry is used to drive the reference inputs, care must be taken to insure that transients caused by these current spikes settle completely during each bit test of the conversion. REF+ 14 ROUT VREF REF – 13 10k TYP LTC1283 EVERY 4 ACLK CYCLES RON 5pF TO 30pF VERTICAL: 0.5mV/DIV LTC1283 • F14 Figure 14. Reference Input Equivalent Circuit VERTICAL: 0.5mV/DIV When driving the reference inputs, three things should be kept in mind: 1. The source resistance (ROUT) driving the reference inputs should be low (less than 1Ω) to prevent DC drops caused by the 300μA maximum reference current (IREF). 2. Transients on the reference inputs caused by the capacitive switching currents must settle completely during each bit test (each 4 ACLK cycles). Figures 15 20 U and 16 show examples of both adequate and poor settling. Using a slower ACLK will allow more time for the reference to settle. However, even at the maximum ACLK rate of 1MHz most references and op amps can be made to settle within the 4μs bit time. 3. It is recommended that the REF – input be tied directly to the analog ground plane. If REF – is biased at a voltage other than ground, the voltage must not change during a conversion cycle. This voltage must also be free of noise and ripple with respect to analog ground. HORIZONTAL: 1μs/DIV W U UO Figure 15. Adequate Reference Settling HORIZONTAL: 1μs/DIV Figure 16. Poor Reference Settling Can Cause A/D Errors 6. Reduced Reference Operation The effective resolution to the LTC1283 can be increased by reducing the input span of the converter. The LTC1283 exhibits good linearity and gain over a wide range of reference voltages (see typical curves of Linearity and Gain Error vs Reference Voltage). However, care must be taken 1283fb LTC1283 APPLICATI S I FOR ATIO when operating at low values of VREF because of the reduced LSB step size and the resulting higher accuracy requirement placed on the converter. The following factors must be considered when operating at low VREF values. 1. Conversion speed (ACLK frequency) 2. Offset 3. Noise Conversion Speed with Reduced VREF With reduced reference voltages, the LSB step size is reduced and the LTC1283 internal comparator overdrive is reduced. With less overdrive, more time is required to perform a conversion. Therefore, the maximum ACLK frequency should be reduced when low values of V REF are used. This is shown in the typical curve of Maximum Conversion Clock Rate vs Reference Voltage. Offset with Reduced VREF The offset of the LTC1283 has a larger effect on the output code when the A/D is operated with reduced reference voltage. The offset (which is typically a fixed voltage) becomes a larger fraction of an LSB as the size of the LSB is reduced. The typical curve of Unadjusted Offset Error vs Reference Voltage shows how offset in LSBs is related to reference voltage for a typical value of VOS. For example, a VOS of 0.5mV which is 0.2LSB with a 2.5V reference becomes 0.5LSB with a 1V reference and 2.5LSBs with a 0.2V reference. If this offset is unacceptable, it can be corrected digitally by the receiving system or by offsetting the “–” input to the LTC1283. Noise with Reduced VREF The total input referred noise of the LTC1283 can be reduced to approximately 200μV peak-to-peak using a ground plane, good bypassing, good layout techniques and minimizing noise on the reference inputs. This noise is