A digitally controlled gain for Op-Amp

[zimbali]

Hi,

Is there anybody there knows how I can amplify a signal with a simple op-amp (CA3140 or 741) with a variable gain which is controlled by a digital signal? Nothing extreme, i.e. signal in the range of mV and gain in the range of couple of hundreds, so output in a range of couple of volts. It should be a MOSFET somewhere in the feedback circuit that its resistance changes with voltage or maybe a more intelligent way ... I'm looking forward to hearing from you.

Thanks for any help, in advanced.

Cheers
Zimbali

 

[VolkerR]

Hi Zimbali,

usually the requirements call for a limited number of gain factors, e.g. 1, 10, 50, 100. In that case you can use analog switches with different attached gain resistor values. Make sure the resistor values are not too extreme, so that the analog switch doesn't introduce too much error.

Another option would be using a digital potentiometer (Xicor, Dallas...), if you need a lot different gain settings. Note that sometimes the digital pot absolute value has a high tolerance, but its ratios are very accurate.

Another option if you have a non-inverting amp configuration with a gain resistor going to GND and single supply operation, you can use simple MOSFETs as switches to turn on differen resistor values.

In theory you could use the resistive characteristics of JFETs, but I've never seen that done before, because then you need to hand-calibrate the circuit due to device tolerances.

Volker

 

[Madcow]

You could try a programmable gain op amp.
Here's one from Microchip that operates over
the spi bus.

http://www.microchip.com/1010/pline/analog/anicateg/linear/pga/index.htm

I would also check analog devices or TI. I remember someone
offering one that was programmed by 4 pins to change the gain.

 

[zimbali]

Thanks for both helps. I'll check them and will be back with the result soon.

cheers

 

[bubulindo]

You can search at

http://www.maxim-ic.com.

They have a chip that simulates the entire resistor network around the instrumentation amplifier. it has a 2 bit ajustable gain stages!!!