Bernd.S wrote:
>
> Dear PIC Specialists,
>
> I use a PIC16C84 based Circuit with Audio (Freqency: 100 - 1000 Hz) Input.
>
> My problem is to construct a AUTOMATIC GAIN CONTROLLED OpAmp
> (with CA3140) using only nonsymmetrical 0V GND and 5.4V Powersupply.
>
> I need this because the input Voltage is a sinodial Wave with a variable
> Amplitude of only 3 to 10 milli Volts. I want to obtain a stabilized Wave with
 a
> Amplitude of 2 Volts whitch has about +2.5V DC Offset (will say: the Voltage
> varies between +0.5 and +4.5 Volt like a +2.5V shifted Sinewave).
>
> The Output of this should go after a Smitt Trigger in the PIC to measure the
> Frequency.
>
> Has anyone Ideas? Where I can obtain help with AGC Circuits?
>
> Thank You for all Help,
>               Bernd
use diodes in reverse parallel in the feedback loop to clip at
(diodes*.7v) to keep the opamp from saturating and then you can supply
sufficient gain to insure a full swing with minimum input and minimum
parts.crude perhaps but it's the edges that count when youre counting.
the 3140 is I believe a bipolar output you'll need a pull-up resistor
on the output or switch to a 3130 (cmos output).
        my two cents    Tony M.