LTC1406I

FEATURES s s s s s s s s s s s LTC1406 Low Power, 8-Bit, 20Msps, Sampling A/D Converter DESCRIPTION The LTC ®1406 is a 20Msps, 8-bit, sampling A/D converter which draws only 150mW from a single 5V supply. This easy-to-use device includes a high dynamic range sampleand-hold with a 250MHz bandwidth. The LTC1406’s full-scale input range is ± 1V. The inputs can be driven differentially or one input can be tied to a fixed voltage and the other input driven with a ± 1V bipolar input. Maximum DC specifications include ± 1LSB DNL and INL over temperature. Outstanding AC performance includes 48.5dB S/(N + D) and 62dB THD with a 1MHz input; 47.5dB S/(N + D) and 59dB THD at the Nyquist input frequency of 10MHz. The unique differential input sample-and-hold can acquire single-ended or differential input signals up to its 250MHz bandwidth. The 60dB common mode rejection allows users to eliminate ground loops and common mode noise by measuring signals differentially from the source. The ADC has an 8-bit parallel output port with separate power supply and ground allowing easy interface to 3V digital systems. The pipelined architecture has five clock cycles of data latency. , LTC and LT are registered trademarks of Linear Technology Corporation. Low Power, 8-Bit, 20Msps ADC 250MHz Internal Sample-and-Hold 7 Effective Bits at 70MHz Input Frequency ± 1LSB DNL and INL Max Single 5V Supply and 150mW Dissipation Power Down to 1µA True Differential Inputs Reject Common Mode Noise Accepts Single-Ended or Differential Input Signals ± 1V Differential or 2V Single-Ended Input Span Analog Inputs Common Mode to VDD and GND 24-Pin Narrow SSOP Package APPLICATIONS s s s s s s s s s Telecommunications Wireless Communications Digital Cellular Telephones CCDs and Image Scanners Video Digitizing and Digital Television Digital Color Copiers High Speed Undersampling Personal Computer Video High Speed Data Acquisition TYPICAL APPLICATION Low Power, 20MHz, 8-Bit Sampling ADC DVDD 12 CLK 24 CLOCK CIRCUITRY 23 22 21 AIN+ AIN– 7 TRACK-ANDHOLD AMP 8-BIT PIPELINE ADC DIGITAL DATA OUTPUT DRIVERS 20 19 18 17 2.2V 2.5k 9 AVDD 4 VBIAS 1.95k 10 AGND 3 SHDN 5 VREF 6 AGND 1 OGND 1406 BD DGND 11 OVDD 2 Effective Bits and Signal-to-Noise + Distortion vs Input Frequency 8 7 OF/UF 6 EFFECTIVE BITS D7 D6 D5 D4 D3 D2 D1 D0 5 4 3 2 1 0 100k 1M 10M INPUT FREQUENCY (Hz) 8 16 15 U U U 50 44 38 S/(N + D) (dB) 32 100M 1406 TA02 1 LTC1406 ABSOLUTE MAXIMUM RATINGS AVDD = OVDD = DVDD = VDD (Notes 1, 2) PACKAGE/ORDER INFORMATION TOP VIEW OGND OVDD SHDN VBIAS VREF AGND AIN+ AIN– AVDD 1 2 3 4 5 6 7 8 9 24 CLK 23 OF/UF 22 D7 21 D6 20 D5 19 D4 18 D3 17 D2 16 D1 15 D0 14 NC 13 NC Supply Voltage (VDD) ................................................. 6V Analog Input Voltage (Note 3) .... – 0.3V to (VDD + 0.3V) Digital Input Voltage (Note 4) .................. – 0.3V to 10V Digital Output Voltage ................. – 0.3V to (VDD + 0.3V) Power Dissipation.............................................. 500mW Ambient Operation Temperature Range LTC1406C................................................ 0°C to 70°C LTC1406I............................................ – 40°C to 85°C Storage Temperature Range ................. – 65°C to 150°C Lead Temperature (Soldering, 10 sec).................. 300°C ORDER PART NUMBER LTC1406CGN LTC1406IGN AGND 10 DGND 11 DVDD 12 GN PACKAGE 24-LEAD PLASTIC SSOP TJMAX = 110° C, θJA = 85°C/ W Consult factory for Military grade parts. CO VERTER CHARACTERISTICS The q denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. (Notes 5, 6) PARAMETER Resolution (No Missing Codes) Integral Linearity Error Differential Linearity Error Offset Error Gain Error (Note 8) With External 2.5V Reference (Note 7) CONDITIONS q q q q MIN 8 TYP ± 0.5 ± 0.25 ±1 ±1 MAX ±1 ±1 ±8 ±5 UNITS Bits LSB LSB LSB LSB A ALOG I PUT SYMBOL PARAMETER VIN IIN CIN (Note 5) The q denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. CONDITIONS 4.75V ≤ VDD ≤ 5.25V Voltage On Either AIN+ or AIN– CLK = 0 CLK = 1 CLK = 0 q q q MIN 0 TYP ±1 MAX VDD ±5 UNITS V V µA pF pF MHz ns psRMS dB V Analog Input Span [(AIN+) – (AIN–)] (Note 9) Input (AIN+ or AIN–) Range Analog Input Leakage Current Analog Input Capacitance Input Bandwidth 4 2 250 3 5 tAP tjitter CMRR VBIAS Sample-and-Hold Aperture Delay Time Sample-and-Hold Aperture Delay Time Jitter Analog Input Common Mode Rejection Ratio Internal Bias Voltage – 2.5V < (AIN– = AIN+) < 2.5V No Load 60 2.2 2 U W U U WW W U U U LTC1406 The q denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. (Note 5) SYMBOL PARAMETER S/(N + D) Signal-to-Noise