I'm using ADC on my STM32F407 to read the signals coming from a piezoelectric sensor. I have written my code in such a way that if input crosses a certain threshold, it starts sampling and storing it in a buffer for 10ms. It seems to work fine when I directly connect a piezoelectric sensor to GND and ADC input pin of the microcontroller. But when I bring in a probe of an oscilloscope in order to check the input for ADC(the input goes to both the oscilloscope as well as to the microcontroller), the reading on the microcontroller does not cross the threshold.But the signal on the oscilloscope shows that this threshold is easily crossed. What's the reason for this?

 

hi 1995,
Most likely cause is loading of the piezo signal by the scope probe and scope input capacitance.
Effecting the amplitude and duration of the pulse.
E

 

hi 1995,
Most likely cause is loading of the piezo signal by the scope probe and scope input capacitance.
Effecting the amplitude and duration of the pulse.
E

Thanks for the response. Can you elaborate on it and suggest any way in which I can get rid of that?

 

What kind of probe are you using? What is the capacitance at the tip?

 

What kind of probe are you using? What is the capacitance at the tip?

I'm using a one which looks like this - https://www.google.co.in/search?q=o...AUICigB&biw=1242&bih=602#imgrc=uOO056f38rNYQM:

I measured the capacitance with respect to ground and it is about 1.27nF.

 

That is extremely high capacitance. Even a low-cost probe should be no more than 20 pF at the tip.

If it is switchable 1:1 or 10:1, are you using it in the 10:1 position? Switchable probes should always be used in the attenuating position except in rare circumstances.

A good understanding of probes is essential to successful use of an oscilloscope. Here are two helpful links from Tektronix. Other major scope manufacturers publish similar information.

http://web.mit.edu/6.101/www/reference/ABCprobes_s.pdf
https://www.tek.com/learning/probes-tutorial

 

That is extremely high capacitance. Even a low-cost probe should be no more than 20 pF at the tip.

If it is switchable 1:1 or 10:1, are you using it in the 10:1 position? Switchable probes should always be used in the attenuating position except in rare circumstances.

A good understanding of probes is essential to successful use of an oscilloscope. Here are two helpful links from Tektronix. Other major scope manufacturers publish similar information.

http://web.mit.edu/6.101/www/reference/ABCprobes_s.pdf
https://www.tek.com/learning/probes-tutorial

There's no such option in this probe. It is not a switchable one.

 

If it isn't switchable, is it a 10:1 attenuator probe? If it is, then there should be a specification for capacitance at the tip, and your measurement must be wrong, unless you are including the capacitance of the sensor. I am concerned only with the probe capacitance.

 

If it isn't switchable, is it a 10:1 attenuator probe? If it is, then there should be a specification for capacitance at the tip, and your measurement must be wrong, unless you are including the capacitance of the sensor. I am concerned only with the probe capacitance.

hmm..it is not a 10:1 attenuator probe. It says 1X on it.

 

I think I solved it by trial and error. I switched the oscilloscope coupling from DC to AC and it works.

 

Not unusual for a 1X probe to have 100 pf of loading.

https://www.google.com/url?sa=t&rct...probes_s.pdf&usg=AOvVaw3jqTqjxTe5xhU8dQaIF70U

Regards, Dana.