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OPA192: Adjusting Vcm for current sense amp

Part Number: OPA192

Hello, 

      I am looking to use the OPAH014-SEP as a current sense amp as shown in the picture below. I am looking to use it to measure the current from power supplies (2V, 2.2V, and 3.7V). The preferred power supply rail I want to used for the Op Amp is 5V. However this causes an issue, as the Vcm is 5V - 3.5V = 1.5V. This leaves the Vcm out of range for any of the power supplies I want to measure currents with. How can I adjust the circuit so that I can adjust the Vcm, so that the Vcm is in the correct range while I am still using a 5V supply for the OPAH014-SEP?

  • Hey Quincy, 

    Are you using the OPA192 as in the title of the post or are you using the OPAH014? 

    Have you seen this circuit detailed as shown in the link below for the OPA192? The OPA192 is rail to rail input. 

    https://www.ti.com/lit/an/sboa310a/sboa310a.pdf?ts=1714772675056&ref_url=https%253A%252F%252Fwww.ti.com%252Ftool%252FCIRCUIT060005

    Best Regards, 

    Chris Featherstone

  • Hi Chris, I am using the OPAH014-SEP. When I tried posting with this part number, it wouldnt let me because it didnt show up in the list, so I just put OPA192, since I am referencing it from a design using the OPA192. 

    Quincy

  • Quincy,

    Here is a potential starting point, and some comments:

    1. Regarding your original circuit.  When you build a diff amp with discrete resistors you will get marginal CMRR.  The best way to get good performance on this kind of circuit is to use matched resistors.  I'm pretty sure you already know this and you are constrained by the space application, but I just mention this for completeness.  
    2. I assume that you do not have any other op amp options that are optimized for 5V and have better Vcm range.  Again, I understand that you have limited options with space applications.
    3. One possible approach is to add a simple divider at the shunt output.  The problem with this is that the divider will be loaded by the difference amp input.  To minimize this gain error I tried to keep the resistors in the divider 1 decade from the difference amp input.  The divider also reduces the circuits overall gain.
    4. Below is a simulation and circuit configuration.  Let me know if this works, or if you want to try a different approach.  

    I hope this helps, Art