Plus Distortion Ratio THD SFDR IMD Total Harmonic Distortion Spurious Free Dynamic Range Intermodulation Distortion CONDITIONS 1MHz Input Signal 10MHz Input Signal 1MHz Input Signal, First 5 Harmonics 10MHz Input Signal, First 5 Harmonics 1MHz Input Signal 10MHz Input Signal fIN1 = 3.500977MHz, fIN2 = 3.598633MHz MIN TYP 48.5 47.5 – 62 – 59 63 60 60 MAX UNITS dB dB dB dB dB dB dB DY A IC ACCURACY (Note 5) The q denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. SYMBOL PARAMETER VIH VIL IIN CIN VOH VOL ISOURCE ISINK High Level Input Voltage Low Level Input Voltage Digital Input Current Digital Input Capacitance High Level Output Voltage Low Level Output Voltage Output Source Current Output Sink Current VDD = 4.75V, IO = – 10µA VDD = 4.75V, IO = – 200µA VDD = 4.75V, IO = 160µA VDD = 4.75V, IO = 1.6mA VOUT = 0V VOUT = VDD q q DIGITAL I PUTS AND OUTPUTS POWER REQUIRE E TS SYMBOL PARAMETER AVDD DVDD OVDD VBIAS VREF OGND IDD PD Analog Positive Supply Voltage Digital Positive Supply Voltage Output Positive Supply Voltage Internal Bias Voltage Reference Voltage Output Ground Positive Supply Current Power Dissipation Power Down Positive Supply Current Power Down Power Dissipation The q denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. CONDITIONS (Note 10) (Note 10) (Note 10) When Externally Driven (Note 10) (Note 10) (Note 10) AVDD = DVDD = OVDD = 5V, fSMPL = 20MHz (Note 13) SHDN = 0V, CLK = VDD or 0 SHDN = 0V, CLK = VDD or 0 q q UW U U WU CONDITIONS VDD = 5.25V VDD = 4.75V VIN = 0V to VDD q q q MIN 2.4 TYP MAX 0.8 ±5 UNITS V V µA pF V V 5 4.5 4.0 0.05 0.10 – 20 30 0.4 V V mA mA (Note 5) MIN 4.75 4.75 2.7 1.9 2 0 30 150 1 5 2.2 2.5 TYP MAX 5.25 5.25 5.25 2.5 3 2 45 225 10 50 UNITS V V V V V V mA mW µA µW 3 LTC1406 TI I G CHARACTERISTICS The q denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. (Note 5) SYMBOL fSMPL(MAX) t1 t2 t3 t4 t5 t6 PARAMETER Maximum Sampling Frequency Clock Period Pulse Width High Pulse Width Low Output Delay Pipeline Delay Aperture Delay Aperture Jitter Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: All voltage values are with respect to ground with DGND, OGND and AGND wired together (unless otherwise noted). Note 3: When these pin voltages are taken below ground or above VDD, they will be clamped by internal diodes. This product can handle input currents greater than 100mA below ground or above VDD without latchup. Note 4: When these pin voltages are taken below ground they will be clamped by internal diodes. This product can handle input currents up to 100mA below ground without latchup. These pins are not clamped to VDD. Note 5: VDD = 5V, fSMPL = 20MHz and tr = tf = 2ns unless otherwise specified. Note 6: Linearity, offset and full-scale specifications apply for a singleended AIN+ input with AIN– tied to VREF = 2.5V. (Notes 11, 12) (Notes 11, 12) (Notes 11, 12) CL = 15pF CONDITIONS q q q q TYPICAL PERFORMANCE CHARACTERISTICS S/(N + D) vs Input Frequency 52 48 44 40 36 32 28 24 20 16 12 8 4 0 100k AMPLITUDE (dB BELOW THE FUNDAMENTAL) SIGNAL-TO-NOISE RATIO (dB) S/(N + D) (dB) 1M 10M INPUT FREQUENCY (Hz) 4 UW UW MIN 20 50 25 25 TYP MAX UNITS MHz ns ns ns 15 5 3 5 25 ns Cycles ns psRMS Note 7: 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 8: Bipolar offset is the offset voltage measured from – 0.5LSB when the output code flickers between 0111 1111 and 1000 0000. Note 9: Guaranteed by design, not subject to test. Note 10: Recommended operating conditions. Note 11: The falling CLK edge starts a conversion. Note 12: At the maximum conversion rate, deviation from a 50% duty cycle results in interstage settling times < 25ns and performance may be affected. Note 13: VIN = – Full Scale. Signal-to-Noise Ratio vs Input Frequency 52 48 44 40 36 32 28 24 20 16 12 8 4 0 100k Distortion vs Input Frequency 0 –10 –20 –30 –40 –50 –60 –70 2ND HARMONIC –80 100k 1M 10M INPUT FREQUENCY (Hz) 100M 1406 G03 THD 3RD HARMONIC 100M 1406 G01 1M 10M INPUT FREQUENCY (Hz) 100M 1406 G02 LTC1406 TYPICAL PERFORMANCE CHARACTERISTICS Spurious-Free Dynamic Range vs Input Frequency 70 SPURIOUS-FREE DYNAMIC RANGE (dB) 60 50 AMPLITUDE (dB) 40 30 20 10 0 100k –30 –40 –50 –60 –70 –80 –90 DNL EOC ERROR (LSB) 1M 10M INPUT FREQUENCY (Hz) Integral Nonlinearity vs Output Code 1.0 COMMON MODE REJECTION (dB) INL EOC ERROR (LSB) 0.5 50 40 30 20 10 0 100k SUPPLY CURRENT (mA) 0 –0.5 –1.0 0 32 64 96 128 160 192 224 256 OUTPUT CODE 1406 G07 PIN FUNCTIONS OGND (Pin 1): Digital Data Output Ground. Tie to analog ground plane. May be tied to logic ground if desired. OVDD (Pin 2): Digital Data Output