3
\$\begingroup\$

Looking at many OpAmp (and InAmp) datasheets, I find the info either confusing or lacking. For example, in one datasheet they just call something Offset Voltage where in others they have two, separated as input and output offset voltages, respectively.

What is going on?


UPDATE

I must apologize from not paying full enough attention to the entire datasheets. The tricky part is that sometimes the data, as stated, are put under a header (a page above) as input), whereas when written in the summary, the offset is to be understood as input offset. However, occasionally the output offset is omitted. Either way I appreciate knowing the difference between the two terms.

\$\endgroup\$
  • 1
    \$\begingroup\$ Please provide links to example datasheets that illustrate your question. \$\endgroup\$ – Elliot Alderson May 2 at 12:29
  • \$\begingroup\$ I wrote an answer to another question about DC offset a few days ago, parts of it might answer your question: electronics.stackexchange.com/questions/436125/… \$\endgroup\$ – Bimpelrekkie May 2 at 12:32
  • 2
    \$\begingroup\$ Input offset voltage is defined as the voltage that must be applied between the two input terminals of an op-ampto null or zero the output. Output offset voltage is the dc voltage between two output terminals (or the output terminal and ground for circuits with one output) when the input terminal(s) are grounded. \$\endgroup\$ – akshayk07 May 2 at 12:33
  • 1
    \$\begingroup\$ @akshayk07 ...when the input terminal(s) are grounded If you make that shorted (i.e. not necessarily grounded) then I would fully agree with your comment. \$\endgroup\$ – Bimpelrekkie May 2 at 12:34
  • 2
    \$\begingroup\$ The JEDEC is indeed trustworthy, but the "when the input terminal(s) are grounded." is only valid when an opamp can actually work in that situation. Not all opamps can when given a single supply. So the statement is true if the common mode input range of the opamp is such that ground is included. So the statement is true but doesn't tell the complete story so isn't universally true. Always be critical of such statements and don't blindly take anyone's word just because they're the JEDEC. JEDEC is also just people and people make mistakes. So stay sharp! \$\endgroup\$ – Bimpelrekkie May 2 at 12:54
0
\$\begingroup\$

Input offset voltage is the voltage source that would be connected to an input of an ideal op-amp to model the behavior of real op-amp.

You can also think of it as the voltage (assumed that both inputs are within the operating ("common mode") range of the chip) that must be applied differentially to cause the output voltage to go to 0V (bipolar supplies) or perhaps midway between Vdd and GND (single supply). AD defines it thusly in the below-linked paper, but it's not inclusive enough for my liking:

Offset Voltage: The differential voltage which must be applied to the input of an op amp to produce zero output

It's a bit hazy what the output voltage condition is (you can think of this as the "output offset"), and open-loop gain is so high that it usually doesn't matter. For example, a precision amplifier might have an open loop gain of 10^6 so a few volts at the output represents a few uV at the input. AD skirts the issue by assuming bipolar supplies. You can certainly test a single supply 3V op-amp by applying +/-1.5V supplies which matches my definition.

Here is one way in the real world to measure input offset voltage, as described in this paper from Analog Devices:

enter image description here

R1 and R2 are chosen to give high gain (but much lower than the open-loop gain) and there is a mirrored pair on the non-inverting input to null out input offset current. R1 is chosen to be very low value to minimize the effects of input bias current. The gain is chosen so that the output will not saturate even for maximum specified input offset voltage and also such that the open loop gain will not unduly affect the accuracy. A 25uV Vos results in an output voltage of about 25mV in this case.

With, say +/-10V supplies, and an op-amp that can swing within 2V of the supplies, this circuit will work with offset voltages of up to +/-8mV, which covers most op-amps. A +/-200nA input offset current will result in an error of +/-2uV in the Vos reading. Most modern op-amps with such a high bias/offset current are bipolar low noise types that run the input transistors at relatively high current, and do not have particularly low Vos, so it's not much of a concern usually.

Here is another method that is somewhat similar but assumes lower input bias currents (potentially more error) and drives the output to zero (via the integrator formed by the auxilliary amplifier) almost exactly (potentially less error due to finite gain of the Device Under Test) Vos is approximately 1/1000 of the voltage at TP1:

enter image description here


In general, the input offset voltage is measured at the output and we apply a calculation to determine the input-referred offset voltage. It's more general for a specification since the output offset depends on the gain in the particular circuit.

There is an analogous situation in the measurement of noise where we measure the noise at the output of an amplifier but divide by the gain of the particular circuit in order to determine the input-referred noise.

If you had a circuit different from a typical op-amp in that the open-loop gain was relatively low, the output offset might not be as well predicted by multiplying a fixed input-referred offset by some variable gain number and you would need two numbers to be specified.

\$\endgroup\$
  • \$\begingroup\$ Wow, one hell of an explanation. Accepted as you also explain why the output offset may be different. \$\endgroup\$ – not2qubit Sep 20 at 20:43
1
\$\begingroup\$

Input offset voltage (Vio) - Voltage across the op amp's inputs that will force the output voltage to be 0V.

Output offset voltage (Voo) - The output voltage of the op-amp when the input terminals are grounded.

These are very different, and output offset voltage is not particularly useful except maybe in certain extremely niche applications. I am using such vague language because I actually cannot think of a usage case where knowing the output offset voltage would be useful, but if you've seen it in a datasheet, maybe it is. Or maybe it was included just in case.

Just to be clear, the output offset is not the output voltage of the op amp when an op amp set up for unity gain has no voltage across its input. This is simply going to be the input offset voltage, just appearing on the output.

The output offset voltage is specifically both inputs grounded, no feedback present. Like this:

enter image description here

Note: Typically this is measured with some kind of specified load resistor, which I've omitted for clarity.

The only possible application I can think of off hand might be for how the op amp behaves under certain fault conditions. If anyone has any other ideas, I'd love to hear it in the comments.

\$\endgroup\$
  • \$\begingroup\$ Nice picture with simple explanation! \$\endgroup\$ – not2qubit Sep 20 at 20:43

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.