Hi Gerhard,
first of all, thanks for your reply.


>> 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.

How true, this is a crucial point that reached my mind shortly after hitting
the "send" button. What I decided then was to isolate the serial port, via an
optocoupler (4N25?). Basicly I would get serial high from DTR and serial low
from RTS, and then use the 4N25(?)'s optotransistor to drive RX through the
current from DTR/RTS. I don't need handshaking anyway. I will set DTR and RTS
to the right values in the my PC application.

Now I'm still not sure if I should use a transformer or if I could use the
DTR/RTS also to power an isolated DC-DC converter, like those nice ones made
by Texas Instruments (e.g. DCR010505), but they may be too noisy, perhaps.
I'm not truly attracted to the idea of a transformer.. but if it proves to
be the best solution, I'll swallow it and forget.

In any case the MAX232 doesn't seem necessary anymore..

Thanks,
PicMan

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