Supply. Normally tied to 5V, can be used to interface with 3V digital logic. Bypass to OGND with 10µF tantalum in parallel with 0.1µF or 10µF ceramic. SHDN (Pin 3): Power Shutdown Input. Logic low selects shutdown. VBIAS (Pin 4): Internal Bias Voltage. Internally set to 2.2V. Bypass to analog ground plane with 10µF tantalum in parallel with 0.1µF or 10µF ceramic. VREF (Pin 5): External 2.5V Reference Input. Bypass to analog ground plane with 10µF tantalum in parallel with 0.1µF or 10µF ceramic. AGND (Pin 6): Analog Ground. Tie to analog ground plane. A IN+ (Pin 7): ± 1V Input. The maximum output code occurs when [(AIN+) – (AIN–)] = 1V. The minimum output code occurs when [(AIN+) – (AIN–)] = – 1V. A IN– (Pin 8): ± 1V Input. The maximum output code occurs when [(AIN+) – (AIN–)] = 1V. The minimum output code occurs when [(AIN+) – (AIN–)] = – 1V. For singleended operation, tie AIN– to a DC voltage (e.g., VREF). UW 1406 G04 Intermodulation Distortion Plot 0 –10 –20 fSAMPLE = 20MHz fIN1 = 3.500977MHz fIN2 = 3.598633MHz 0.5 1.0 Differential Nonlinearity vs Output Code 0 –0.5 100M –100 0 1 2 34567 FREQUENCY (MHz) 8 9 10 –1.0 0 32 64 96 128 160 192 224 256 OUTPUT CODE 1406 G06 1406 G05 Input Common Mode Rejection vs Input Frequency 70 60 Supply Current vs Sampling Frequency 35 30 25 20 15 10 5 0 100k 1M 10M INPUT FREQUENCY (Hz) 100M 1406 G08 1M 10M 20M SAMPLING FREQUENCY (Hz) 1406 G09 U U U 5 LTC1406 PIN FUNCTIONS AVDD (Pin 9): Analog 5V Positive Supply. Bypass to analog ground plane with 10µF tantalum in parallel with 0.1µF or 10µF ceramic. AGND (Pin 10): Analog Ground. Tie to analog ground plane. DGND (Pin 11): Digital Ground for Internal Logic. Tie to analog ground plane. DVDD (Pin 12): Digital 5V Positive Supply. Bypass to DGND with 10µF tantalum in parallel with 0.1µF or 10µF ceramic. NC (Pins 13, 14): No Internal Connection. AVDD = DVDD = VDD PIN 1 2 3 4 5 6 7 8 9 10 11 12 13 to 14 15 to 22 23 24 NAME OGND OVDD SHDN VBIAS VREF AGND AIN+ AIN– AVDD AGND DGND DVDD NC D7 to D0 OF/UF CLK DESCRIPTION Ground for Output Drivers Supply for Output Drivers Shutdown Input, Active Low Internal Bias Voltage External Reference Input Analog Ground, Clean Ground Positive Analog Input, ± 1V Span Negative Analog Input Analog Supply Analog Ground, Substrate Ground Digital Ground Digital Supply No Connect, No Internal Connection Data Outputs Overflow/Underflow Output Clock Input OGND OGND 0 OVDD OVDD VDD – 0.3 – 0.3 – 0.3 VDD + 0.3 VDD + 0.3 10 4.75 0 0 4.75 5 0 0 5 5.25 2.7 0 1.9 2 2.2 2.5 0 VDD VDD 5.25 MIN TI I G DIAGRA ANALOG SIGNAL N–1 CLOCK t2 DATA OUT N–6 N–5 N–4 N–3 t5 N–2 N–1 t4 N 1406 TD 6 W U U UW U D7 to D0 (Pins 15 to 22): Digital Data Outputs. The outputs swing between OVDD and OGND. OF/UF (Pin 23): Overflow/Underflow Bit. OF/UF high with D7 to D0 all high indicates an overrange, OF/UF high with D7 to D0 all low indicates an underrange condition. OF/UF low indicates a conversion within the normal input range. The outputs swing between OVDD and OGND. CLK (Pin 24): Clock Input. Internal sample-and-hold tracks the input signal when CLK is high and samples the input signal on the falling edge. NOMINAL (V) TYP 0 3 or 5 5.25 VDD 2.5 3 MAX ABSOLUTE MAXIMUM (V) MIN – 0.3 – 0.3 – 0.3 – 0.3 – 0.3 – 0.3 – 0.3 – 0.3 – 0.3 – 0.3 – 0.3 – 0.3 MAX VDD + 0.3 6 10 VDD + 0.3 VDD + 0.3 VDD + 0.3 VDD + 0.3 VDD + 0.3 6 VDD + 0.3 VDD + 0.3 6 t6 N N+1 N+2 N+3 N+4 t3 t1 N+5 N+6 LTC1406 FUNCTIONAL BLOCK DIAGRA CLK 24 CLOCK CIRCUITRY 23 22 21 OF/UF D7 D6 D5 D4 D3 D2 D1 D0 AIN+ AIN– 7 TRACK-ANDHOLD AMP 8-BIT PIPELINE ADC 8 2.2V 2.5k 9 AVDD 4 VBIAS 1.95k 10 AGND 3 SHDN 5 VREF 6 AGND 1 OGND APPLICATIONS INFORMATION Conversion Details The LTC1406 uses an internal sample-and-hold circuit and a pipeline quantizing architecture to convert an analog signal to an 8-bit parallel output. With CLK high the input switches are closed and the analog input will be acquired on the input sampling capacitors CS (see Figure 1). On the falling edge of CLK the input switches open, capturing the input signal. The sampling capacitors are then shorted together and the charge is transferred to the hold CLK CH CS AIN+ CLK CLK + CS CLK TO NEXT STAGE AIN– CLK – CH 1406 F01 CLK Figure 1. Input Sample-and-Hold Amplifier W DVDD 12 DGND 11 OVDD 2 DIGITAL DATA OUTPUT DRIVERS 20 19 18 17 16 15 1406 BD U W U U U U capacitors CH resulting in a differential DC voltage on the output of the track-and-hold amplifier that is proportional to the input signal. This differential voltage is fed into a comparator that determines the most significant bit and subtracts the result. The residue