Re: ACG for PC soundcard input?

"David King" <daveking@xxxxxxxxxxxxx> wrote in message
news:54u5l1lenrqdihlnnnq8cj0h1980huen43@xxxxxxxxxx
> I have a notebook computer with line level input. I would like to use
> shareware PC based oscilloscope software designed to input to the
> soundcard ADC via a switchable attenuating amp. Yes, I do know the
> limitations with this.
>
> The problem with these setups is how to avoid inadvertently damaging
> the card with over voltage. IOW if the attenuator is manually set too
> low or the signal level suddenly changes.
>
> Can someone possibly recommend a circuit for a suitable automatic gain
> control such that it would not significantly alter the acquired
> signal?
>
> My frequency of concern is 20-500Hz at 1-5Vpp. Typically these devices
> run off 5V available at the games or USB ports.
>
> There are several circuits for the basic principle on the net, some
> use a clipping indicator, or zeners for crude limiting, but I always
> wondered why no ACG. That would be a logical and welcome addition.
>
> Can anyone help?
>
> Dave King

AGC and clipping indicators aren't much use with a PC oscilloscope. AGC
would make all signals look the same size, thus hiding all the amplitude
information. Clipping is immediately visible on screen as the signal hits
the limits.

You won't blow up your sound card if you have enough series resistance,
because the input circuit will almost certainly safely take 10 mA to +5V or
ground when the signal goes outside those limits. 1K ohm in series, for
example, would let you go 10 volts off scale, and 10K would make it good for
100V.

If that is not enough peace of mind, I suggest an external opamp driving the
series resistor. The opamp can only swing as far as its power supplies
permit. For example, a low voltage +5V powered opamp could *not* go outside
0 to 5V, or a +15V powered opamp with 2K2 of series resistance could only
deliver 6mA etc. Even with a 5V powered opamp, I'd still want some series
resistance, to handle the instant of connection etc.

I'm sure you can find a circuit on the Internet with an opamp and series
resistance. A switched attenuator on the input would be nice too.

Roger Lascelles



.

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