Datasheet AD7091R-2, AD7091R-4, AD7091R-8 (Analog Devices) - 21

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Data Sheet AD7091R-2/AD7091R-4/AD7091R-8
Use an external filter on the analog input signal paths to the • For ac applications, the driver must have a THD AD7091R-2/AD7091R-4/AD7091R-8 VINx pins to achieve the performance that is commensurate with the AD7091R-2/ specified performance. This filter can be a one-pole low-pass AD7091R-4/AD7091R-8. RC filter, or similar. • If the buffer is placed between MUXOUT and ADCIN, the driver Connect the MUX amplifier and the AD7091R-2/AD7091R-4/AD7091R-8 OUT pin directly to the ADCIN pin. Insert a buffer amplifier in the path, if desired. When sequencing analog input circuit must settle for a full-scale step onto channels, do not place a filter between MUX the capacitor array at a 12-bit level (0.0244%, 244 ppm). OUT and the input to any buffering because doing so leads to crosstalk. If buffering is In an amplifier data sheet, settling at 0.1% to 0.01% is more not employed, do not place a filter between MUX commonly specified and may differ significantly from the OUT and ADCIN when sequencing channels because doing so leads to crosstalk. settling time at a 12-bit level. Be sure to verify the amplifier settling time prior to driver selection.
DRIVER AMPLIFIER CHOICE
Although the AD7091R-2/AD7091R-4/AD7091R-8 are easy to
Table 9. Recommended Driver Amplifiers1
drive, a driver amplifier must meet the fol owing requirements:
Product Description
• ADA4805-1 Low noise, ultralow power, wide bandwidth amplifier The noise generated by the driver amplifier must be kept as AD8031 Low voltage, low power, single channel amplifier low as possible to preserve the SNR and transition noise AD8032 Low voltage, low power, dual channel amplifier performance of the AD7091R-2/AD7091R-4/AD7091R-8. AD8615 Low frequency, low voltage amplifier The noise from the driver is filtered by the one-pole, low- pass filter of the AD7091R-2/AD7091R-4/AD7091R-8 1 For the latest recommended ADC driver products, see the AD7091R-2/ analog input circuit, made by R1 and C2, or by the external AD7091R-4/AD7091R-8 product pages. filter, if one is used. Because the typical noise of the AD7091R-2/AD7091R-4/AD7091R-8 is 280 µV rms, the SNR degradation due to the amplifier is      280  SNR = LOSS 20 log  π  2 280 + 2 f−3dB(NeN )   2  where: f−3dB is the input bandwidth, in megahertz, of the AD7091R-2/ AD7091R-4/AD7091R-8 (1.5 MHz), or the cutoff frequency of the input filter, if one is used. N is the noise gain of the amplifier (for example, gain = 1 in buffer configuration; see Figure 45). eN is the equivalent input noise voltage of the op amp, in nV/√Hz. Rev. C | Page 21 of 42 Document Outline FEATURES APPLICATIONS FUNCTIONAL BLOCK DIAGRAM GENERAL DESCRIPTION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS TIMING SPECIFICATIONS Timing Diagram ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE ESD CAUTION PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS TERMINOLOGY THEORY OF OPERATION CIRCUIT INFORMATION CONVERTER OPERATION ADC TRANSFER FUNCTION REFERENCE POWER SUPPLY DEVICE RESET TYPICAL CONNECTION DIAGRAM ANALOG INPUT DRIVER AMPLIFIER CHOICE REGISTERS ADDRESSING REGISTERS CONVERSION RESULT REGISTER CHANNEL REGISTER CONFIGURATION REGISTER ALERT INDICATION REGISTER CHANNEL x LOW LIMIT REGISTER CHANNEL x HIGH LIMIT REGISTER CHANNEL x HYSTERESIS REGISTER SERIAL PORT INTERFACE READING CONVERSION RESULT WRITING DATA TO THE REGISTERS READING DATA FROM THE REGISTERS POWER-ON DEVICE INITIALIZATION MODES OF OPERATION NORMAL MODE POWER-DOWN MODE ALERT (AD7091R-4 AND AD7091R-8 ONLY) BUSY (AD7091R-4 AND AD7091R-8 ONLY) CHANNEL SEQUENCER DAISY CHAIN OUTLINE DIMENSIONS ORDERING GUIDE