is then amplified by two and passed to the next stage via a similar sample-and-hold circuit. This continues down the eight pipeline stages. The comparator outputs are then combined in a digital error correction circuit. The 8-bit word is available at the output, five clock cycles after the sampling edge. Dynamic Performance The LTC1406 has excellent wideband sampling capability. The sample-and-hold amplifier has a small-signal input bandwidth of 250MHz allowing the ADC to undersample input signals with frequencies well beyond the converter’s Nyquist frequency. FFT (Fast Fourier Transform) test techniques are used to test the ADC’s frequency response, distortion and noise at the rated throughput. By applying a low distortion sine wave and analyzing the digital output using an FFT algorithm, the ADC’s spectral content can be examined for frequencies outside the fundamental. Figure 2 shows a typical LTC1406 FFT plot. 7 LTC1406 APPLICATIONS INFORMATION 0 –10 –20 AMPLITUDE (dB) fSAMPLE = 20MHz fIN1 = 1.000977MHz SFDR = 64.8dB SINAD = 48.6dB –30 –40 –50 –60 –70 –80 EFFECTIVE BITS –90 –100 0 1 2 3 4567 FREQUENCY (Hz) 8 9 10 1406 F02a Figure 2a. Nonaveraged, 4096 Point FFT Input Frequency = 1MHz 0 fSAMPLE = 20MHz –10 f = 28.99902MHz IN1 –20 SFDR = 54.9dB SINAD = 47.0dB –30 –40 –50 –60 –70 –80 –90 –100 0 1 2 3 4567 FREQUENCY (Hz) 8 9 10 AMPLITUDE (dB) 1406 F02b Figure 2b. Nonaveraged, 4096 Point FFT Input Frequency = 30MHz Signal-to-Noise Ratio The signal-to-noise plus distortion ratio [S/(N + D)] is the ratio between the RMS amplitude of the fundamental input frequency and the RMS amplitude of all other frequency components at the ADC output. The output is band limited to frequencies above DC to below half the sampling frequency. The effective number of bits (ENOBs) is a measurement of the resolution of an ADC and is directly related to the S/(N + D) by the equation: ENOB = [S/(N + D) – 1.76]/6.02 8 U W U U where ENOB is the effective number of bits and S/(N + D) is expressed in dB. At the maximum sampling rate of 20MHz the LTC1406 maintains near ideal ENOBs up to and beyond the Nyquist input frequency of 10MHz (see Figure 3). 8 7 6 5 4 3 2 1 0 100k 1M 10M INPUT FREQUENCY (Hz) 100M 1406 TA02 50 44 38 S/(N + D) (dB) 32 Figure 3. Effective Bits and Signal-to-(Noise + Distortion) vs Input Frequency Total Harmonic Distortion Total harmonic distortion is the ratio of the RMS sum of all harmonics of the input signal to the fundamental itself. The out-of-band harmonics alias into the frequency band between DC and half the sampling frequency. THD is expressed as: THD = 20 log 2 2 2 2 V2 + V3 + V4 + . . .Vn V1 where V1 is the RMS amplitude of the fundamental frequency and V2 through Vn are the amplitudes of the second through nth harmonics. THD vs Input Frequency is shown in Figure 4. The LTC1406 has good distortion performance up to the Nyquist frequency and beyond. Intermodulation Distortion If the ADC input signal consists of more than one spectral component, the ADC transfer function nonlinearity can produce intermodulation distortion (IMD) in addition to THD. IMD is the change in one sinusoidal input caused by the presence of another sinusoidal input at a different frequency (see Figure 5). LTC1406 APPLICATIONS INFORMATION AMPLITUDE (dB BELOW THE FUNDAMENTAL) 0 –10 –20 –30 –40 –50 –60 –70 2ND HARMONIC –80 100k 1M 10M INPUT FREQUENCY (Hz) 100M 1406 G03 THD 3RD HARMONIC Figure 4. Distortion vs Input Frequency 0 –10 –20 fSAMPLE = 20MHz fIN1 = 3.500977MHz fIN2 = 3.598633MHz AMPLITUDE (dB) –30 –40 –50 –60 –70 –80 –90 –100 0 1 2 34567 FREQUENCY (MHz) 8 9 10 1406 G05 Figure 5. Intermodulation Distortion Plot 70 SPURIOUS-FREE DYNAMIC RANGE (dB) 60 50 40 30 20 10 0 100k 1M 10M INPUT FREQUENCY (Hz) 100M 1406 G04 Figure 6. Spurious-Free Dynamic Range vs Input Frequency U W U U If two pure sine waves of frequencies fa and fb are applied to the ADC input, nonlinearities in the ADC transfer function can create distortion products at the sum and difference frequencies of mfa ± nfb, where m and n = 0, 1, 2, 3, etc. For example, the 2nd order IMD terms include (fa ± fb). If the two input sine waves are equal in magnitude, the value (in decibels) of the 2nd order IMD products can be expressed by the following formula: IMD fa ± fb = 20 log ( ) Amplitude at fa ± fb Amplitude at f a ( ) Peak Harmonic or Spurious Noise The peak harmonic or spurious noise is the largest spectral component excluding the input signal and DC. This value is expressed in decibel relative to the RMS value of a full-scale input signal (see Figure 6). Input