Re: Buffer-OP on the input of the ADC

From: Tim Wescott (tim_at_wescottnospamdesign.com)
Date: 01/17/05 

Date: Mon, 17 Jan 2005 09:36:09 -0800

Frida wrote:
> I am using an ADC from texas instruments, ADS8344, and I have been
> recomended to use an op-amplifier on the inputs of each channel (I use the
> ADC as single ended), to work as a buffer or drive amplifier. I have seen
> on some general notes on the internet that the distributor usually have
> som op's recomended for different purposes regarding the circuit. But I
> can't find anything on Texas's homepage. Is there someone that have used
> the ADS8344 (or ADS8345) with op's on the input?? Is there someone who can
> recomend a specific op for the purpose?? I guess the op should just be an
> amplifier with gain=1, and a non-inverting amp. (I have a amplifier
> together with a balancing part and a filter before this.)
>
> Thank you in advance!
>
This would be a good question for sci.electronics.design, if you can
find your replys among all the post-election bickering.

Most new, fast ADC (I assume the ADS8344 is one of them) have a
switched-capacitor structure, and whenever the ADC finishes a conversion
it switches things back to the "ground" state and starts acquiring.
This causes a voltage glitch on the ADC input which can persist through
the acquisition interval and get sampled as noise.

Your op-amp circuit needs to be able to absorb the glitch and settle to
within your desired system error in the ADC's acquisition interval. The
ADC data sheet should tell you the expected amount of charge that will
be injected into it's input pin, and the input pin's capacitance. This,
plus details about the acquisition timing (which will vary depending on
how you drive the ADC's command pins), will determine the environment in
which your amplifier will have to work -- then you just need to design
the darn thing.

I'd be astounded if someone out there didn't have a white paper on this
-- if you don't find one on the TI website you could check Analog
Devices, or other high-performance ADC manufacturers. 

-- 
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Relevant Pages

  • Re: Buffer-OP on the input of the ADC
    ... > I am using an ADC from texas instruments, ADS8344, and I have been ... > recomended to use an op-amplifier on the inputs of each channel (I ... > recomend a specific op for the purpose?? ... The purpose of such an op-amp is to eliminate the effects of source ...
    (comp.arch.embedded)
  • Buffer-OP on the input of the ADC
    ... ADC as single ended), to work as a buffer or drive amplifier. ... som op's recomended for different purposes regarding the circuit. ... recomend a specific op for the purpose?? ...
    (comp.arch.embedded)
  • Re: A question on practical receiver
    ... If RMS thermal noise is equal to or greater than one LSB of the ADC, ... I suspect that such an ADC would have a healthy amount of self noise as is, making the last few bits more marketing than actual adders of precision to the device. ... Wescott Design Services ... "Applied Control Theory for Embedded Systems" was written for you. ...
    (comp.dsp)
  • Re: Help interfacing Current Transformers to ADC
    ... with an analog multiplexer going into one ADC? ... Wescott Design Services ...
    (sci.electronics.design)
  • Re: New to ADCs, newbie question
    ... I have a project I'm working on where I need to interface 12 sensors ... While you're learning about ADCs, sooner or later you'll learn that nearly every ADC has a certain number of bits of precision that come from the engineering department, and a few more that come from marketing. ... Wescott Design Services ... "Applied Control Theory for Embedded Systems" was written for you. ...
    (sci.electronics.design)