Offset Voltage and Open-Loop Gain—they’re cousins

RE: Offset Voltage and Open-Loop Gain—they’re cousins

Bruce Trump — Thu, 16 Jan 2014 15:52:12 GMT

Carter,

Your questions would be challenging topics for blogs, each requiring diagrams for full explanation (which I cannot include here in this reply). I am now retired from TI and no longer writing blogs so I'll provide brief answers here:

I believe you are asking how open-loop gain (Aol) is tested. The graph shown in this blog is generally produced by connecting the op amp in a two-op-amp loop configuration that controls its output voltage while measuring offset voltage. This is a DC parameter so the output voltage is stepped slowly, making DC offset measurements throughout the output voltage swing range. Various references discuss this technique and you can probably find more information on-line.

To test gain-bandwidth product, connect the op amp in a non-inverting gain of 10 or greater. The GBP is the product of measured -3dB bandwidth and the closed-loop gain of the circuit (such as 10). Use an even higher closed-loop gain if the op amp is not unity-gain stable. Set the signal amplitude low to avoid slewing. Measurement at G=1 will generally produce an erroneous result as it is affected by the subtle changes in gain and phase near the unity-gain crossing. Current-feedback op amps (such as the OPA694) do not exhibit the classical nearly constant gain-bandwidth behavior of voltage-feedback op amps. For these devices, bandwidth is generally specified and tested with specific gain and feedback network values.

Regards-- Bruce

RE: Offset Voltage and Open-Loop Gain—they’re cousins

Carter Liu — Thu, 16 Jan 2014 06:16:26 GMT

Hi Bruce

How do you test the Vol of a certain OpAmp?what signal source do you use and how to eliminate the influence of Vos when testing?

And I also want to know how to test the GBP of a certain OpAmp,because each time I tested an Amp I cannot get the value of GBP specified in the datasheet,especially for those with big GBP products such as OPA847,OPA694...

My email :carter-liu@ti.com;409204017@qq.com wish your response!

Regards

Carter Liu

RE: Offset Voltage and Open-Loop Gain—they’re cousins

Bruce Trump — Tue, 21 Aug 2012 17:30:54 GMT

Yes, Stephen, that is a good way to think of it. You could also add a term for the offset induced by common-mode rejection errors Vcm / CMRR, where CMRR is expressed in V/V, not dB. Vcm is the deviation away from the specified zero common-mode point. And you can add another temperature-dependent term for the offset voltage drift with temperature.

Regards-- Bruce

RE: Offset Voltage and Open-Loop Gain—they’re cousins

Stephen Power — Tue, 21 Aug 2012 12:18:07 GMT

Hi Bruce

Would a more correct model of Input Offset Voltage be (Vos + Vo/AOL) in series with the non-inverting op amp input rather than just Vos as sometimes depicted in text books? (Vos is the Input Offset Voltage for Vo=0V, Vo is output voltage and AOL is of course open-loop gain)

regards

Steve