Bandwidth The input bandwidth is that input frequency at which the amplitude of the reconstructed fundamental is reduced by 3dB for a full-scale input signal. The LTC1406 has been designed for wide input bandwidth (250MHz), allowing the ADC to undersample input signals with frequencies above the converter’s Nyquist frequency. The noise floor stays very low at high frequencies; S/(N + D) becomes dominated by distortion at frequencies far beyond Nyquist. Analog Inputs The LTC1406 has a unique differential sample-and-hold circuit that allows rail-to-rail inputs. The AIN+ and AIN– inputs are sampled at the same time and the ADC will always convert the difference of [(AIN+) – (AIN–)] independent of the common mode voltage. Any unwanted signal that is common to both inputs will be rejected by the common mode rejection of the sample-and-hold circuit. The common mode rejection holds up to extremely high frequencies (see Figure 7). The inputs can be driven differentially or single-ended. In differential mode, both inputs are driven ± 0.5V out of phase with each other. In single-ended mode, the negative input is tied to a fixed voltage and AIN+ is used as the 9 LTC1406 APPLICATIONS INFORMATION 70 COMMON MODE REJECTION (dB) 60 50 40 30 20 10 0 100k 1M 10M INPUT FREQUENCY (Hz) 100M 1406 G08 Figure 7. Common Mode Rejection vs Input Frequency single input providing a ± 1V bipolar input range centered around AIN–. Likewise, AIN+ can be tied to a fixed voltage and AIN– used as the single input. In any configuration the maximum output code (1111 1111) occurs when [(AIN+) – (AIN–)] = 1V and the minimum output code (0000 0000) occurs when [(AIN+) – (AIN–)] = – 1V. Each analog input can swing from ground to VDD but not beyond. Therefore, the input common mode voltage can range from 0.5V to 4.5V in differential mode and from 1V to 4V in single-ended mode. As an example, with AIN– connected to the VREF pin (2.5V) the input range will be 1.5V to 3.5V (see Figure 8a). To achieve other ranges the input may be capacitively coupled to achieve a 2V span with virtually any common mode voltage (see Figure 8b). The 2V input span requires a 2.5V external reference be connected to the VREF pin. The LT1460-2.5 micropower precision series reference is recommended. To achieve other input spans, the reference voltage (VREF) can vary between 2V to 3V. The VREF pin can also be driven with a DAC or other means. This is useful in applications where the peak input signal amplitude may vary. The input span of the ADC can then be adjusted to match the peak input signal, maximizing the signal-to-noise ratio. The analog inputs of the LTC1406 are easy to drive. The inputs draw only one small current spike while charging the sample-and-hold capacitors following a rising CLK edge. 10 U W U U ANALOG INPUT 1.5V TO 3.5V AIN+ LTC1406 AIN– ANALOG INPUT 2V SPAN AIN+ LTC1406 AIN– 2.5V VREF 1406 F08a 2.5V VREF 1406 F08b Figure 8a. DC Coupled Figure 8b. AC Coupled While CLK is low the analog inputs draw only a small leakage current. If the source impedance of the driving circuit is low, then the LTC1406 inputs can be driven directly. As source impedance increases, so will acquisition time. For minimum acquisition time with high source impedance, a buffer amplifier should be used. The only requirement is that the amplifier driving the analog input(s) must settle after the small current spike before the next conversion starts (settling time must be 25ns for full throughput rate). Choosing an Input Amplifier Choosing an input amplifier is easy if a few requirements are taken into consideration. First, to limit the magnitude of the voltage spike seen by the amplifier from charging the sampling capacitor, choose an amplifier that has a low output impedance (< 50Ω) at the closed-loop bandwidth frequency. For example, if an amplifier is used in a gain of 1 and has a unity-gain bandwidth of 50MHz, then the output impedance at 50MHz must be less than 50Ω. The second requirement is that the closed-loop bandwidth must be greater than 70MHz to ensure adequate small-signal settling for full throughput rate. The following list is a summary of the op amps that are suitable for driving the LTC1406. More detailed information is available in the Linear Technology Databooks and on the LinearViewTM CD-ROM. LT®1223: 100MHz Video Current Feedback Amplifier. 6mA supply current. ± 5V to ± 15V supplies. Low noise. LT1227: 140MHz Video Current Feedback Amplifier. 