# Why is input offset voltage so bad? (op amps)

Take a typical application of an op amp - a photo amplifier:

In the circuit above, since input offset voltage is a dc characteristic, it is modeled as a dc source near the inverting pin of the op amp. This representation of offset voltage is also traditionally seen in many other noise analyses of op amps.

the output voltage V of the op amp as a function of the current i is, by KVL, KCL, and the two golden op amp rules, is

$$V = V_\text{os} + Ri$$

Which is very similar to the equation we'd get with an ideal op amp:

$$V = Ri$$

So, why not just measure the current by letting Vos be the reference voltage in our voltmeter? How about other easy ways to calibrate-out the effect of the offset voltage? Wouldn't those ways be much cheaper than buying a high-quality op amp?

A good answer would include a way to more accurately characterize the negative impact of offset voltage on a typical op amp circuit.

• I'm not trying to use the source - just graph the behavior of the output, and you'll get an y=mx+b graph – Dave Jul 7 '15 at 15:56

why not just measure the current by letting Vos be the reference voltage in our voltmeter?

It's not really clear what you mean by this, but Vos is entirely internal to the op-amp, and it might be a combination of different errors in different stages of the amplifier. We only model it with a voltage source in series with the input. But there's no one physical place you could put a multimeter probe to measure it.

How about other easy ways to calibrate-out the effect of the offset voltage?

Usually you can calibrate it out, for example by measuring the amplifier response to 2 or 3 different fixed input voltages.

One problem, though, is that an op-amp with high Vos is also likely to have a higher drift in Vos with temperature.

Wouldn't those ways be much cheaper than buying a high-quality op amp?

Calibration requires additional test operations in manufacturing. These might require additional operator handling. That adds significantly to manufacturing cost. If you need to calibrate over temperature, it could add dollars (not pennies) to your manufacturing cost.

Then the calibration data needs to be stored in the device somehow, and retrieved to apply the calibration to each measurement. For some (many) products that's no extra cost, but for others it might mean adding an EEPROM and uC that weren't needed before.

If there's an error in the calibration process, or the stored data is corrupted, you get field returns, which are costly.

Error due to offset voltage of an amplifier can, and often is, calibrated out. For example, if you had a gain of 50 amplifier with Vos = +/- 3mV maximum (and suppose we have an ADC and micro or a trimpot) you could simply short the input read the output and either diddle the trimpot or digitally subtract +/-150mV from the output (perhaps storing the output offset value in an EEPROM). That costs some programming, there is danger of the calibration being lost (EEPROM gets corrupted or a bad case of 'screwdriver drift' afflicts the trimpot) and it adds a production step. EEPROM corruption could be perceived as badly as a complete device failure, so it's not something to be sniffed at.

However, there generally is a correlation between high offset voltage and high TCVos and drift of offset voltage with time. Calibrating out TCVos using a temperature controlled chamber is a total pain, and I would not wish it on anyone who does not without doubt need it (it also tends to do badly with dynamic changes of temperature). Drift with time is something that cannot be handled directly- only by periodic re-calibration (which could be automated, but again that involves some additional expense and may not be acceptable in a product that has to work continuously).

In some cases, there is some external source of offset that far exceeds what the op-amp contributes (as well as being more variable) and you can just calibrate it out for each reading. An example of that would be some Hall current sensors (eg. DC clamp-on meters) and load cells used for measuring weight. In that case, Vos can be high- there is no point in paying for something you don't need (but maybe you need high gain for the desired precision and the low Vos comes for free).

So, horses for courses, but very good op-amps are very cheap these days and it's often worth it to use a good part and almost always worth evaluating the trade-offs. Some vacuum tube op-amps had an offset voltage exceeding 1 volt, we've come a long way with microvolt Vos and sub-0.1uV/°C TCVos parts widely available for reasonable price.