insignificant with a 2.5V reference but will become a larger fraction of an LSB as the size of the LSB is reduced. The typical curve of Noise Error vs Reference Voltage shows the LSB contribution of this 200μV of noise. For operation with a 2.5V reference, the 200μV noise is only 0.08LSB peak-to-peak. In this case, the LTC1283 U noise will contribute virtually no uncertainty to the output code. However, for reduced references, the noise may become a significant fraction of an LSB and cause undesirable jitter in the output code. For example, with a 1V reference, this same 200μV noise is 0.2LSB peak-topeak. This will reduce the range of input voltages over which a stable output code can be achieved by 0.2LSB. If the reference is further reduced to 200mV, the 200 μV noise becomes equal to one LSB and a stable code may be difficult to achieve. In this case averaging readings may be necessary. This noise data was taken in a very clean setup. Any setup induced noise (noise or ripple on VCC, VREF, VIN or V –) will add to the internal noise. The lower the reference voltage to be used, the more critical it becomes to have a clean, noise-free setup. A “Quick Look” Circuit for the LTC1283 Users can get a quick look at the function and timing of the LTC1283 by using the following simple circuit. REF + and DIN are tied to VCC selecting a 3V input span, CH7 as a single-ended input, unipolar mode, MSB-first format and 16-bit word length. ACLK and SCLK are tied together and driven by an external clock. CS is driven at 1/64 the clock rate by the CD4520 and DOUT outputs the data. All other pins are tied to a ground plane. The output data from the DOUT pin can be viewed on an oscilloscope which is set up to trigger on the falling edge of CS. Scope Trace of LTC1283 “Quick Look” Circuit Showing A/D Output of 0101010101 (155HEX) CS DOUT DEGLITCHER TIME MSB (B9) LSB (B0) FILLS ZEROES VERTICAL: 1V/DIV, HORIZONTAL: 5μs/DIV 1283fb W U UO 21 LTC1283 TYPICAL APPLICATI CHO CH1 CH2 CH3 CH4 CH5 CH6 VIN CH7 COM DGND LTC1283 TO OSCILLOSCOPE SNEAK-A-BITTM The LTC1283’s unique ability to software select the polarity of the differential inputs and the output word length is used to achieve one more bit of resolution. Using the circuit below with two conversions and some software, a 2’s complement 10-bit + sign word is returned to memory inside the MPU. The MC68HC05C4 was chosen as an example; however, any processor could be used. SNEAK-A-BIT Circuit 10μF 3V CHO OTHER CHANNELS OR SNEAK-A-BIT INPUTS VIN –3V TO 3V CH1 CH2 CH3 CH4 CH5 CH6 CH7 COM DGND LTC1283 VCC ACLK SCLK DIN DOUT CS REF + REF – V– AGND 0.1μF –3V DIN 1 DIN 2 DIN 3 SNEAK-A-BIT is a trademark of Linear Technology Corp. LTC1283 • TA05 { 22 UO S A “Quick Look” Circuit for the LTC1283 3V 4.7μF f/64 VCC ACLK SCLK DIN DOUT CS REF + REF – V– AGND CLOCK IN 500kHz MAX f CLK EN Q1 Q2 Q3 Q4 RESET VSS CD4520 VDD RESET Q4 Q3 Q2 Q1 EN CLK LTC1283 • TA03 0.1μF Two 10-bit unipolar conversions are performed: the first over a 0V to 3V span and the second over a 0V to – 3V span (by reversing the polarity of the inputs). The sign of the input is determined by which of the two spans contain it. Then the resulting number (ranging from – 1023 to 1023 decimal) is converted to 2’s complement notation and stored in RAM. SNEAK-A-BIT Code 1MHz CLOCK DOUT from LTC1283 in MC68HC05C4 SIGN LOCATION $77 B10 B9 B8 LSB LOCATION $87 B2 B1 B0 