Tool/software:
Dear everyone:
When we recently used the BQ79731 chip, we found that when the VF pin voltage is 0V(measured with a 4-bit semi-multimeter), the software will read 36mV voltage. However, the GPIO pin does not have this problem when used as a voltage acquisition. May I ask what causes this? In addition, when the voltage of VF pin is close to 0V, can the sampling accuracy reach 0.1mv
Hello Huajian,
How are you taking this measurement? Are you taking it directly at the pin?
VF1 and VF2 should be from 0.3 to 5V in reference to AVSS. It has an accuracy of +/- 3mV, so it being at 0V is within its specification.
Are all the filtering components placed close to the pin? Are there any long traces or weak GND pours that would effect the accuracy?
Also please refer to the datasheet for accuracy specifications. All the relative accuracies for VF are located on page 10 on the BQ79731 datasheet.
Best,
Quentin
Hi Quentin：
I measured the voltage at the pin directly with a multimeter. I also found two posts on the forum about VF pins having bias voltage issues. And all the filtering components placed close to the pin. The ground plane is a large area of copper coating in the second layer, so the pressure difference on the ground should be relatively small. 0~1V voltage VF pin sampling accuracy is not accurate, do you have any suggestions?
Hello Quentin:
How to perform digital filtering when GPIO pin is used as ADC? And what's the difference between “Absolute voltage measurement on GPIO” and “Ratio metric input measurement on GPIO” ？
Hello Quentin:
I found that GPIO does not have a digital filter. The register with the filter configured for GPIO is also not found in the datasheet. Is it right?
Hello Huajin,
Yes you are correct, I believe I was referring to the incorrect datasheet.
Here it clearly states that GP ADC path does not have a DLPF.
However there is a DLPF on the VF input. How is the LPF_VF currently configured on your setup? 
Best,
Quentin
