ThePicMan wrote:

> It's a "poor man's very low frequency scope", because I need to monitor
> an analog low voltage (from 0.0V to 1.5V) input that changes slowly, and
> record and log this waveform on my PC via RS232

> (some devices I will monitor may be at a different ground potential than
> the PC).

I think this is the crucial point that you need to address before deciding
anything else. How much isolation do you want? And how do you want to
achieve this?

Basically you can either put your circuit on the PC's ground and create a
separation in the analog input, or you can put your circuit on the input's
ground and create a separation in the serial link between the circuit and
the PC.

Creating a separation in the input may be as simple as a difference
amplifier (a few up to a few ten volts of ground difference) or as
expensive as a full-blown isolation amplifier (a few hundred up to a few
thousand volts). 

Creating a separation in the serial link can be done with optos and either
a separate power supply (transformer-based) or a dc-dc converter (with
separate grounds for input and output, of course).


The answers to most of the other questions (input amplifier, serial
circuit, ground connections) depend on the decision about how you deal with
the ground isolation.

Gerhard
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