Help with DC Servo for Current Feedback Opamp

[klewis]

Since the non-invert resistor has a much smaller value on a CF amp vs VF amp, I thought I might reduce the resistor values in the servo by an order of magnitude in the inverting servo. I've made a run with this configuration and it seems to correct the offset. I will try to build this configuration tonight and see what happens.

[klewis]

here is the plot for the prior schematic

[klewis]

Well, that didn't work> I got +4.3v DC offset when i fired it up. Tried several resistor values in place of r22, though r20 was set at 1k.

[klewis]

any other thoughts? help...

[pacificblue]

The more I think about it the more I come to the conclusion that a DC servo won't work with a current feedback amp. The DC servo gives a voltage feedback, not a current feedback. You will have to inject a current into the non-inverting input to achieve an offset compensation.

I. e. connect a potentiometer (trimmer) between + and -. To the wiper connect a small capacitor to ground and a resistor to the non-inverting input. With the potentiometer you set the voltage to which the capacitor will be charged. The capacitor discharges through the resistor and amplifier's input stage to ground, which leads to the compensation current. Adjust with the potentiometer.

[analog_sa]

Originally posted by pacificblue
The more I think about it the more I come to the conclusion that a DC servo won't work with a current feedback amp.

Seems to work plenty well with AD815 judging by the number of servoed designs...

[klewis]

Originally posted by analog_sa



Seems to work plenty well with AD815 judging by the number of servoed designs...

Analog,

can you point me to a one?

thanks,

Ken

[klewis]

Originally posted by pacificblue
The more I think about it the more I come to the conclusion that a DC servo won't work with a current feedback amp. The DC servo gives a voltage feedback, not a current feedback. You will have to inject a current into the non-inverting input to achieve an offset compensation.

I. e. connect a potentiometer (trimmer) between + and -. To the wiper connect a small capacitor to ground and a resistor to the non-inverting input. With the potentiometer you set the voltage to which the capacitor will be charged. The capacitor discharges through the resistor and amplifier's input stage to ground, which leads to the compensation current. Adjust with the potentiometer.

Pacificblue,

Thanks for thinking about this. I got a servo to work, the values for the resistors are quite different from servos for vf amps. But the resistor value changes move the Fc way up to around 1Khz. Which then boost the lower frequencies - not good. So, I'm currently simulating a third stage in the servo to bring the voltages back up... it's sort of working. I'll build one next weekend.

Ken