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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?
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
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:
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
