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# OPA855: OPA855

Part Number: OPA855
Other Parts Discussed in Thread: LMH32401

We are using the LMH32401 and are interested in converting the differential output to single-end and driving a 50 ohm load. At the same time we want to cancel the common mode output voltage. The circuit needs to be D.C. coupled. We ran a test with a OPA855 configured as a differential amplifier and using feedback and input resistors of 1k ohms, so the differential gain was 1. The circuit appeared to be stable. The datasheet says a gain of 7V/V for stability. Does 7V/V also apply when configured as a differential amplifier?

Thank you for the help,

David

• Hi David,

like this?

david_opa855_3.TSC

When a minimum gain for stability is specified in a datasheet, this is always the noise gain. And the noise gain of this circuit would be 2V/V:

Kai

• Kai,

Yes, the circuit is as you show. Even though the circuit appears to be running stably, should we increase the gain to ensure that the noise gain is > or = to 7V/V?

Thanks, David

• Hi David,

this is an often asked question: Why is my circuit not showing stability issues, allthough I violate the minimum gain recommendation?

Because in this frequency range parasitic impedances of board and components can have a considerably impact on the phase margin. Assume these parasitic parallel capacitances of resistances, for instance. These capacitances can come from the resistor itself and/or from the solder pads sizes:

david_opa855_4.TSC

A phase margin of 43.6° can mean a stable performance. But if you alter a bit the parasitic capacitances you may end up with this:

And this circuit would no longer be stable.

But with a higher gain, the circuit would again have sufficient phase margin:

So, when using a too low gain, the risk of instability increases considerably. This must be kept in mind, when violating the minimum gain recommendation

Kai