Offset voltage for an charge amplifier

Hello Thomas,

I think you refer to

https://www.analog.com/en/resources/reference-designs/circuits-from-the-lab/CN0350.html

U1C is necesseray, because the output for Vref on AD7091R shall not be be loaded by the voltage divider (see fig. 14 in the datasheet of AD7091R).

U1B is necessary, if Href should be loaded (e.g. if R8 should be placed in the eval-board).

best regards

Achim

Hello Achim,

thank you very much for your answer, it was very helpful!

One more thing though I´m not sure about yet: Why isnt it possible to get HREF inbetween R1 and R2? The output for Vref on AD7091R would not be effected. Wouldn´t U1C provide enough current, even if HREF is loaded? 

I suppose the voltage divider is significant again. Probably the impendance of R1 would effect HREF I guess.

Greetings,

Thomas

Hello Achim,

thank you very much for your answer, it was very helpful!

One more thing though I´m not sure about yet: Why isnt it possible to get HREF inbetween R1 and R2? The output for Vref on AD7091R would not be effected. Wouldn´t U1C provide enough current, even if HREF is loaded? 

I suppose the voltage divider is significant again. Probably the impendance of R1 would effect HREF I guess.

Greetings,

Thomas

Hello Thomas,

If you load a voltage divider, its output voltage will shift.To avoid this U1B is used as buffer.

best regards

Achim

Hi, ThomasRWTH .

I agree with Achim 's insights. One thing also to take note of is HREF comes from VREF. Thus, you need to isolate this section to noise. HREF is for signal conditioning of the AC signal. While VREF is the point of reference during the ACQ and CNV phase in the ADC. Charge redistribution on the binary weighted capacitors in the ADC causes current to be drawn from or sunk by the reference. With all these details, you can perceive that U1B and U1C is important for a more stable and optimize system.

Regards,
Jo