Testing Op-amps performance

Hello Nath,

You are asking a very broad question...but for a buffer...with just an input and an output, there are just a few parameters you can measure, but there are several error sources that come into play.

The major error sources will be initial offset, offset drift over temp and common mode error...and maybe a little open loop gain error if you are driving a heavy load. But the dominant error source will be the offset voltage.

The initial offset is different for each part.

Offset voltage will shift as the input voltage changes. You can see this in Fig. 25 and 26 "dVOS vs CMR" graphs. Since it is a follower, Vcm = Vin.

Offset voltage will also change by about 1uV per °C. If the offset was 1mV at room temp (25°C), then it will be about 1.060mV at 85°C.

Near the rails, output swing limits can cause errors near the rails. Gain errors in the linear range can be seen in the "Input Voltage vs Output Voltage" graphs (AKA, Open Loop Gain) in Figs 26 and 27. As the gain drops due to output loading, more input voltage is required to move the output, creating more error.

For a buffer, the error measurement is fairly simple...Vout over Vin. The perfect buffer would have the output exactly the same as the input. The measurement can be as simple as placing a sensitive (floating) voltmeter between the input and the output to measure the slight difference between them, and sweeping the input voltage across the expected range. This will give you total error vs output voltage. Do this over temperature, too.

For the gory details, you can spend a few hours reading this document...:^)

Op Amps for Everyone Design Guide (Rev. B)

http://www.ti.com/lit/an/slod006b/slod006b.pdf

Regards,

Thanks that's very helpful.

Will go through the document :)