CN0510/AD5941 Loss of accuracy after amplifying disturbance current

Question

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=4.8Ohm waveform 
 
 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: 
 
 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

MLambe · Accepted Answer

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.)

Electronic Design Services - PartnerZone

Products Mentioned

CN0510/AD5941 Loss of accuracy after amplifying disturbance current