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CC1312R: ADC input voltage problem

Part Number: CC1312R

Hi

I used the following circuit to connect to the ADC input pin.

1. R13:10M R12:2.43M

When BATTERY_DET1 input voltage is 2.1V, the ADC reading is only 1.7V.

When BATTERY_DET1 input voltage is 0.9V, the ADC reading is  1.1V.

And when the PIR (connect to another ADC pin) is triggered, ADC readings float from 0.7 to 1.4V
(Measuring with an oscilloscope, the voltage is stable 0.9V)
2.  Change the resistance R13:100K R12:20K
ALL ADC readings are correct and stable.
Is there a way to make the ADC read correctly and stable with R13:10M R12:2.43M? Because there is still power consumption.

  • The picture is missing from your post.

    As you can see from the datasheet the ADC has a given input impedance meaning that if you use a resistor ladder with high impedance, the input impedance of the ADC will have an impact. Consider using a transistor that turns the resistor ladder off when not measuring the ADC. Decoupling cap could also help some.
  • I tried to add a 0.1uF capacitor. It seems that the ADC reading value is very stable.

    The reading value is very similar to the value obtained by my calculation of the voltage dividing resistor. The PIR trigger does not affect it.

    The calculated value is 2.46V, and the ADC reading is also 2.46V, but when measured with a voltmeter, the actual voltage is 2V, and the ADC reading becomes  2V.

    My guess is that the voltmeter internal resistance causes this result. Is my guess correct?

  • The full figure is missing in the original post meaning that I don't know which voltage you use on the resistor ladder.

    Have you checked the spec regarding the impedance on the voltmeter you are using?
  • I uploaded the figure again.

    V_BAT is connected to the battery, the range is 5V~13V. 

    BATTERY_DET is connected to the ADC pin of the MCU.

    At present, a 0.1uF capacitor is connected in parallel with R12. Is this the recommended value?

    What is the effect of the capacitor value being too large or too small?

    I will confirm the internal resistance of the voltmeter.

  • The size of a decoupling cap will be dependent on the size of the resistors used. Basically the cap has to be able to deliver the current charging the input of the ADC when this is sampling that the resistor ladder is not capable of delivering.

    When you calculated BATTERY_DET = 2.46 V, what was the value of V_bat?

    As long as you are measuring a constant value the size of the cap could be as large as you need.
  • When I calculated BATTERY_DET = 2.46 V, the value of V_bat was 12.72.
    I confirmed that the SPEC Input Impedance of the meter only shows >10M ohm.
    I used another voltmeter to measure the resistance of this voltmeter = 10.9 M ohm.
    Use this resistance value to recalculate BATTERY_DET1=2.1V.
    Very close to the measured value of the previous meter:2V.
    I understand the size of the capacitor you said.
    Thank you very much for your reply, very helpful to me.