10mA supply current. ± 5V to ± 15V supplies. Low distortion. Low noise. LinearView is a trademark of Linear Technology Corporation. LTC1406 APPLICATIONS INFORMATION LT1229/LT1230: Dual and Quad 100MHz Current Feedback Amplifiers. ± 2V to ± 15V supplies. Low noise. 6mA supply current each amplifier. LT1259/LT1260: Dual and Triple 130MHz Current Feedback Amplifiers. ± 2V to ± 14V supplies. 5mA supply current. Low distortion. Low noise. LT1363: 70MHz Voltage Feedback Amplifier. ± 2.5V to ± 15V supplies. 7.5mA supply current. Low distortion. LT1364/LT1365: Dual and Quad 70MHz Voltage Feedback Amplifiers. ± 2.5V to ± 15V supplies. 7.5mA supply current per amplifier. Low distortion. Input Filtering The noise and the distortion of the input amplifier and other circuitry must be considered since they will add to the LTC1406 noise and distortion. The small-signal bandwidth of the sample-and-hold circuit is 250MHz. Any noise or distortion products that are present at the analog inputs will be summed over this entire bandwidth. Noisy input circuitry should be filtered prior to the analog inputs to minimize noise. A simple 1-pole RC filter is sufficient for many applications. For example, Figure 9 shows a 220pF capacitor from AIN+ to AIN– and a 75Ω source resistor to limit the input bandwidth to 9.6MHz. The 220pF capacitor also acts as a charge reservoir for the input sample-andhold and isolates the ADC input from sampling glitch sensitive circuitry. Larger value capacitors may be substituted to further limit the input bandwidth. High quality capacitors and resistors should be used since these components can add distortion. NPO and silver mica type dielectric capacitors have excellent linearity. Carbon surface mount 75Ω 220pF OUTPUT CODE ANALOG INPUT 1.5V TO 3.5V AIN+ LTC1406 AIN– 2.5V VREF 1406 F09 Figure 9. RC Input Filter U W U U resistors can also generate distortion from self-heating and from damage that may occur during soldering. Metal film surface mount resistors are much less susceptible to both problems. Input/Output Characteristics Figure 10 shows the ideal input/output characteristics for the LTC1406. The code transitions occur midway between successive integer LSB values (i.e., – FS + 0.5LSB, – FS + 1.5LSB, – FS + 2.5LSB...FS – 1.5LSB, FS – 0.5LSB). The output is straight binary with 1LSB = FS – (– FS)/256 = 2V/ 256 = 7.8125mV. The OF/UF bit indicates that the input has exceeded full scale and can be used to detect an overrange or underrange condition. A logic high output on the OF/UF pin with an output code of 0000 0000 indicates the input is less than the negative full scale. A logic high output on the OF/UF pin with an output code of 1111 1111 indicates that the input is greater than the positive full scale. A logic low output on the OF/UF pin indicates the input is within the full-scale range of the converter. In applications where absolute accuracy is important, offset and full-scale errors can be adjusted to zero. Offset error must be adjusted before full-scale error. Zero offset is achieved by adjusting the offset applied to the AIN– input. For zero offset error, apply a voltage equal to the input OF/UF BIT 1111 1111 1111 1110 1111 1101 1000 0001 1000 0000 0111 1111 0111 1110 0000 0010 0000 0001 0000 0000 –FS –1 0 1 LSB LSB INPUT VOLTAGE (V) FS – 1LSB 1406 F10 Figure 10. Transfer Characteristics 11 LTC1406 APPLICATIONS INFORMATION common mode voltage minus 3.90625mV (i.e., – 0.5LSB) and adjust the offset at the AIN– input until the output code flickers between 0111 1111 and 1000 0000. For full-scale adjustment, an input voltage equal to the input common mode voltage plus 988.28125mV (i.e., FS – 1.5LSBs) is applied to AIN+ and the VREF input is adjusted until the output code flickers between 1111 1110 and 1111 1111. Digital Inputs and Outputs The LTC1406 is designed to easily interface with either 3V or 5V logic. The digital input pins, SHDN and CLK, have thresholds of nominally 1.9V and will accept a 3V or 5V logic input. The data output pins, including OF/UF, are connected to a separate supply and ground (OVDD and OGND respectively). OVDD is normally connected to DVDD but can be connected to an external supply as low as 2.7V. OGND is normally connected to DGND but can be connected to an external ground or an external voltage source as high as 2V. Clock The LTC1406 requires a 50% duty cycle clock. The duty cycle should be timed from the nominal threshold of the CLK input which is 1.9V. At conversion speeds below the maximum conversion rate of 20MHz, the duty cycle can deviate from 50% with no degradation in performance as long as each clock phase is at least 25ns long. At the maximum conversion rate, deviation from a 