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THS3491: THS3491+VCA824 circuit problem

Part Number: THS3491
Other Parts Discussed in Thread: VCA824

Hi,

I would like to ask you to give me your advice about THS3491-VCA824 circuit.

The problem is the gain difference between design value and measured value.

I designed the THS3491 gain as around x4.91 but measured gaind is around x4.5.

(THS3491 gain = 1 + 560/143 = 4.91)

That is to say, input is 0.22Vpp and output is around 0.99Vpp.

And I tried to set register 1K ohm between THS3491 and VCA824.

As a result, THS3491 output became 0.96Vpp and after 1K ohm the voltage became 0.65Vpp.

I do not understand why THS3491 behaves as this.

As I attached my schematics and block diagram, will you check it please?

THS3491-VCA824 circuit.pdf

Thank you very much for your support.

Best Regards,

Takumi