Driving ADC AD7631 using AD8421 and AD8271

Question

Hi, 
 Please refer the attachment for schematic. 
 I am working on multi channel Data acquisition system. Each channel is sampled and quantized in 25 us. 
 AD7631, 18 Bit ADC, is used in this design. As per the information given in the ADC datasheet, a RF filter is needed in front of ADC. It has R(12ohm) and C(2.7nF). Currently, there is no buffer placed in between IA and ADC. The ADC is driven differentially using AD8421 and AD8271(IA reference generator). I can see some overshoots at the output of IA in simulations. And also in AD8271 datasheet, it is mentioned that the series resistance should be greater than 20 ohm. 
 Qns: 
 1. Can AD8421 and AD8271 drive AD7631 [ RF filter(R=12 ohm & C =2.7 nF) ] without any stability and settling time issue ? 
 2. Is that RF filter in really needed in front of AD7631. 
 3. The buffers suggested for driving AD7631 is costly. (AD829AQ - $5.5). Can anybody suggest a low cost buffer. 
 Note: 
 Supply voltage - +/-15V 
 Input signal - +/-10V 
 Thanks, 
 Jebas.

jebaspaul · Accepted Answer

Thanks Maithil for your detailed reply.)

maithil · Answer

Hi, 
 Indeed, we do recommend an RC filter between the driver and ADC and we have answered your query about the RC filter in the couple of other threads: 
 http://ez.analog.com/thread/38254 
 http://ez.analog.com/message/123397#123397 
 From the AD8421 and AD8271 datasheets, it appears that you will need a buffer to drive an 18-bit AD7631 SAR ADC. You may be able to drive it at lower sampling rates. Having said that you do have to verify this on your setup. 
 I'd suggest the AD8021, ADA4898-1 as the low cost buffers to drive the AD7631. 
 The other option is to use the AD8475 to drive the AD7631. Have a look at the circuit note below. 
 http://www.analog.com/static/imported-files/circuit_notes/CN0180.pdf 
 Hope that helps. 
 Regards, Maithil

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Driving ADC AD7631 using AD8421 and AD8271

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