Choice of nulling trimmer and slew rate with CA3140T Op-Amp

[Ohmicide]

I'm in the middle of a restoration project that involves a sample/hold circuit. The circuit is based on a CA3140T operational amplifier. In the datasheet for this part, there are several circuits and the nulling trimmers differer in their effective values from 10K to 100K. Would the 100K trimmer have a slower slew rate than the 10K trimmer? I ordered some 12 turn trimmers at 100K and am hesitant to install them if the circuit works significantly better with 10K trimmers. I'm asking this question because every schematic I have found for this circuit has a 100K trimmer listed but the actual circuit itself has 10K trimmers between pins 1 and 5 and the wiper connected to the negative rail. The circuit I am working with is unity gain with a 120p cap from 1 to 8 and a .22uf cap on pin 3. The input voltage sample range is from 0 to 4 volts. Here is the datasheet: https://www.intersil.com/en/products/amplifiers-and-buffers/all-amplifiers/amplifiers/CA3140.html Any help will be appreciated.

 

[richie00boy]

If the original circuit used 100k then it should be fine. If the datasheet showed 100k and you used 10k then that could be a problem, but the other way round the problems aren't likely to happen.

 

[Mooly]

The only difference that using different values of trimmer will cause is in the 'feel' and resolution of the way the trimmer works. A 100k will give fine resolution as long as the imbalance is slight. If it needs to be near one end to trim the error then a lower value preset will seem to work better.

You need to understand how it works, then you'll see the absolute value makes little difference. All you are doing is altering the value of one or other of the internal 500 ohm resistors in the input stage current mirror by paralleling with the trimmer value. If the trimmer is 10k and set mid point then the 500 ohms become 500 ohm + 5k in parallel with it. If the trimmer is 100k then its 500 ohm + 50k.

 

[Ohmicide]

Thanks.