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How accurate can ADS8881 sense differential voltage?

zhen zhang2
Intellectual 370 points
Other Parts Discussed in Thread: ADS8881, REF102, OPA277

Hi Everyone,

I am planning to use ADS8881 to perform current sensing. (Sensing the diff voltage cross the precision resistor).

The idea to replace the R in the below circuit with a programmable resistor to generate a programmable bias current (1uA to 1mA)

However, due to the accuracy of the programmable resistor, I have no way of getting a error less than 3% .

My idea to have a precision resistor before the load and sense the current with a ADC (ADS8881) and adjust the programmable resistor with that feedback..   Let us say the precision resistor is 1kOhm. 

The voltage cross the resistor will be 1mV to 1V, (bias current 1uA to 1mA).

Will  be realistic to expect ADS8881 be able to sense as low as 1mV voltage difference?

Thanks a lot for the help,

Eddy

  • 0
    TI__Intellectual 1205 points
    Hello, Eddie!

    Be careful with this circuit. While the ADS8881 can easily resolve the differential voltages that you describe, keep in mind that the REF102 has a 10V output, at up to 10mA. If your Load has a small resistance, you could place a current sense resistor on the load-end of "R". In that case, the absolute voltage on the load-end of the resistor "R" will be close to GND, so the ADS8881 will be fine. HOWEVER, if the Load has a high resistance or is disconnected, the absolute voltage on the current sense resistor will go high. The ADS8881 needs to have its analog input voltages between GND (0V) and your reference voltage.

    How high the voltage will go depends on the rest of the circuit. The OPA277 will be limited by its input Common Mode Voltage (Vcm) in this configuration, and will be able to increase to 2V below its positive supply voltage. The REF102 will then try to produce a voltage that is 10V above that, but will be limited by its supply voltage.

    With that information in mind, the ADS8881 can use a Reference Voltage (Vref) of up to 5V, which would allow the analog inputs to each be anywhere between 0V and 5V. Your resolution would be 2 * Vref / 2^n = 38uV (where Vref = 5V and n = 18 bits). So you will have plenty of resolution. The challenge is limiting the common mode voltage on the inputs of the ADS8881. Placing the sense resistor at the load end of "R" will give you a common mode voltage that is about the same as the voltage on the load. To limit the voltage on the load, keep the supply to the OPA277 to no greater than Vref + 2V and -Vref - 2V, or +/- 7V. Then Pin 4 of the REF102 will be at Vref at its maximum condition, making pin6 at 15V.

    Let us know if there are more questions.