Where are the Trim Pins?

RE: Where are the Trim Pins?

Marian Stofka — Mon, 10 Sep 2012 14:06:55 GMT

Bruce, you yourself name "lower offset amplifiers", but as if very carefully.

Why? Nowadays also TI produces a variety of op amps, which have typical input voltage offset of the order of 50 micro-volts.

Trimming is done by the manufacturer; by laser-trimming within a chip.

Fig. 2 brings us back; when we were young and built circuits using uA709s'.

Moreover, trimming as in left of Fig.2; could change the desired voltage gain by as much as (R1Rp)/(2R3R3) =5E(-3); or 0.5%.

RE: Where are the Trim Pins?

Leendert Prevo — Mon, 10 Sep 2012 07:17:22 GMT

IF you like to trim the offset, you have to know the heat-flow on the circuit board as to be sure that the trimming circuit does not 'untrim' because the trimming-resistors get at different temperature.

RE: Where are the Trim Pins?

Tim Sobering — Sun, 09 Sep 2012 18:57:12 GMT

I've always been of the opinion, probably due to Pease's influence, that the trim pins are best left alone and methods like those you describe in Figure 2 are best. However, one recommendation in the circuit on the right side of figure 2 would be to consider a large cap across R4 to reduce (eliminate) the noise contributions due to Rp, R3, R4 and I_n+.

RE: Where are the Trim Pins?

Bruce Trump — Sun, 09 Sep 2012 18:09:20 GMT

Good point, Edwin. The temperature effect when using the trim pins is the reason I don't recommend trimming other system offsets, just the op amp offset. It you limit the use of trim pins to null only the offset of the first op amp in a signal chain the effect is minimal. In fact, when using the trim pints (figure 1) with bipolar (BJT) op amps you are statistically more likely to improve the temperature drift as you null its offset. The effect on temperature drift is actually more significantly affected by the change induced in transistor characteristics as the input stage is skewed--not so much by resistor matching.

RE: Where are the Trim Pins?

Edwin Pettis — Sun, 09 Sep 2012 15:45:47 GMT

I remember the Fig. 2 trimming circuits well, been around practically since op amps were invented. One other thing you didn't mention, when using external resistors to trim the offset, they will cause a shift in the temperature coefficient of the offset voltage due to the difference in TCR between the on chip resistors and the off chip resistors. This usually is not of much concern unless the circuit's offset drift is a significant factor.