piclist 2000\04\29\133256a >
Thread: [EE] 24-bit A/D. Are We in the Twilite Zone Here?
face BY : Dan Michaels email (remove spam text)

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].

{Quote hidden}

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"


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