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ADC0_DP3 drawing more current than ADC1_SE5a

Question asked by Alexander Farley on Feb 22, 2017
Latest reply on Feb 23, 2017 by Robin_Shen

I'm using a FRDM-K22F with two pins configured as ADC inputs using Processor Expert. I've stuck with the default settings (longest conversion time, low-power not checked, high-speed not checked, single-ended mode, sharing not checked).

 

I'm testing a 5V-output sensor through a (50%) resistor divider which should give a voltage swing between 0-2.5V at the ADC input. While ADC1_SE5a seems fine (readings agree with a multimeter, etc) the ADC0_DP3 input seems to be drawing a significant amount of current, despite being configured in the same way as ADC1_SE5a. 

 

When the sensor is outputting 4.7V, the resistor divider gives about 2.3V through the divider if it's not connected to any ADC inputs. If I wire it to ADC1_SE5a, I still get 2.3V when sampling. If I wire the 2.3V node to ADC0_DP3, it gets pulled down to .9V. 

 

It appears that the resistor divider I'm using creates a sensor output impedance which is too high for the ADC0_DP3 input impedance. Why is ADC1_SE5a not having the same issues?

 

ADC0_DP3 gives reasonable readings using a low-impedance source (power rails or gnd) so it's not just a conversion/scaling issue. If ADC1 was sampling from more than 1 channel this could be caused by the inrush current when the channel switches but since it's only connected to 1 channel, this probably isn't the explanation. 

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