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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 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!

Reply to
Frida
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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
http://www.wescottdesign.com
Reply to
Tim Wescott

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

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Reply to
Joerg

snip

Hence giving new meaning to the term "mosey".

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Reply to
Tim Wescott

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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Reply to
CBFalconer

Hi Tim,

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

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Reply to
Joerg

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