FILLED WITH 0’s LTC1283 • TA07 B7 B6 B5 B4 B3 MC68HC05C4 SCK MOSI MISO CO DIN Words for LTC1283 MSBF MUX ADDR. (ODD/SIGN) 0 0 0 0 1 0 1 1 1 1 1 1 1 1 1 1 1 1 UNI WORD LENGTH 1 1 1 1 1 1 1283fb LTC1283 • TA08 LTC1283 TYPICAL APPLICATI MNEMONIC LDA STA LDA STA #$50 $0A #$FF $06 CONFIGURATION DATA FOR SPCR LOAD CONFIGURATION DATA INTO $0A CONFIGURATION DATA FOR PORT C DDR LOAD CONFIGURATION DATA INTO PORT C DDR BSET 0, $02 MAKE SURE CS IS HIGH JSR READ –/+ DUMMY READ CONFIGURES LTC1283 FOR NEXT READ JSR READ +/– READ CH6 WITH RESPECT TO CH7 JSR READ –/+ READ CH7 WITH RESPECT TO CH6 JSR CHK SIGN DETERMINES WHICH READING HAS VALID DATA, CONVERTS TO 2’s COMPLEMENT AND STORES IN RAM READ – / +: LDA #$3F LOAD DIN WORD FOR LTC1283 INTO ACC JSR TRANSFER READ LTC1283 ROUTINE LDA $60 LOAD MSBs FROM LTC1283 INTO ACC STA $71 STORE MSBs IN $71 LDA $61 LOAD LSBs FROM LTC1283 INTO ACC STA $72 STORE LSBs IN $72 RTS RETURN Read + / –: LDA #$7F LOAD DIN WORD FOR LTC1283 INTO ACC JSR TRANSFER READ LTC1283 ROUTINE LDA $60 LOAD MSBs FROM LTC1283 INTO ACC STA $73 STORE MSBs IN $73 LDA $61 LOAD LSBs FROM LTC1283 INTO ACC STA $74 STORE LSBs IN $74 RTS RETURN TRANSFER: BCLR 0, $02 CS GOES LOW STA $0C LOAD DIN INTO SPI. START TRANSFER LOOP 1: TST $0B TEST STATUS OF SPIF BPL LOOP 1 LOOP TO PREVIOUS INSTRUCTION IF NOT DONE LDA $0C LOAD CONTENTS OF SPI DATA REG. INTO ACC STA $0C START NEXT CYCLE STA $60 STORE MSBs IN $60 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. UO S MNEMONIC LOOP 2: TST BPL $0B LOOP 2 DESCRIPTION TEST STATUS OF SPIF LOOP TO PREVIOUS INSTRUCTION IF NOT DONE CS GOES HIGH LOAD CONTENTS OF SPI DATA REG. INTO ACC STORE LSBs IN $61 RETURN LOAD MSBs OF +/– READ INTO ACC OR ACC (MSBs) WITH LSBs OF +/– read IF RESULT IS 0 GOTO MINUS CLEAR CARRY ROTATE RIGHT $73 THROUGH CARRY ROTATE RIGHT $74 THROUGH CARRY LOAD MSBs OF +/– READ INTO ACC STORE MSBs IN RAM LOCATION $77 LOAD LSBs OF +/– READ INTO ACC STORE LSBs IN RAM LOCATION $87 GOTO END OF ROUTINE CLEAR CARRY SHIFT MSBs OF –/+ READ RIGHT SHIFT LSBs –/+ READ RIGHT 1’s COMPLEMENT OF MSBs 1’s COMPLEMENT OF LSBs LOAD LSBs INTO ACC ADD 1 TO LSBs STORE ACC IN $72 CLEAR ACC ADD WITH CARRY TO MSBs. RESULT IN ACC STORE ACC IN $71 STORE MSBs IN RAM LOCATION $77 LOAD LSBs IN ACC STORE LSBs IN RAM LOCATION $87 RETURN Sneak-A-Bit Code for the LTC1283 Using the MC68HC05C4 DESCRIPTION BSET 0, $02 LDA $0C STA RTS CHK SIGN: LDA ORA BEQ CLC ROR ROR LDA STA LDA STA BRA MINUS: CLC ROR ROR COM COM LDA ADD STA CLRA ADC STA STA LDA STA RTS $61 $73 $74 MINUS $73 $74 $73 $77 $74 $87 END $71 $72 $71 $72 $72 #$01 $72 $71 $71 $77 $72 $87 END: 1283fb 23 LTC1283 PACKAGE DESCRIPTIO .300 – .325 (7.620 – 8.255) .008 – .015 (0.203 – 0.381) +.035 .325 –.015 +0.889 8.255 –0.381 ( ) INCHES MILLIMETERS *THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .010 INCH (0.254mm) NOTE: 1. DIMENSIONS ARE 24 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7487 (408) 432-1900 ● FAX: (408) 434-0507 ● TELEX: 499-3977 U N Package 20-Lead Plastic DIP (Reference LTC DWG # 05-08-1510) 1.060* (26.924) MAX 20 19 18 17 16 15 14 13 12 11 .255 ± .015* (6.477 ± 0.381) 1 .125 – .145 (3.175 – 3.683) 2 3 4 5 6 7 8 9 10 .045 – .065 (1.143 – 1.651) .020 (0.508) MIN .065 (1.651) TYP .120 (3.048) MIN .005 (0.127) MIN .018 ± .003 (0.457 ± 0.076) .100 (2.54) BSC N20 0405 1283fb LT/CGRAFX 0407 REV B • PRINTED IN USA © LINEAR TECHNOLOGY CORPORATION 1994