Re: question about op-amps

From: andy (news4_at_earthsong.free-online.co.uk)
Date: 07/21/04 

Date: Wed, 21 Jul 2004 14:23:26 +0100

On Wed, 21 Jul 2004 02:22:08 -0700, russ wrote:

> John Popelish <jpopelish@rica.net> wrote in message news:<40FDD518.4A63EA7A@rica.net>...
>> russ wrote:
>> >
>> > Hi
>> >
>> > I am trying to read external data into the serial port of a computer.
>> > Basically if 'something' is at a few millivolts, then to input a '1'
>> > or +10 V with respect to ground into the computer.
>> >
>> > This means I want to amplify my input. I tried a standard '741'
>> > op-amp wired up with resistors etc to make an inverting input with a
>> > gain of a couple of hundred times. I had a lot of trouble getting it
>> > to work consistently though, what with bad soldering, no decent signal
>> > generator or oscilloscope and probably interference due to the high
>> > gain.
>> >
>> > What I wanted to know was, is there an easier way of going about this?
>> > Is there what I want sold as a unit in an electrical shop?
>> >
>> > Thanksd in advance
>> >
>> > Russ
>>
>> You have confused me. A serial port accepts binary data that is
>> either +3 to +12 volts ( which is a logic zero) or -3 to -12 volts
>> (which is a logic one). Normally, these states make sense only in
>> they are patterned into a sequence that represents characters.
>>
>> So how does your millivolt signal fit into a serial port, regardless
>> of how much you amplify it?
>
> Basically, my signal of interest is either zero or some small signal
> (mV). I wanted a circuit that will put +3 to +12 V on one of the pins
> when the signal, is zero and -3 to -12 V on the pin when my signal is
> non-zero. ie amplify the signal such that it is either max voltage or
> min voltage when the signal is above or below some threshold.

you might be better using the parallel port - serial ports expect to
receive data in blocks of 8 bits (give or take), so you would have to add
a serial port driver chip to your circuit. You can read individual lines
of the parallel port in software, by reading the whole port and then just
masking out the bits you're not using. 

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