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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)

Joe, Grif, Wagner, Dave, thank you all for the responses.
I think I have been around the track a couple of times with
pretty much everything you all mentioned.

Bypass caps everywhere, as much physical separation between
analog and digital sections as possible given a 3"x5" pcb,
"separate gnd busses" for analog and digital, analog gnd
single point connection returned directly to power entry point,
as much gnd planing as possible on a 2 layer pcb, wide power
traces, separate analog regulators, lowest feasible R values
on opamp ckts, 2 opamp inverting stages in series, not using
non-inverting stages, ferrite bead in power in lead and Rs-232

Also, pcb is in an EMI-shielded ABS case, and with case
removed, the noise *is* always 2x to 3x to 5x higher. [I
wonder if this isn't "the" crucial clue?]. I assume this
means pickup from local digital sections of pcb is what is
being shielded. I don't think it's coming from outside.

Some specifics:
- 2 mV noise is referenced to input, with input grounded
 directly and measured by reading A/D output, not with scope.
- for this measurement, input range = +/-1.25v, so 2 mV
 represents only about 9 bits - so am losing ~3 A/D LSBs.
- Power train:
 - wall wart -> 7812 --> 7805 (digital).
                 "   --> 78L06 (analog +).
                 "   --> 7662 (-12v) --> 79L06 (analog -).
 - all regs bypassed, filtered high side, low side, etc.
- Gnd planes present under noisy 7662 (766x derivative). Noise
 referred to above is broadband noise, not correlated to 7662
- Analog circuitry: 2 inverting op amp stages (superior CMR to
 non-inverting), currently using TLE2072, 4 gains set via '4052.
 - Lowest range: +/-1.25v, Av1=0.4, Av2=4.
 - R values: 1st stage - 1M in, T-section feedback to lower
   effective R, with 82K-21K-68K. 2nd stage 12K in, 50K fb.
 - both stages switch gain via '4052 mentioned above.
 - output to A/D thru small R.
 - opamp/CMOS switch bypassed directly at pins.
- A/D: LTC1400, SOIC, 12-bit, 8-pin, internal Vref, SPI control
 at 6.4 Mhz. Vref bypassed directly at chip. Vdd, Vss bypassed
 close to chip.
- Digital control: PIC76 @ 20 Mhz, bypass caps in several places,
 gnd planes under chip, gnd island around xtal.
 - no hi-speed signals passing near analog circuitry.
 - 2 DC control signals from PIC to '4052, bypassed.
- RS-232 connection to pcb: lots of noise was originally measured
 on the TX line from the PC, and now has a ferrite bead inline.

Note - overall, this is a "system-level" problem. Mixed
 analog-digital, small space, noisy 7662, R values as small as
 I can make them, RS-232 to PC, wall wart power, etc. I have
 isolated and dealt with any number of noise sources already.

Working on next pcb layout:
- moving analog regs further away from PIC.
- removing 7812 from inline with 7805.
- considering moving single-point analog gnd connection from
 78L06/79L06<->power_entry_point to under A/D.

Note - I have several times fiddled with the opamp ckt design
 and R values, and they are about as low as I can go now, and
 still keep Rin = 1M.

I may need to go to a completely different front-end design, ie
FET follower amp - unfortunately this is much more complicated,
requires lots more space, it drifts, etc/etc. I was hoping to
keep the front-end design simple and straightforward, since
BW is only 1 Mhz, but I am beginning to wonder.

Re what Dave said:
>Decouple your supplies into the op-amp.  A series R and local C
to ground
>should work well.

Bypass caps are "everywhere", but I haven't tried series R in
the opamp power leads. However, they have their own regulators,
bypassed and separate from the digital regs, plus a separate
analog gnd, too. So I don't think a series R here will help much.

>Check the op-amp PSRR AT the frequency of your main interference.
>Use a spectrum analyzer to check everything, not just a scope.

Measured noise is pretty wideband, not correlated to PIC xtal
or 7662 - [xtal is outside passband, 7662 has been dealt with].

>In the circuit itself, lower the impedances if you can, keep
layout tight,
>don't ground or decouple to any foreign points.

I already have lowered the R values in the analog section as
far as I think I can go. The 2 mV noise was measured with input
BNC shorted.

"... gnd or decouple to a foreign point". What does "foreign"
point" mean?

Re what Joe mentioned:
>And yes, you may be missing something.  You can trade off bandwidth for
>noise! The narrower the bandwidth, the lower the noise figure. Every time
>you reduce bandwidth to 50% noise is decreased  to 35%.  Its one of those
>square law formulas I learned in Signals.

I am pretty familiar with signal theory, but this is a wideband ckt,
and the analog has to be made to tow the line. BW here is 1 Mhz, and
I wish I could make it 20 Mhz!!  [I could probably get Walter's 0.12
uV, if I LP filtered down to 3 hz. Yeah, right].

------->  My wondering was whether there isn't some magical formula
saying something like "for a 1 Mhz BW, no matter how damn hard you
try, you aren't ever gonna get the noise below ___ mV, unless you
do ........". [as in, "make the Rs all <= 1K", whatever].

>There are other exotic methods for squeezing the last little bit out of a
>signal path, synchronous detectors, digital filtering, extremely long (over)
>sampling, etc.

My case is 1-shot waveform captures, and I want to make the ckt
work as well as possible *before* the DSP stuff comes in [which it
does later on].

RE what Wagner mentioned, I am still mulling over your comments.
Lots of good info there. The Vref on my LTC1400 A/D is inside
the chip, bypass caps outside. Noise measured via A/D binary
output, with front-end amp tied hard to gnd.

>Here again, expensive and good polypropylene caps makes a hell of a
>difference from cheap ones.

My only caps are ceramic bypass and electrolytic P/S filtering.
Where did you use polypropylene? For coupling?

Whew, what a marathon. Given all this, if there is something
I have clearly overlooked, ....... ????????

Best regards,
- Dan Michaels


See also: www.piclist.com/techref/io/atod.htm?key=a%2Fd
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