DC on output

[udip]

BTW: I'm not using the LT1115, I'm using the OPA627,
and the feedback capacitor is 10k, and not 2k5.

Also the servo is not LT it's OPA134......

Maybe that got something to do with it.....

 

[sajti]

sajti, I think you are right..

I checked on scope and I can see a triangular wave on the output
of servo with input, but only with low frequency.

That's servo oscillation?

If i put a non-inverting servo to the - input of opamp, should I
use some sort of bias to the + input in order the eleminate the
feedback to ground capacitor?


Thanks...

If You use non inverting DC servo, just connect the input transformers secondary to the ground. It's not necessary to use any bias. Use DC servo, is to avoid the feedback capacitor.

sajti

 

[udip]

and one more.....

Should the servo output AC??

It doesn't make sense because the pole of it is within 0.5 - 2 hz??

 

[peranders]

Yes, frequencies up to 1 kHz or so slips through.

Why don't you play with my simulation files? LTSpice is very educational when it comes to understand a DC servo and it's very accurate because of the low frequencies.

Download the freeware LTSpice from www.linear.com and start play.

 

[udip]

Hi peranders,

Thanks,

I'v been playing with the simulations....and it's ok...
but if you noticed in my preamp the servo is in a completely different configuration, as it's output goes to ground via resistor
and changing the bias on the + input of the main opamp (I think)

In the simulation there is no indication of AC output the comes
back to the main opamp.....

So I guess this it's not the common use of servo in the
IN and NI configurations..

 

[udip]

Anybody knows if its ok that an opamp like the OPA627-BP
will have a dc offset between 0.01 - 2v on output with
no servo or capacitors??
(the feedback resistoris 10k, and gain changing from 2 to 50)

 

[udip]

peranders, Do you know the difference between those two
configurations of the servo - the one like in your files vs the
one that is shown on the schematics i posted....?

 

[udip]

Another one:

I rechecked the offset and now it's between 0 - 3mv with no
servo, with servo its 0.

Do I need the servo??

 

[udip]

I'm sorry about all those questions I'm asking.....

I have 2 more:

1)Should I check for offset with inputs shorted?

2)The servo output is a triangular only when input is distorted.
Is this ok?



Thanks again for your help....