[EE] 24-bit A/D. Are We in the Twilite Zone Here?
Dan Michaels email (remove spam text)
Dave Van Horn wrote:
>Check the op-amp PSRR AT the frequency of your main interference.
Dave, it hadn't escaped my attention that you mentioned this the
other day. This may be much of the problem. I am currently using
the TLE072 dual op amp, with gain-BW product = 10 Mhz. It's about
the best thing I could find that looked comparable to the ckt reqs.
I upgraded from the slower LF353 when I started pushing the BW up.
I just re-checked the specs. The CMRR of the TLE072 drops to 50
dB at 1 Mhz, the PSRR+ to 50 dB, and the PSRR- to only 20 dB.
Note, I earlier canned the LF353 because its PSRR- drops way down
to 2 dB at 1 Mhz! Also, I originally went with the inverting
opamp config because of the CMRR dropoff.
This may be the real problem with my board. So what is the
1. Anyone know of a good dropin DIP-8, dual op amp replacement
with better PSRR?
2. Would it help to work on the analog -V regulator [79L06] a
little harder? I already have bypass and filter caps there,
and on the opamp, plus 50 mil power traces. OTOH, it does
get its power from the noisy ole 7662, and on the present
pcb, its fairly close to the PIC. [on the new pcb I had
already moved it away].
3. Maybe something other than 79L06 would be better.
- Dan Michaels
See also: www.piclist.com/techref/io/atod.htm?key=a%2Fd
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