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TLV333 vs. OPA333 (or other?) for low current sense application

Gerard Visser
Prodigy 165 points
Other Parts Discussed in Thread: TLV333, OPA333, INA333

I am looking for a precision opamp with low supply current for precisely sensing currents in the range of 0-10 uA on a -60 V rail. The opamp would be powered by shunting some current from this rail to ground and the supply current must be less than about 30 to 50 uA. (Because of limitations of the -60V source, and need to minimize power consumption.)

The sense resistor should be fairly low value, I am baselining to use 1 kOhm. So the opamp should have low offset voltage and low frequency noise. The TLV333/OPA333 seems pretty good. Probably can get precision to a few dozen nA with this.

I am thoroughly confused what the difference is between these two parts. Are they the same chip only different testing? It would be nice to have a summary of their differences (it would be even better if such a thing was mentioned in the datasheets).

Is there any other opamp (or other solution besides a traditional "high side current monitor" opamp circuit flipped over to a negative supply rail, that I should consider.

  • 0
    TI__Mastermind 25825 points

    Hello Gerard,

    Looking at low-power, RRI/O devices with low offset voltage it looks like the OPA333 and TLV333 are the best choices. Please keep in mind that the input bias currents (specifically the input offset current) will create an offset voltage based on the shunt resistor that you use. For example, an additional offset voltage of +/-0.14uV may be introduced due to a 1kohm shunt and typical offset current of +/-140pA (TLV333). The OPA333 has a maximum offset current specification of +/-400pA, which corresponds to +/-0.4uV.

    Concerning the differences between the parts there are not many as you noted. The typical performances are very similar (save CMRR and Aol), but the TLV333 has looser/fewer maximum specifications compared to the OPA333 which corresponds to less testing time...of course this corresponds to less cost.

    The only other devices that you may want to consider are instrumentation amplifiers because they have high input impedance. One instrumentation amplifier that you may want to look at is the INA333. It has low offset voltage and input bias current, but you have to take into consideration the Vcm vs. Vout behavior of instrumentation amplifiers. It is unlikely that current shunt monitors and difference amplifiers will work in this application due to input bias current magnitude with respect to the current range you're trying to measure.

    Hope this helps!

  • 0
    Prodigy 165 points
    hi Pete, Christopher,
    Thank you very much for your replies, I understand it more clearly now. OPA333 should be the way to go here, indeed I would choose the maximum performance; within reason the cost doesn't matter.
    Pete I think the positive input bias current (so +/-200 pA at most at 25 deg C) simply adds directly to the current being measured, so it gives +/-200 pA error. This is fine (I need to have roughly a few 10's of nA error at most). The offset voltage and LF noise (and thermoelectric voltages, etc.) will produce the dominant error of ~10 nA for ~10 uV on my 1 kOhm sense resistor. The negative input bias current just feeds to the low impedance output of the amp / transistor and doesn't contribute.
    As far as I can see, an instrumentation amp doesn't really add any value in the low-side current sense circuit. Unless I am really confused.
    Anyway, I will prototype things with the OPA333 and hopefully get good results. Thanks for your help, and in general thanks for this fine product!