My colleague Soufiane recently published an article, “Pushing the Precision Envelope.” In it, he discussed various technologies we use to “trim” or adjust the offset voltage of our amplifiers to very low values. It got me thinking about offset voltage trim pins—where did they go?

Most newer op amps lack the offset voltage trim pins once found on virtually all op amps. There are many factors at work in this change. Better, lower offset amplifiers, auto-calibrated system designs, pressure to reduce assembly and adjustment costs, tiny surface-mount packages—all combine to reduce the use of offset trim pins. Still, many of our best selling op amps have trim pins and knowledge and best practices of how to use (or not use) them are fading.

This much is easy—if you don’t use the trim pins, leave them open-circuit, no connection—do not connect them to ground.

Figure 1 shows a common type of internal trim circuitry. Trim pins connect to a tapped portion of the input stage load circuitry. Adjusting the potentiometer skews the balance of the load ± a few millivolts of input offset voltage. Data sheets generally recommend a value for the pot but it is not critical. A much higher resistance potentiometer will cause the change in offset voltage to occur toward the extremes of rotation. Too low a value will reduce the adjustment range. Pots in the range of +100% to -50% of the recommended value will likely function satisfactorily.

Notice that the trim circuitry in this example is referenced to the V+ supply. Some op amps have trim circuitry referenced to the V- supply terminal. Connecting the wiper of the pot to the wrong rail or to ground on a ± supply will surely cause problems. Some designers attempt tricky active circuitry to drive these pins. While this is possible, ground-referred circuitry connected to the trim pins can create power supply rejection problems.

It’s best to use the trim pins only to null the offset of the first amplifier in a signal chain. Generally that stage has some gain and its offset dominates that of the complete signal chain. If used to correct other large sources of offset in the chain, you could introduce an unwanted temperature drift.

Lacking trim pins, there are other ways to trim offsets in your system. Variable voltages from a potentiometer or other control signal can be injected or summed into various points in your signal chain. Examples are shown in figure 2. Notice that these trimming voltages are shown here to be derived from the power supplies. Regulated supplies are probably sufficient.  Unregulated supplies such as batteries may not be sufficiently constant or stable.

The improved offset voltage of modern amplifiers often eliminates the need for trimming. Still, there are times when some type of offset adjustment is required. You can be ready with techniques, whether with trim pins or add-on circuitry.

Do you trim offset voltage? How? Please tell us in your comments below?

Bruce          email:  thesignal@list.ti.com

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Anonymous
  • 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%.

  • 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.

  • 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+.  

  • 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.

  • 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.