AC constant curretn source

Matthias Ikeda

Other Parts Discussed in Thread: OPA132, OPA561, BUF634, INA103

Hello,

I want to design a AC current source of current variable up to 30mA rms.
The frequencyrange of the current source should be from 1Hz to 100KHz. The load is mounted in a nitrogen-flow-cryostat an therfore will change it's resistance with temperature between 20 and 100Ohms.

The AC input signal originates from a lockin amplifiers Oscillator output, is referenced ro ground and can be chosen between 0 and 4Vrms. the outputcurrent will be sourced to ground. features of this current source should be a very low excitation of higher harmonics, low noise, very low offset.

I thought about a howland current source. my questions are: which opamp would be the most adequat choice for my requirements? Is there even a more convinient way of realising such acurrent source?

Thanks.

over 13 years ago

0 Bruce Trump over 13 years ago

Guru 21975 points

Matthias,

This appears to be a one-of-a-kind science experiment, not a production design. Is that correct?

A Howland current source is probably the best approach.

It may suffice to use an OPA132 op amp. It is available in DIP package which would facilitate prototype construction techniques. Its output current may be sufficient, though its production testing would not assure this. Typical output current is over 30mA if you can keep it cool enough. You could use the minimum power supply voltage required to drive the highest load resistance to minimize power dissipation.

If you need a production-worthy design, I would recommend using an amplifier with a higher output current capability. One possibility would be to use the BUF634 buffer inside the feedback loop of a precision op amp such as the OPA132. Another possibility is to use a power op amp such as the OPA561. The offset voltage of this op amp is not so low. You have not provided specific information on accuracy or offset requirement so I'm not sure if this op amp would be suitable.

I hope these ideas are helpful. You are welcome to provide more details and we may be able to help refine these ideas.

Regards, Bruce.

0 Matthias Ikeda over 13 years ago in reply to Bruce Trump

Prodigy 50 points

Hello,

Thanks a lot for your information. At the moment, you are right, the current source will be used for a science experiment. Maybe in future, the board i'm planing could be used as a production design as well. The chip used for the current source doesn't necessarily have to be available in DIP package.

Anyway, for the moment i'll try to build the current source using the OP132 op amp.

I have an other question concerning INA103 Instrumentation amplifier. Could the output signal be phaseshifted (and how much) in comparison to a 30mVpp 100kHz input signal for G=1 or 100.

Thanks again for your effort,

matthias.

0 Bruce Trump over 13 years ago in reply to Matthias Ikeda

Guru 21975 points

Matthias,

It would not be good to try to alter the phase response of the INA103. It's not entirely clear what you are attempting to do but this should probably be accomplished in a separate filter circuit of some type, following the INA103.

If you have further questions on this topic, it would be best to start a new thread with more detail.

Regards, Bruce.