Hello all,

I am trying to build a load transient board where the DUT has floating outputs. For that I'm using an NMOS which is driven by an op-amp (OPA27) and a 1Ohm sense resistor. The voltage across the resistor is fed back in the negative input terminal of the op-amp.
The circuit in this configuration is working. However I wanted to isolate the input signal and for that I used an 1:1 isolation transformer (T6437-DL ). And it appears that I can't use this transformer at 10 kHz, and if I increase the frequency I have a problem with the op-amp. I tried everything I could imagine of, so now I think I need your help. I have attached image I took during tests.

Thank you for your help,
dD

 

I don't know why, but I can't see your attachments.

 

I can open them, I try to attach them again here.

 

What is the difference between the two sets of pictures?
Post your circuit.

 

In the first the frequency of the pulse is 10kHz and in the second it is 150kHz. Here is the circuit

 

I think I would use an opto isolator instead of the transformer. Then you can run down to DC.
Maybe something like this:
http://my.mouser.com/ProductDetail/...=sGAEpiMZZMtd3yBnp8bAgMhsn5Iee23BHHcET6vmrgc=

 

The transformer appears to be saturating at the lower frequency.
I agree with ronv. An opto isolator would likely work better.

 

What I don't understand is why it is saturating, the transformer has a volt-time ratio of 134us and the pulse I am applying has a frequency of 10kHz and an amplitude of 1V.

I thought at first to use an optocoupler too, but during the simulations I saw that they would be too slow and I wanted a rise/fall time less than 1 us. I have also the linear optocoupler HCNR200 but I didn't manage to make it work.

 

I think the one I posted is less than a usec.
It's hard to get to the data sheet. But you can copy the part # from Mouser into the Avco search box and get it fairly easy.