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THS4531: THS4531 as CT amplifier

Ian Benton
Prodigy 240 points
Part Number: THS4531
Other Parts Discussed in Thread: THS4631, OPA810, INA181, INA187, THS3470

Tool/software:

Would the THS4531 make a good transresistance amplifier for use with a low current current-transformer (i.e. with less than 10mA output).
I imagine the circuit would have resistors between In+ and Out- and between In- and Out+ and the CT across In+ and In-.

Am I correct in thinking it would deal with the DC offset problems in CT transresistance amplifiers due to the high gain at DC being equal to R divided by the resistance of the CT, as discussed in AD's application note

www.analog.com/.../transimpedance-amplifier-buffers-current-transformer.html

  • 0
    TI__Mastermind 24079 points

    Hello Ian,

    I can certainly help you think through this application.  I would like to confirm you are considering a fully-differential amplifier (FDA), such as the THS4531, which has two outputs as a differential pair.  Is this in line with your application needs?

    One key item for considering FDAs is the relatively low input impedance at VIN+ and VIN-, compared the the high input impedance seen on typical voltage feedback op-amps.  Some voltage-feedback op-amps are even described as high-input impedance, such as ones with a JFET input stage.  Our OPA810 and THS4631 are examples.

    FDAs do allow for shifting the DC common mode voltage to whatever the setting is for the VOCM (output common mode voltage) pin.  Examples are included in the device datasheets.

    Did you have a specific set of requirements or design considerations you wished to work through together?  I can make sure we avoid any common mistakes and select the correct part to solve your signal chain.

    Best,

    Alec

  • 0
    Prodigy 240 points

    Having done some measurements, I think that our friends at AD are being a bit over-careful. My CT has a DC resistance of 250Ω and the feedback resistor for the Transresistance amplifier is only 120Ω, so the DC gain will be about 0.5.

  • 0 in reply to Ian Benton
    TI__Mastermind 24079 points

    Hi Ian,

    That is good information to know, let me take another look at realizing the circuit.

    Are you able to share if you are looking to replicate the AD document or if you want to extend that topic to use a fully-differential output like on THS4531?

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    TI__Mastermind 24079 points

    Hello Ian,

    Have you made progress with your design?  If you have any feedback, results, or additional questions I would be glad to hear an update from you.

    Best,

    Alec

  • 0
    Prodigy 240 points

    No. I gave up. The problem with using a transresistance amplifier with a CT is that it needs to source the same amount of current as the CT. That can be tens of mA. As a CT is only limited by Voltage*time, it can output very high current for short periods, which the amp then has to match, and what happens if it can't? I went back to burden resistors, and am looking at the INA187 as an improvement on the INA181, because of its ability to withstand negative-going transients.

  • 0 in reply to Ian Benton
    TI__Mastermind 24079 points

    Hello Ian,

    It is frustrating to change gears and find a new approach, but I hope you find your new solution is effective.

    As a side-note, we do have a recent device which can handle large currents: THS3470.  It may be interesting to consider for any high current transient circuitry you encounter or design-around going forward.

    Best,

    Alec