ADC_SE9 pulls circuit high to 3.3v

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ADC_SE9 pulls circuit high to 3.3v

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robinsterling
Contributor I

Hi,

I am working on a project in which I am using a MKL03Z8VFG4 to periodically measure the output from a transistor amplifier. However, when initializing the ADC inputs, ADC_SE8 is always high. I also use ADC_SE9 which is at ground and works fine, as you would expect. They are both configured exactly the same.


The pin remains high whether the amplifier is connected or left floating, so it is not the external circuit.

I have checked that pull enable is off and pull select is low.

Would anyone be able to let me know why this might be happening?

Thanks

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mjbcswitzerland
Specialist V

Robin

What is the impedence of the circuit at the input to ADC_SE8? Don't forget that there will always be a small input bias current. In fact you may prefer to enable the pull-down at the input to help bias the input at 0V when nothing is connected.

I have seen the same effect when using the ADC inputs ued to scan a touch screen:

1. When the touch screen is pressed there is a low impendence path and the input is equal to this voltage (changes from 0V..3V depending on the touch location)

2. When the touch screen is not pressed the low impedance patch is removed and the input is 'floating'

3. If the touch screen was previously pressed at a value < 1.75V the ADC input would "float" to 0V

4. If the touch screen was previously pressed at a value > 1.75V the ADC input would "float" to 3.3V

5. When a weak pull-up or pull-down is connected the ADC input "floats" to either 0V or 3.3V, depending on the pull-up/down.

This suggests that there is a feedback path between the ADC's "floating" input and its last output value (note that it is a successive approximation design if I remember correctly and so will be tuning an internal reference point, which may well be feeding back through a weak input bias path).

To check this you could  temporarily ground the input to see whether it then changes to its second stable state ('0' rather than '1') and remain there when removed. If you confirm this you will probably need to lower the input source impedance.

Since you write that it is an amplifier it should have very low impedance already, which would point to a different problem.

Regards

Mark

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