Dan Michaels wrote:
[snip]
> My only caps are ceramic bypass and electrolytic P/S filtering.
> Where did you use polypropylene? For coupling?
> ==============

As I said before, working with Volts, even mV, it doesn't care so much
what type of capacitor you use, but goint to microVolts or even
NanoVolts, things change drastically. Very low leakage filter capacitors
are very important.  At 500nV, how many electrons do you think would be
necessary to leak from one capacitor's plate to another to bouce the
voltage to 510nV? not a bunch.  How many bits into the A/D it means? a
24 bits ADC would suffer with that change at the vref. Some
Polypropylene Film ECQP(U) (Panasonic) offers a minimum isolation of 45
GigaOhms, probably your circuit board will have less isolation between
tracks.

Just a question: THe 2mV readed at the A/D output is a jumping noise or
a steady noise, that can be confused with a/c or d/c A/D input offset?

Another point is that I never use a 16 bits ADC to read 16 bits
signals... I just ignore last 3 or 4 bits, so I pay more for a steady
conversion... using an expensive 22 bits AD7713 to really read 18
bits... going now to the LTC2408 (24 bits that in real are 28 bits) to
use just 20 bits... that's the deal, come on, it is a 6 digits counter
with 20 bits!!! I can have a resolution of 10nV at 10mV range...
(9.999,99)

> 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.

What about short to ground the LTC1400 inputs? not just the front end
amp inputs...  You need to isolate and make sure where is the origin of
the noise.

Wagner.