Dave Van Horn wrote:
>
>Try a different op-amp.
>
>I just got back from a field trip where I discovered that my major noise
>source was the op-amp itself rectifying RF. No tracks needed, the chip was
>acting as an AM demod, and giving a very nice replication of the
>transmitted pulses.
>
>LMC6464 got rid of the problem, rectified pulses now unmeasurably low.
.......

Sounds like you could use some front-end RF filtering too. Maybe
ferrite beads in the input signal lines, or inductors in the Vcc
lines, etc.

And I've looked at dozens of op amp specsheets in the past couple
of days. Turns out all of the JFET types have very bad PSRR- at 1
Mhz, only 0 to 20 dB. Even true for Natl LM6152 which is 75 Mhz GBW.

Maxim has nothing usable. Linear Tech has a lot of interesting
new op amps. The LT135x series have enormous slew rates [400-1000
V/us] at nominal GBW [12-70 Mhz], plus nice PSRR [50 dB @ 1 Mhz],
and fairly low Icc ~1-2 mA. Really nice amps.

Unfortunately, these things are very costly [$7, qty 1, Digikey],
and the I[input_bias] is up near 1 uA, so now I need a fancy way
to control DC offset jumps when changing between the 4 amp gains.
What fun!!

Well, in a few days, I'll get an LT1355 and plug it in.

I'm just starting to look at the Burr-Brown amps. No current
catalog here, so have to download datasheets and search thru for
the PSRR, etc, values.

Anybody know a nice opamp with GBW >= 10Mhz, CMRR and PSRR
>= 50 dB at 1 Mhz, low I(input_bias), and under $7? [Ha, see
comment below re Holy Grail].
==================

>>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].
>
>Couldn't hurt, (see my other note on op-amps as AM demodulators!)
>It's probably layout driven, can you probe around with a spectrum analyzer
>and see what surfaces? It may be that you can get 3-6dB just by re-routing
>some tracks.
>
>>3. Maybe something other than 79L06 would be better.
>
>Hmm.. 79L10 followed by 79L06, with intermediate filtering?
>Cheap, brute force, probably effective. Got that much headroom?
>

On my new pcb layout, I've already moved the analog v.regs further
from the digital cktry, improved their hi-side bypassing, and
increased the area of gnd planing around them. I'll see if this
helps when I get in the new pcbs.

And there isn't really too much headroom on the 79L06.
7812 -> 7662 (with -10 -> -11v out under load), and 79L06 requiring
about -9 for regulation. If I go to a 7815 and higher Vin, then
I have even more heat problems dropping Vin down to 5v for the
7805 to digital.

I can't see anything left to do, viz-a-viz layout/etc, short of
going to a totally different P/S design.
============

Ha, this whole thing is beginning to look like the Quest for the
Holy Grail :-). Just call me Percival. I'm trying to improve the
analog system overall ENOB from 9.5 bits, and *at best* will
probably get only 10.5-11 bits. Spend 3 months, countless $$$$,
just to get pittance in improvement. Ahhh, wonderful engineering!!

Best regards, and thanks for the help,
- "Percy"
===================