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# AMC1211-Q1: Why is the Maximum Vp-Vn of AMC1211-Q1 only 2.88V even with 5V VDD2 ?

Part Number: AMC1211-Q1

I see from the datasheet of this isolated opamp that the VCMout has only the value of 1.44V.

Does that means I can only get the maximum Vp-Vn of 1.44Vx2=2.88V?

In addition, the figure31 below also shows this phenomenon.

• Hi Jankel,

figure 20 of datasheet sheds some light on the issue:

Kai

• Hi Kai,

but still the question.

Why can't it output 5V or nearly 5V of Vp-Vn even with a VDD2 of 5V.

What's more, you can see from the datasheet, this figure 20 is under the test condition of 3.3V VDD2 rather than 5V. This figure has no data under the VDD2 of 5V.

• Hi Kai,

Another queation:

What's the meaning of Fail-Safe Output?

Does it mean when fail-safe output is active, the output(Vp-Vn) would be negative?

• Hi Jankel,

the AMC1211 produces differential outputs which swing arround VCMout = 1.44V. As VCMout is nearly constant for VDD2 = 3...5.5V it makes no sense at all to produce output signals which are bigger then 2 x VCMout. This would ruin the symmetry of the differential output signals.

Kai
• Hi Kai,

Do you mean that although the opamp can output 5V, but the linearity is not good above 2.88V, so outputing 5V is not recommended although we can?

• Hi Jankel,

The output of the AMC1211 is differential and Kai is pointing out that the negative output of the device VOUTN cannot be driven below GND.

VOUTP swings from 1.44V to 2.88V, and VOUTN swings from 0V to 1.44V.

Kai,

Thank you for your support as always!
• Hi Alex,

Could you give me the structure of the output stage inside IC?

Kai just said the output will be not so linear if exceed 2.88V, so I think it can output maybe 4V although it's not so linear to the input.

Or the ic totally can't output 4V maybe because the structure of the output stage inside IC, like the voltage drop of the drain-source of the mosfet causes the limitation of the output voltage?

• Hi Jankel,