50% duty cycle clock results in interstage settling times of < 25ns and performance may be affected. With the CLK pin high, the ADC will track the difference of the two analog inputs. On the falling edge of CLK the input is sampled and the conversion begins. At the end of five clock cycles (on the fifth falling CLK edge following the start of conversion) the data from the conversion will be available at the digital outputs until the next falling CLK edge. Each falling edge of CLK starts a new conversion so successive conversion results are available on successive falling CLK edges. While the falling edge starts the conversion, both rising and falling edges are used internally during the conversion. It is therefore important to provide a clock signal that has low jitter and fast rise and fall times (< 2ns). Much of the internal circuitry operates dynamically limiting the minimum conversion rate to 10kHz. To ensure proper operation after power is first applied, or the clock stops for more than 100µs, typically 20 clock cycles must be performed at a sample rate above 10kHz before the output data will be valid. 10 9 8 7 DNL (LSBs) 12 U W U U fSAMPLE = 20MHz 6 5 4 3 2 1 0 28 32 36 40 44 48 52 56 60 64 68 72 DUTY CYCLE (%) 1406 F11 Figure 11. Typical DNL vs Duty Cycle Power Shutdown The quiescent power of the LTC1406 can be further reduced between conversions by taking the SHDN pin low. This powers down all of the internal amplifiers and bias circuitry and the part draws only a small quiescent current of 1µA from the 5V supply. There is a nominally 4k internal resistor between VREF and AGND that will continue to draw current during shutdown as long as VREF is driven. It should also be noted that the data output drivers are not threestate devices and do not go into a high impedance state during shutdown. If the data output pins will remain connected to a load during shutdown, current may be drawn through the OVDD supply pin. This can be prevented by including a FET switch in series with OVDD or OGND controlled by SHDN. If the data bus will remain active during LTC1406 APPLICATIONS INFORMATION shutdown. It may also be desirable to isolate the data output pins from the bus to reduce the load capacitance. To resume normal operation the SHDN pin must be brought high and typically 20 clock cycles must be performed at a sample rate above 10kHz before the output data will be valid. Board Layout and Bypassing Wire wrap boards are not recommended for high resolution or high speed A/D converters. To obtain the best performance from the LTC1406, a printed circuit board with ground plane is required. Layout for the printed circuit board should ensure that digital and analog signal lines are separated as much as possible. In particular, care should be taken not to run any digital track alongside an analog signal track or underneath the ADC. An analog ground plane separate from the logic system ground should be established under and around the ADC. Pin 1 (OGND), Pin 6 (AGND), Pin 10 (AGND) and Pin 11 (DGND) and all other analog grounds should be connected to this single analog ground point. The VCM, VREF, DVDD and OVDD bypass capacitors should also be connected to this analog ground plane. No other digital grounds should be connected to this analog ground plane. In some applications it may be desirable to connect the OVDD to the logic system supply and OGND to the logic system ground. In these cases OVDD should be bypassed to OGND instead of the analog ground plane. Low impedance analog and digital power supply common returns are essential to low noise operation of the ADC and the foil width for these tracks should be as wide as possible. In applications where the ADC data outputs and control signals are connected to a continuously active microprocessor bus, it is possible to get errors in the conversion results. These errors are due to feedthrough from the microprocessor to the comparators. The problem can be eliminated by forcing the microprocessor into a wait state during conversion or by using three-state buffers to isolate the ADC data bus. The LTC1406 has differential inputs to minimize noise coupling. Common mode noise on the AIN+ and AIN– leads will be rejected by the input CMRR. The LTC1406 will hold and convert the difference voltage between AIN+ and AIN–. The leads to AIN+ (Pin 7) and AIN– (Pin 8) should be kept as short as possible. In applications where this is not possible, the AIN+ and AIN– traces should be run side by side to equalize coupling. Supply Bypassing High quality, low series resistance ceramic, 10µF