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INA199: Unstable reading

Marvn Arcangel
Genius 10745 points
Part Number: INA199
Other Parts Discussed in Thread: OPA325

Hi Team,

Can you please help check the circuit below?

The circuit uses INA199A1DCKR to monitor the current charging for a battery. But the issue is that the raw ADC reading was very noisy and when the battery is at >3.7V, the ADC reads fluctuates between 0 - 200.

Please let me know if you need more information.

Thanks in advance.

Marvin

  • 0
    TI__Intellectual 1235 points

    Hi Marvin,

    Can you help answer following questions:

    1. What is the expected current measurement range?

    2. What is the expected common-mode voltage on the INA input pins? What is the full voltage value the battery is being charged to?

    3. What ADC is being used in this circuit? What is the LSB of the ADC? Can you clarify the units or what is meant by reading of '0-200'? Can you also provide the sampling rate?

    It is also possible that the INA is struggling to drive the ADC, please see our app note on driving SAR ADCs: https://www.ti.com/lit/an/sboa443/sboa443.pdf 

    Thanks,

    Cierra

  • 0 in reply to Cierra Cowley
    Prodigy 50 points

    Hi Cowley.

    Pls refer bellow information for your question:

    1. Expect current measurement range: 0-500mA

    2. Common mode voltage on the INA input voltage: 10mV@500mA. Full battery voltage: 4.2V

    3. ADC used in this curcuit is internal ADC, single end input, 10bit. Vref = internal MCU = 4.2V.

    unit 0-200, this is raw ADC value I have read from output of INA when Battery voltage >3.7V, but it is not stable, it fluctuate from 0-200.

    Thanks.

  • 0 in reply to Anh Hoang
    TI__Intellectual 1235 points

    Hi Anh,

    Are you able to provide a scope screenshot when probing at the input pins of the INA when the battery is >3.7V? I would like to verify whether the fluctuations are caused by the INA or occur before entering the INA.

    What is the purpose of  R21 and C38? Are these meant to filter high frequency noise in addition to the input filter? You may need to decrease the values of R21 and C38 in order to drive the ADC effectively. 

    Please see Current Shunt Amplifier Considerations for Driving SAR ADCs to calculate the R/C values needed for the charge bucket filter to drive a SAR ADC. 

    Thanks,

    Cierra

  • 0
    Expert 1235 points

    Hi Marvin
    As your circuit above, i think your issue is place sensing resistor to pack- of battery although INA199 support both low and high sense.

    Will your load on circuit get power from battery?

    Battery structure (protection circuit on battery) will affect to GND.

    I tried this topo on last time, and saw that need move to power rails to sense ( although current sense will be sum of charge and load current, but it will be easy to calibration with firmware)
    Thanks.

  • 0 in reply to Alone173
    Guru 140200 points

    Hi,

    the INA199 is very precise but not very fast. According to figure 14 of datasheet its settling time is about 100µs. This may conflict with the aquisition time of ADC, if the output of INA199 is not able to fully settle within the aquisition time window of ADC. And the charge bucket filter at the output INA199 furtherly slows down the settling by introducing a "5 x tau" delay of 500µs.

    So, if this is the error source, increasing the aquisition time of ADC can help or speeding up the settling time by adding a fast ADC driver with suited charge bucket filter (if needed). The OPA325 gives an excellent ADC driver.

    Kai