Hello,
I am looking for an ADC 16+ bit resolution that would sample 64 chanels
at the
rate of 25 Khz each. switching is planned to be done with a mux.
It is rather difficult to find a sigma-delta ADC for that kind
of sampling frequency even for a single channel. Of course,
many inexpensive audio converters claim something like
"192 ks/s, 24 bits", but this is not the complete truth.
If you want to use such a converter in a multiplexed system,
you'll face the fact that the bandwith limitation (96 kHz)
effectively limits the settling time when multiplexing.
The worst case is when you have two multiplexed signals
which alternate between minimum and maximum. The square
wave produced this way has very significant high frequency
components. If you want to sample this signal down to
16 bits, the bandwidth has to be way larger than the multiplexing
frequency.
So, the first problem is the bandwidth-limiting nature of
sigma-delta converters. Other converter types (SAR, flash)
don't have this problem, their bandwidth may be much wider
than the sampling frequency (which in many cases is a problem
per se).
Another significant problem with sigma-delta converters
is their bad DC behaviour. The "el cheapo" audio converters
have rather impressive dynamic performance, but when it comes
to measuring DC levels, there may be hundreds of LSBs of
error and drift, as those parameters are insignificant in
audio processing.
There are fast DC-accurate sigma-deltas as well. For example,
the TI (BB) ADS1606 seems to offer 16 bits at 5 Ms/s and
2.45 MHz bandwidth. Oh, the price is $30 each, and you'd still
need many of these in parallel (check the data sheet to
get the idea of the settling time).
There are some less expensive converters with dozens of
kilosamples per second. However, they could sample only one
channel at a time, and the price is still well above
that of an inexpensive audio sigma-delta.
I'd recommend using the approach of one converter per channel.
This makes the sampling requirements much easier. If you want
to sample something at 64 x 25 kHz = 1.6 MHz at 16 bits, the
input impedance has to be low, but at 25 kHz there should be
no problems.
If you really want to multiplex, then SAR converters are better.
There are 16-bit SAR converters with megasample-range throughput.
Using one of these could possibly solve the problem with a
single converter and a huge multiplexer. (Beware, there are even
SAR converters with poor DC performance!)
The solution this way would be less expensive than with per-channel
converters, but the design is more complicated (multiplexers,
buffers, etc. with 16-bit accuracy). In any case, you'll need
to study the converter data very carefully, as very often
the datasheets are rather shy when it comes to the deficiencies
of the converters.
- Ville