CN0510/AD5941 Loss of accuracy after amplifying disturbance current

Hi, I'm using the CN0510 to measure a single 18650 lithium battery.

For some purpose, I need to amplify the sense current to 2A maximum from the Darlington pair of CN0510.

The schematic of CN0510 is here:

PDF

After I replace the emitter resister which is named R21 in the schematic, and the value is 62Ohm, into 4.8Ohm,

the current flow out from the battery increase from 23.835mA(mean) and 11.062mA(peak-peak) to 269.86mA(mean) and 133.81mA(peak-peak).

This is the same as circuit simulation.

The actual waveform is like below, and the channel configuration is in the picture,

R21=61Ohm waveform

R21=61Ohm Vdc=3500 Vpp=300

R21=4.8Ohm waveform

R21=4.8Ohm Vdc=3500 Vpp=300

It seems good but after I open the terminal such as Putty, the result from CN0510 gets strange.

When I use 62Ohm as R21, the impedance of measurement is like the lithium battery impedance theory as below:

R21=61Ohm EIS

But after the 4.8Ohm resistor installed, the result is distortion:

I'm wondering if the current is too large so the input stage of AD5941 gets over the range.

So I measured the AIN2 pin and AIN3 pin after AD8694, the voltage is like below:

CH1: AIN2

CH2: AIN3

Is this value over the range of the AD5941 input stage?

Or there is something I missed, so I can get the right value of EIS of lithium battery?

Thanks for your apply, hope you have a nice day Slight smile

  • +1
    •  Analog Employees 
    on Nov 17, 2020 5:57 PM 11 days ago

    Hi,

    Yes, your analysis is correct. Increasing the current through the battery is increasing the differential voltage across the cell and also across RCAL. The gain stage implemented by the AD8694 is increasing the current beyond the limits of the AD5941. You can reduce the gain of the AD8694 circuit by changing R13, R14, R28 and R29.

  • 0
    •  Analog Employees 
    on Nov 17, 2020 5:57 PM 11 days ago

    Hi,

    Yes, your analysis is correct. Increasing the current through the battery is increasing the differential voltage across the cell and also across RCAL. The gain stage implemented by the AD8694 is increasing the current beyond the limits of the AD5941. You can reduce the gain of the AD8694 circuit by changing R13, R14, R28 and R29.

  • Hi, thanks for your reply.

    I want to know is there any information about the voltage threshold of AIN2 and AIN3?

    Is the datasheet mentions it?

    I try to increase the current into 2A by replacing R21 with 0.6Ohm, and the AIN2 and AIN3 pin voltage seem like below:

    As you can see, the instrument amplifier amplifies the battery voltage (or RCAL voltage when the device is in the calibrating process).

    Those two signals had already been distorted (or saturate?), so I know the measurement result is not acceptable.

    But in the previous post, the waveform of AIN2 and AIN3 haven't been distorted, they still are sine waves, but the measurement results are not correct, too.

    I need to know the voltage threshold to design the ratio of the instrument amplifier to match my large disturbance current.

    Is there any information? 

    P.s: I had already check out the HSTIA chapter of the datasheet, but I can't understand is there any relationship between High-Speed TIA and ADC?

  • 0
    •  Analog Employees 
    on Nov 20, 2020 8:41 AM 9 days ago in reply to HabonRoof

    Hi,

    Yes, this information can be found in the specification table of the AD5941 datasheet. The max voltage is +-0.9V. The DC level is set by the LPDAC at 1.2V so the max voltage on AIN2/3 should be 1.2+0.9 and minimum should be 1.2V -0.9.

    The high speed TIA is not used for the battery impedance measurement.