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Original question:

INA333: microvolt bridge measurement

INA333: 1mV measurement possible?

Emanuel Soares
Intellectual 530 points
Part Number: INA333
Other Parts Discussed in Thread: TINA-TI, INA331, INA828

Hello,
I have a INA333 based project, which goal is to reach 1uV resolution in voltage measurement.
I plan to use a 0.002 Ohm shunt resistor at the down side of a half-bridge so that with a gain of G=500 a 1mA current may be measured.
I would like to know if it is possible with INA333, considering the noises and other parameters. I am not totally sure about what to consider in this calculations around noise and so on, so I sincerely request a help to show me how to move on this calculations as I didn't get all ideas from original question.
Below, an ideal scheme of the system I want to use:

  • 0
    Guru 294870 points

    The INA333 has a specified input offset voltage voltage of 25 µV. With the 2 mΩ shunt, this corresponds to a measurement error of 12.5 mA.

    How much of a voltage drop over the shunt can you afford?

  • 0 in reply to Clemens Ladisch
    Intellectual 530 points
    Clemens Ladisch said:
    How much of a voltage drop over the shunt can you afford?

    The resistor is 5W rated, so max current would be 50A. But the problem is that I need a precise current resolution, of at least 1mA. Is this 25 µV a offset meaning I can eliminate it with a calibration routine or is this an error?

  • 0
    TI__Guru 64840 points

    1mA current through 2mohm shunt means you are attempting to measure input voltage of 2uV, which is much lower than the INA333 maximum input offset voltage of ~+/-25uV.  Thus, I assume you plan to calibrate out the initial offset error to alleviate the point made by Clemens and you are only concerned here with achieving 1uV resolution and not absolute precision.

    Assuming dual +/-2.5V supply, this would allow you IG1 current range measurement up to +/-2.5A - see below.

    However, in the gain of 500 the total output noise is 1.58mV and thus 3 time higher than 500uV (1uV*500) output voltage resolution you try to achieve - see below.  

    Thus, in order to be able to make the measurement you would need to add a low-pass filter at the output as shown above.  Using 10k||1uF filter brings the total output RMS noise down to 192uV, which should be acceptable to obtain 500uV resolution.

    Running a small-signal transient analysis confirms stable operation of the circuit - see below.

  • 0 in reply to Marek Lis
    Intellectual 530 points

    Hi Marek, this is really clear!
    In fact this chip is not appropriate to me, as your simulation shows a limited bw. Can TINA-TI be set to show 3 decimal digits, cause it is not clear it can reach 1uV.

    Regards,
    Emanuel

  • 0 in reply to Emanuel Soares
    TI__Guru 64840 points

    Emanuel,

    The bandwidth is a function of gain and for INA333 in Gain of 500 is about 700Hz - see below.

    But until now you only mentioned required resolution and said nothing about the required speed. For higher bandwidth, you could use INA331 (80kHz in G=500) or INA828 (50kHz in G=500) - see below.