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THS4120: THS4120CDGN

Richard Arnold1
Intellectual 310 points
Part Number: THS4120
Other Parts Discussed in Thread: TINA-TI, THS4551

I am running an half bridge to a resistive load.

I am currently supplying 30VDC and 1Amp.

I using the THS4120CDGN to measure the voltage drop across R13 and R12 to determine the current going through the resistive load (forward and backwards).

The output of the amplifier is always 2V.  I have replaced R19/R20 with a 3.9K and R21/R71 with 62Ohm

 

Is there something I am missing?  I have rad the datasheet a few times and also other documentation and can't seem to find what I am doing wrong.

Any suggestions?

  • 0
    Guru 140200 points

    Hi Richard,

    you have chosen a very high gain. This can result in issues in combination with the input offset voltage of THS4120: 8mV x 62.5 = 0.5V.

    I would run a TINA-TI simulation. This is the fastest way to find out what's going on and to solve the problems.

    Kai

  • 0
    Guru 140200 points

    Hi Richard,

    I see another issue: If for a brief period both Q3 and Q4 are turned-off, the input voltage becomes so low that you violate the common mode input voltage range of THS4120. See the VICR specification in the datasheet.

    Kai

  • 0 in reply to kai klaas69
    TI__Genius 17306 points

    Hello Richard,

    Could you provide an update when you have had the time to consider Kai's recommendations?  Violating the common mode input voltage range would certainly cause undesired output affects.  If you require assistance with performing a TINA-TI SPICE simulation, we are available to help.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    Intellectual 310 points

    I downloaded the software.  I am trying to figure how to use the software.  Does the VICR talk about the VOCM voltage?  My VOCM is 1.63V.  I see that the VICR min is .65V.  

    If you can help get me up to speed on the software that would be great.  

  • 0 in reply to Richard Arnold1
    TI__Genius 17306 points

    Hello Richard,

    I can help you out; I will see what I can write up & mock up tomorrow morning; I can send it over and we can discuss.  

    Best,

    Alec

  • 0 in reply to Richard Arnold1
    TI__Genius 17306 points

    Hello Richard,

    I am building up a SPICE file to share for TINA-TI and can write up step-by-step blurbs to help with use of the file.  If you are okay with this approach, I would like to send this over today; we can always hop on a call or exchange emails if you require live assistance or have more lengthy questions.

    Please expect an update on this thread.  Thanks!

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    Intellectual 310 points

    Alec,

    I am ok with this approach.  I tend to work 9AM-4PM EST so I can work in a call.

  • 0 in reply to Richard Arnold1
    TI__Genius 17306 points

    Hello Richard,

    I have a slightly different update than promised: I think it may be good to try using a different amplifier, such as the THS4551.  The THS4120 is a CMOS amplifier and as Kai mentioned, the input common mode range will be violated by whichever branch/resistor is not being 'sensed'.  The minimum of 0.65V will be surpassed and the device may operate as you observed, having an output set at 2V.  I am going to continue my investigation to see what outputs I can observe with 16A into the amplifier, as your setup describes.

    However, I mention the THS4551 as the device has a negative rail input and would be able to handle the input swinging close to GND.  If this solution interests you, I recommend giving the THS4551 a look for this application.  There are PSPICE & TINA-TI models for the THS4551.

    Please allow me some time to do the further investigation steps into your application with the THS4120; feel free to use or simulate the THS4551 as a replacement or exploration of the difference in operation/architecture.  

    Best,

    Alec