Buffer-OP on the input of the ADC

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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!


Re: Buffer-OP on the input of the ADC
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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
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Re: Buffer-OP on the input of the ADC
Hi Frida,

Just to add to Tim's good advice: Another company that is excellent for
finding information as well as the amplifiers/buffers you need is Linear
Technology.

If you don't need gain but just want to buffer 1:1 look for an opamp
that is meant to drive video cables and, most of all, is 'unity gain
stable'. A low output impedance is important for the reasons Tim
mentioned (spikes coming back out of the ADC's input). Also, make sure
to give the amp enough supply voltage. Even though the advertising
language might say something like 'rail to rail' it may not be able to
drive hard enough when the ouput voltage gets too close to either supply.

Regards, Joerg

http://www.analogconsultants.com

Re: Buffer-OP on the input of the ADC

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snip
Hence giving new meaning to the term "mosey".

--

Tim Wescott
Wescott Design Services
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Re: Buffer-OP on the input of the ADC
Hi Tim,

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Yeah, I believe that term had to do with rails. Or more with the
railroad, when they mosey the train up to the end of the portion where
the current clearance ends, then wait for clearance for the next portion.

Regards, Joerg

http://www.analogconsultants.com

Re: Buffer-OP on the input of the ADC
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The purpose of such an op-amp is to eliminate the effects of source
impedance.  If you already have an amplifier/filter you already are
controlling the source impedance, which should be low.  Many ADCs
do not represent a constant load.

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