bypass capacitors should be used at the VDD, VCM and VREF pins as shown in the Typical Application on the first page of this data sheet. Surface mount ceramic capacitors such as Murata GRM235Y5V106Z016 provide excellent bypassing in a small board space. Alternatively, 10µF tantalum capacitors in parallel with 0.1µF ceramic capacitors can be used. Bypass capacitors must be located as close to the pins as possible. The traces connecting the pins and the bypass capacitors must be kept short and should be made as wide as possible. Example Layout Figures 12a, 12b, 12c and 12d show the schematic and layout of an evaluation board. The layout demonstrates the proper use of decoupling capacitors and ground plane with a 2-layer printed circuit board. U W U U 13 JP12 JP1 4 5 2 3 + JP2 U1 LT1460-2.5 (MS8) JP3 6 E4 SHDN R1 10Ω CLK U2 74HC74 (OPT) 3 VCC C3, 0.1µF E5 EXT REF 2.5V REF 2 OUT IN GND C4 22µF, 10V R3 50Ω 4 C5 0.1µF C2 10µF 10V 1 APPLICATIONS INFORMATION J2 AIN+ R4 50Ω C7 10µF 10V 1 J5 (OPT) U3 LTC1406 OGND OVDD SHDN 4 5 6 7 8 9 R2 10Ω R19 51Ω 2 7 5 JP4 C9 0.1µF JP5 CLK OF/UF D7 24 23 22 1 2 JP6 JP7 2 3 5V C16 C11 10µF 10V 10µF, 10V R5 1k (OPT) C10 0.1µF 2 7 R6 C8 1k 100pF (OPT) OE D0 R11 21 VCC Q0 VBIAS VREF AGND AIN+ AIN– AVDD D6 D5 D4 D3 D2 D1 3 20 19 18 17 16 4 5 6 7 8 20 19 1 2 OF/UF D7 D1 D2 D3 D4 D5 D6 15 JP8 (OPT) – 6 4 R7 1k (OPT) R8 50Ω 3 JP9 + U5 SO8 (OPT) R12 R13 R14 R15 R16 R17 Q1 Q2 Q3 Q4 Q5 Q6 10 11 18 17 16 15 14 13 3 4 5 6 7 8 D6 D5 D4 D3 D2 D1 R9 1k JP10 C12 0.1µF 5V C15 0.1µF R10, 1k NOTE: ALL RESISTORS ARE IN OHMS, 1/8W, 5%, 0805 C13 0.1µF C14 10µF, 10V AGND DGND 12 D0 NC DVDD NC 9 14 13 10 D7 GND R18 Q7 CK U4, 74HC574 (OPT) 12 11 9 10 11 12 13 14 1406 F12a D0 CLK OGND OGND CLK OGND J3 AIN– 2 7 J6 (OPT) – 6 4 3 JP11 + U6 DIP8 (OPT) Figure 12a. Suggested Evaluation Circuit Schematic U C6 (OPT) + 4 W U C1 22µF 10V U7 TC7SH04F J4 (OPT) J1 CLOCK U 14 E1 ANALOG SUPPLY 5V E3 E2 OUTPUT SUPPLY VCC C17 0.1µF OUTPUT GND E6 ANALOG GROUND LTC1406 LTC1406 APPLICATIONS INFORMATION U W U U Figure 12b. Suggested Evaluation Circuit Board—Component Side Silkscreen Figure 12c. Suggested Evaluation Circuit Board—Component Side Layout Figure 12d. Suggested Evaluation Circuit Board—Solder Side Layout PACKAGE DESCRIPTION 0.015 ± 0.004 × 45° (0.38 ± 0.10) 0.007 – 0.0098 (0.178 – 0.249) 0.016 – 0.050 (0.406 – 1.270) * 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 0° – 8° TYP 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. U Dimensions in inches (millimeters) unless otherwise noted. GN Package 24-Lead Plastic SSOP (Narrow 0.150) (LTC DWG # 05-08-1641) 0.337 – 0.344* (8.560 – 8.738) 24 23 22 21 20 19 18 17 16 15 14 13 0.229 – 0.244 (5.817 – 6.198) 0.150 – 0.157** (3.810 – 3.988) 1 23 4 56 7 8 9 10 11 12 0.053 – 0.068 (1.351 – 1.727) 0.004 – 0.0098 (0.102 – 0.249) 0.008 – 0.012 (0.203 – 0.305) 0.025 (0.635) BSC GN24 (SSOP) 1197 15 LTC1406 TYPICAL APPLICATION Low Power, 20MHz, 8-Bit Sampling ADC LTC1406 5V 1 2 10µF 2.5V REFERENCE 3 4 5 10µF 6 7 ANALOG INPUTS 8 9 10µF 10 11 12 OGND OVDD SHDN VBIAS VREF AGND AIN+ AIN – AVDD AGND DGND DVDD CLK OF/UF D7 D6 D5 D4 D3 D2 D1 D0 NC NC 24 23 22 21 20 19 18 17 16 15 14 13 NC NC 8-BIT PARALLEL BUS CLOCK INPUT OVERFLOW/UNDERFLOW OUTPUT RELATED PARTS PART NUMBER ADCs LTC1196/LTC1198 LTC1197/LTC1199 LTC1410 LTC1415 LTC1419 LTC1604 LTC1605 DACs LTC1446/LTC1446L LTC1448 LTC1458/LTC1458L Dual 12-Bit VOUT DACs in SO-8 Package Dual 12-Bit Rail-to-Rail Output DAC in SO-8 Package Quad 12-Bit Rail-to-Rail Output DACs LTC1446: VCC = 4.5V to 5.5V, VOUT = 0V to 4.095V LTC1446L: VCC = 2.7V to 5.5V, VOUT = 0V to 2.5V VCC = 2.7V to 5.5V, Output Swings from GND to REF, REF Input Can Be Tied to VCC LTC1458: VCC = 4.5V to 5.5V, VOUT 0V to 4.095V LTC1458L: VCC = 2.7V to 5.5V, VOUT = 0V to 2.5V Single Supply, 8-Bit, 1Msps/750ksps ADCs Single Supply, 10-Bit, 500ksps/450ksps ADCs 12-Bit, 1.25Msps Sampling ADC with Shutdown Single 5V, 12-Bit, 1.25Msps ADC 14-Bit, 800ksps Sampling ADC with Shutdown 16-Bit, 333ksps ADC Single 5V, 16-Bit, 100ksps ADC Single 3V or 5V Supply, Low Power, Serial Interface, SO-8 Package Single 3V or 5V Supply, Low Power, Serial Interface, SO-8 Package Best Dynamic Performance, THD = 84dB and SINAD = 71dB at Nyquist Single Supply 55mW Dissipation 81.5dB SINAD, 150mW from ± 5V Supplies 90dB SINAD, 100dB THD, 250mW Dissipation Low Power, ± 10V Inputs DESCRIPTION COMMENTS 16 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408)432-1900 q FAX: (408) 434-0507 q www.linear-tech.com U 1406 TA03 1406f LT/TP 0299 4K • PRINTED IN USA © LINEAR TECHNOLOGY CORPORATION 1998