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Thread: [EE] 24-bit A/D. Are We in the Twilite Zone Here?
face BY : Dan Michaels email (remove spam text)

Russell McMahon wrote:

>1.    It's vital that your op-amps have good gain at the common mode
>ie - even if you are amplifying a dc or low frequency signal, if you have
>digital (eg clock, uP data, SPI signals etc) appearing as common mode
>signals then your op-amps MUST be able to reject these actively . ie you
>should be using amps with a 10MHz plus gain bandwidth and really the more
>the better! A low gbw opamp will pass the common mode signals as noise in an
>uncontrolled way.
>2.    Consider using parts like the AD7730 or the newer AD7731 with internal

My present opamp, TLE072, has 10 Mhz GBW but only 20 dB PSRR at 1 Mhz,
and I've been looking for a better drop-in replacement. Natl has the
LM61x2 series, going to 75 Mhz GBW, but amazingly, the PSRR on every
one of these goes into the mud (ie, <= 20 dB) right near 1 Mhz <-- this
seems to be a magical number.

I checked the AD7730, $28/ea from Arrow. Ugh.

I'm beginning to think you just can't do it all with a simple opamp
ckt lie I am using. To do better than 2 Mv noise, would need to go
with FET follower front-end feeding onto expensive low-impedance,
differential-mode opamp ckts, like they do it inside the scopes.

>3.    If I was doing this (and I'm glad I'm not just at present :-)) I would
>consider building a minimum component configuration just to see how good I
>could make it with the IC used.
>ie get rid of everything possible
>Once you are sure of how well you can make the simplest possible circuit
>perform you can then extend it and see where it starts to degrade.

Yeah, Grif and Wagner suggested this too. I'm going to try building
up one of my new pcbs [when I get one] one circuit at a time, and
test, test. This way it will more or less be "in system", maybe I
can pinpoint the key problem aspect this way.

Thanks and best regards,
- Dan Michaels


See also: www.piclist.com/techref/io/atod.htm?key=a%2Fd
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Subject (change) [EE] 24-bit A/D. Are We in the Twilite Zone Here?

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