modified 10 years ago

MCU input ADC pin with long wire

00:51:57

Noise on ADC input is rectified by ESD diodes and makes an offset voltage.
 
The 1uF filter cap also has an ESL (1nH shown) that forms a high impedance notch for a small range of RF noise to get through. 

If I use another cap ... say .1uF it will also have a notch, I can only hope they don't line up with each other, anyway that is one of the problems with long wires and ADC inputs.

published 10 years ago

hurz

10 years ago

Its in reality not that ugly with 10V at 5MHz zero impedance interference which then cause 600mA cutting current at lower ESD protection, but a unsymmetrical wiring is definitive problematic. Think about a differential line at least from sensor to ADC if ADC does not provide diff input. Or use two ADC as input. BTW placing big caps at input to ground lowers the measurement frequency dramatical. Its together with line impedance a LPF low pass filter. Keep it up!

flowDAQ

10 years ago

Made adjustment to show notch better and reduce 10v coupling signal to something more reasonable. 1k is sensor impedance. I did not even think of line impedance.

hurz

10 years ago

I think about this as solution. But you have to lift the sensor to a differential signal or tell us a little more about it, if you try this in real on a breadboard? This is only the principle and not a full implementation http://everycircuit.com/circuit/4514973869932544

flowDAQ

10 years ago

Someone on the Arduino forum got me thinking about this. They wanted a long wire (100 meter) from a sensor to the ADC, it sounded like a bad idea.

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