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Null detector; current sense 1nA-300mA; multiple sense IC

VT
Genius 3300 points
Other Parts Discussed in Thread: INA121, INA116, PGA280, INA111, INA110, TIPD135, REF200, TIPD101, TIPD107, ADS1248, THS4521

Problem:
1. I have to make a null detector with MCU.
2. Input voltage maximum is 30V dc.
3. Current to detect is 1nA to 300mA.
4. Galvanometer resistor selected is 100ohm. As this is standard in many analog null detector I have. And it also limits the current through power supply in case two opposite are zero & other two are open. In this case entire 30V will appear across 100 ohm. 
5. Current measured should be bidirectional.

6. I have attached the circuit.

7. I can use multiple IC with different ranges i.e. 1nA-500nA , 500nA - 1000nA , 1uA-500uA , 500uA-1000uA , 1mA-300mA.

 

 

3660.8156.ckt.pdf

  • 0
    TI__Expert 6930 points

    Thank you for your question.  Presently it is a major US holiday.  Please expect a delay with our review.  We will reply more thoroughly on Monday.

  • 0 in reply to Edward Mullins
    Genius 3300 points

    Update on it?

  • 0 in reply to VT
    TI__Genius 14825 points

    Vindhyachal,

    A traditional current shunt monitor from our portfolio will certainly not have the capability to measure nA.  We will barely have the ability to measure 300mA accurately as a 100ohm shunt resistor will add errors to the measurements.  Our bias currents are already in the uA range, which dwarf nA measurements.

    True instrumentation amplifiers have front ends that are designed to have very low leakage currents and would be more likely to be able to measure nA of current.  There is a discussion currently about a 1uA-150mA measurement circuit here: http://e2e.ti.com/support/amplifiers/precision_amplifiers/f/14/t/385110.aspx that you may find rather helpful.

    The main issue I see with this circuit is that there could be 30V across the shunt and while many of our current shunt monitors are designed to handle that kind of voltage, I am concerned that some instrumentation amplifiers may not.  I'm going to move this discussion over to the precision amplifiers team and have them take a look at your circuit and make a suggestion for one (or several) devices to handle your design target.


    Thank you,

    Jason Bridgmon

  • 0 in reply to Jason Bridgmon
    TI__Mastermind 25795 points

    Hello,

    1nA to 300mA is more than 8 decades of load current. Therefore I recommend evaluating the use of a programmable gain amplifier, or PGA. Please take a look at TIPD135. Even with +/-15V supplies, however, you will still need to use a smaller value shunt resistor. Also, you will require a separate solution for the nA current range because the PGA280/1 have bias currents of 2nA max. Therefore you should consider an instrumentation amplifier with pA's or fA's of input bias current, such as the INA110, INA111, INA121, or INA116. However, please be aware of the input common-mode range of these devices. While you clearly state the maximum differential voltage is 30V, I'm unsure of the commmon-mode voltage since the values of R1-R4 are not given in the attached schematic. Assuming they're equal, your common-mode voltage would be mid-supply which will yield the largest output swing for the instrumentation amplifier.

    I hope this helps.

  • 0 in reply to Pete%20Semig
    Genius 3300 points
    @Pete Semig:

    1. I am reading amplifier suggested by you & will check their suitability.

    To clarify some points:
    1. Galvo resistor is 100ohm & dc voltage is 30V.
    But I can use combination of resistor like 30+40+30 ohms or even 60+40+.010ohm.
    I can make different combinations & take drop across those individual resistors.


    2. R1-R4 can vary, user can change them according to need.
    All I have to do is when bridge is balanced by 0.1uV i.e 1nA flows across 100ohm resistor I should be able to detect.
  • 0 in reply to Pete%20Semig
    Genius 3300 points

    @Pete Semig:

    As I had said earlier,  R1-R4 can vary user can give them any value. Actually this circuit is used for finding unknown resistance.

    All I have to do is when bridge is unbalanced by 0.1uV i.e 1nA flows across 100ohm resistor I should be able to detect.

    Based on your input & reading I have selected INA121 & attached is circuit configuration .

    1. Plz check the ckt, i have reduced the bridge voltage to +15V dc

    2. Two worries, ina121 has high INPUT OFFSET VOLTAGE +-500+-500/G compaed to input voltage I am giveing when current is 1nA across 100 ohm i.e 0.1uV so what will happen when offset noise is larger than signal to be measured.

    3. Am I voilating CMR range here.

    ckt.pdf

  • 0 in reply to Pete%20Semig
    Genius 3300 points
    Any update on it @Pete
  • 0 in reply to VT
    TI__Mastermind 25795 points
    Hello,
    We have recently upgraded our E2E forums and I have been having an issue with reply to posts until now. I am currently on international travel and will get back with you as soon as I can. Sorry for the delays.
    Thanks,
    Pete
  • 0 in reply to VT
    TI__Mastermind 25795 points

    Hello,

    Thank you for the additional information and your patience as we worked through some technical difficulties.

    I would also use a single 15V supply to power the INA. That would place the common-mode voltage of your bridge (assuming R1=R2=R3=R4) at mid-supply which would maximize your output swing.

    Your right...the offset voltage of any INA you select will have to be calibrated out because your trying to measure only 100nV, which is much less offset than any of our instrumentation amplifiers. You should actually perform a 2-point calibration to also calibrate out any gain error due to resistor tolerances.

    Since your ultimate goal is to determine the value of a resistor, you may want to consider using a precision current source (e.g. REF200). Since you know the current supplied by the device you could measure the voltage and ultimately obtain the resistance.

    Alternately, perhaps you could use a V-to-I converter (e.g. TIPD101 or TIPD107). You may have to also calibrate these circuits depending on the accuracy you require. This could probably be done by switching in known precision resistors and measuring the output.

    I hope this helps.

  • 0 in reply to Pete%20Semig
    Genius 3300 points

    Hi Pete,

    Sorry for late reply. Its more than 1 month.

    I have attached a circuit. Since I have to measure the voltage using ADS1248.

    Now ADS1248 is connected to second board & it have single supply. (AVDD = DVDD = 3.3V)

    Now output of INA121 in attached circuit is single ended. So I had used THS4521 to convert it to differential signal.

    1. Now I have selected gain = 1 for THS4521.

    2. Ground of both boards is common.

    3. I have left Vocm of THS4521 open . Will it put the signal to mid level of supply voltage to that its in Vocm range of ADS1248

    or I have to additionally connect 1.67V to THS4521.

    1033.ckt.pdf