Hello everyone,

I'm trying to design an Howland pump which should drive an inductive load (1H, 50ohm) with a variable current (-100 to 100 mA). I have simulated the circuit using high CMRR op amps (~80dB) and to decouple the input and the output I have placed a buffer in the feedback path (see schematic). I'm having troubles in selecting the correct opamp for the feedback (as long as I use an ideal op amp I get the right current, while when I switch to a another amp I start seeing probelms).

Thanks for the help
Vinnie

 

You need a high power op amp that can supply 100mA.
Standard op amps usually have a maximum of about 20mA.
You could add a current booster circuit to the output of a standard op amp, if you prefer.

 

You need a high power op amp that can supply 100mA.
Standard op amps usually have a maximum of about 20mA.
You could add a current booster circuit to the output of a standard op amp, if you prefer.

Thanks for the quick answer. Well, the current is provided by U6 which can give 500 mA. U1 doesn't seem to output a lot of current because it's just buffering the voltage.

 

Post your .asc simulation file.

 

Post your .asc simulation file.

I attached the .asc file. Apparently when I don't use an ideal op amp the voltage at the inverting input doesn't follow the voltage generated at the non inverting input. I also checked the current that the buffer provides and it is in the order of the uA.

 

You have the power supply polarities reversed on U1.
It doesn't like that.

Also C1 and C2 do nothing.

 

You have the power supply polarities reversed on U1.
It doesn't like that.

Also C1 and C2 do nothing.

Oh man you right!! Now it's working smoothly! C1 was supposed to be in parallel to the negative feedback resistor to filter out the noise

Thanks again