Pull up Resistor and Amplifier

[MouradR]

Hi,

I am working on an ECG circuit.

It contains an instrumentation amplifier, two high pass filters, an amplifier and pull up resistor.

Initially, I had the pull up resistor after the amplifier and it worked.

I came today to test the circuit and it didn't work so I put the pull up resistor before the amplifier and it worked.

My question is why is it so unstable and which method works best (is there a theory I don't know about).

I don't want to keep changing from a pull up resistor before/after an amplifier.


Cheers,

 

[johnjoe]

With amplifier, do you mean your instr. amp.? Maybe post a schematic.

 

[MouradR]

Just a operational amplifier.
Sorry, I don't have the schematic on me at the moment.

 

[johnjoe]

Maybe you need some dc current path, but it would be really helpful to draw (also by hand) a schematic and specify the components.