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Thread: [EE] 24-bit A/D. Are We in the Twilite Zone Here?
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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
>frequency.
>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

<1.5.4.16.20000427124238.39c734d4@lynx.sni.net>

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Subject (change) [EE] 24-bit A/D. Are We in the Twilite Zone Here?

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