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# LMH6551: Differential input voltage

Part Number: LMH6551

Hi,

I would like to use a fully differential amplifier because I want to interface with a sigma-delta ADC.

I need to know if there is a limit to the differential input voltage.

The voltage supply that I have is +-5V.

Can I have Vid=(Vin+)-(Vin-)=5-(-5)=10V ?

Thank you

Bertoncello Filippo

• Hello Bertoncello,

Limit as in damaging the device or for the performance of the amplifier? When you talk about a differential input voltage do you mean driving inverting & non-inverting input pins of the device? Or rather just the input preceding the gain setting resistors RG?

Best,

Hasan Babiker

• Hello Hasan,

I mean the voltage between driving inverting & non-inverting input pins of the device.

I would like to know the difference voltage limit for the performance of the amplifier.

Thank you,

Filippo Bertoncello

• Hello Filippo,

FDA's aren't designed to be driven directly at the inverting and non-inverting pins, so inputting any voltage will give an unexpected response at the output. During normal operation, the FDA is designed to bring the voltages of the inverting & non-inverting pins to the same potential.

Can you clarify what you are trying to achieve & why you are driving the FDA inputs directly? A schematic or a simulation file would help.

Best,

Hasan Babiker

• Hello Hasan,

I don't want to give the voltage directly to the inverting and non-inverting pins of the FDA.

The circuit that I want to use is a classic FDA circuit with differential inputs.

In my case, the input voltage could be a sinusoid with Vpp=12V or a DC voltage of 6V.

the datasheet of LMH6551 indicates the Common-mode input voltage but I don't find anything about differential input voltage. Before buying this FDA I would like to know if there is a differential voltage limit.

Best,

Filippo Bertoncello

• Hello Filippo,

Thanks for the clarification, your concern here seems to be in the case that you are increasing the differential input voltage further after the output has already been saturated.

For this particular device the differential input voltage isn't a concern, however you will still need to be wary of the common-mode input voltage & input current limitations. So in regards to your example, a differential input with 12Vpp is still within the range of the device & you should be able to operate normally. If you meant 12Vpp single-ended... or in other words you have a 24Vpp differential sine wave input then this would be a concern, as you would be right along the edge of the abs max common mode limitations. See below:

The output will saturate at 3.9V according to the typical rating of the datasheet & so the IN- pin will be very close to the positive supply rail and hitting the abs max limit damaging the device.

Best,

Hasan Babiker

• Hello Hasan,

Thank you very much!

Best,

Filippo Bertoncello