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# AD8009 to drive a variable capacitive load

Hello,

I would like to use the AD8009 to drive a variable capacitive load over a SMA cable.

According to the datasheet, I need to  "insert a resistor in series between the op amp output and the capacitor." It is required to "to prevent instability and improve settling time when driving a capacitor"

My circuit can be summarized like that :

My goal:

for some reason, I need to apply a constant voltage at the point “A”.

My issue:

According to the capacitor variation, the voltage at the point “A” will not be constant (voltage divider effect).

My question:

What do you recommend to avoid or limit the voltage divider effect?

Thank you!

• edit: I need to apply a constant AC voltage at the point “A”.

• Hi,

I can see that the input frequency and capacitance can varying from your circuit summary. Does AC constant voltage mean constant Vrms or Vpeak at point A?

Voltage division in your case will depend on the impedance of the capacitor. You may vary the input frequency as the capacitance changes to ensure a constant Vpeak or Vrms at point A. For further reading about impedances, you may go to this link.

All the best,

Gerdan

• Thank you for answering me and for the link. AC constant means Vpeak at point A. Actually, you are right: the Ziso and Zcapa form a low-pass filter and I can work before the cutoff frequency to ensure a constant Vpeak at point A. But if I want to work above the cutoff frequency, I would like to know if there is a design pattern to limit the voltage divider effect.

• Actually, the effect of the voltage divider is limited when the capacitance variation is about delta_C = 20pF (difference between blue curve and green curve is lesser than 3dB @100MHz). So it should works. After clarifying my ideas, the answer is obvious...