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Weird behavior in current monitor circuit

Other Parts Discussed in Thread: OPA705, OPA704, LM317, INA138, INA210, INA168, OPA192, OPA251, OPA244, INA282, INA225, INA270, INA200

My application involves controlling fans to keep device temps within specs. I need to monitor high side current in this application, ie, the current powering the fans. Current span is 10mA to 500mA. Vout will feed directly ADC of CMOS uP.

I've been simulating this circuit below, as per snoa620, AN-32 Fet circuit applications, pg 12:

Previous inaccuracies made me switch op amp to OPA704 (which will be OPA705 in real circuit, but there is no model for 705).

Also used 2N3904 replacing JFET 2n3684, since I found no JFET model in TINA and no easy model for 2n3684 around. R1 and R3 have been set to resistor values that I already have. Relationship of 1/1000 was kept intact. R4 = 8k2 instead of 5k gives about 4.1V for Vout when Iload is 500mA, which is nice.


TINA circuit attached. high side.TSC

Power source for fans (VS2 above) is an LM317 with modified feedback network, so I can apply a wide input voltage range. 

I have compiled a sweep list for different VS2 values and found:

Vin Iload Vout V/A
12 0,591 4,891 8,274
10 0,493 4,080 8,282
8 0,394 3,268 8,293
6 0,296 2,456 8,309
4 0,197 3,930 19,944
2 0,099 0,828 8,407

For Vin (=VS2) = 4V and values around, circuit goes crazy. Vout is much higher than expected. It goes back to normal when Vin = 2V or lower.

Same thing happened for other sensitivities, ie, other values of R4, although at different values of Vin.

Why is this circuit behaving like this near Vin = 4V??? 

Side question1: why circuit overall sensitivity (V/A in above list) slightly increases when Iload decreases?? 

Side question2: is the only difference from this circuit shown to balanced circuit below the bias current compensation or is there any other reason for the 200ohm resistor at op amp V- below??

Thank you.

  • Hi,

    I suspect there can be several issues you are facing with the circuit. If you are convinced to use an opamp and build your own current monitor, the opamp applications engineers can help you with similation issues and real issues with the circuit. My question for you is have you considered using an actual current shunt monitor? Devices you might consider are INA138, INA168 or INA210 family. These INA's are designed to do exactly what you are trying to do by using the opamp and transistor in the feedback. If you need support for the INA products I will be your point of contact. If you will stay on the path of building the function from the opamps one of the other apps engineers will help you, alghough with the holidays I am not sure of their vacation schedule. If this issue cannot wait, please send me an email and I will go through the details of why your circuit may not be working as desired. My email is
  • Ed,

    Thank you. I've sent you an email.
  • Hi Thiago,

    Sorry for the long delay to my reply...just getting back into the swing following the Holidays.  I have several comments/concerns regarding your design...

    • I would like to point out to you that the OPA704/OPA705 is a 12V opamp, with ABS MAX of 13.2V for the power supply voltage.
    • The maximum for the Common Mode Input Range for the OPA70x is given as VS+0.3V This means that the maximum voltage that can be applied to the -IN terminal of the OPA70x will be 0.3V greater the power supply applied to the OPA70x.  For a maximum of 12V power supply this means your input voltage has a maximum of 12.3V, beyond which the OPA70x is not recommended. This is well below your intended operating range for Vin which I believe is 24V to 36V.
    • I was able to repeat the weird simulation results you described at VIN = 4V.  In your simulation you used a 15V power supply voltage for the OPA70x...when I reduced it to 12V, the circuit converged properly with VIN at 4V and gave the expected result.  FYI...when you had the 15V supply applied for whatever reason the circuit converged with the expecatation that that transistor was saturated...this is turn lead to a very large base current to the transistor and the simulator found this "alternate" operating point...I think this can be ignored, especially as I have already pointed out you cannot apply 15V to the OPA70x in the first place.

    If you want to stay on the opaamp, build your own route you will need a higher volatge opamp capable of handling the 36V inputs...I suspect you also will prefer a single supply solution which means you will need a rail-to-rail input opamp (or a wide input range opamp for the st possible dynamic range)...please consider:

    OPA192 (Preview status) contact me regarding sample availability




    Please reconsider a current shunt monitor for this function...several choices for the 36V range you described (in the email) come to mind...

    INA282 - Precision current shunt monitor up to 80V, analog voltage output, a variety of gains across the family

    INA225 - Precision current shunt monitor up to 36V, analog voltage output with pin selectable gain options

    INA270 - Low cost current shunt monitor up to 80V, analog voltage output

    INA200 - Low cost current shunt monitor up to 80V, analog output voltage, includes comparator output for overcurrent detection