ADC Impedance matching

lowpowermcu · ‎2012-11-12

Posted on November 12, 2012 at 10:27

Hi all,

I have a pressure sensor whose the impedance is in the range [1400W 3000W].

I would understand and know if the ADC impedance in stm32f1xx devices matches my sensor impedance or not?

MCU LÃ¼fter

frankmeyer9 · ‎2012-11-12

Posted on November 12, 2012 at 10:50

There are good explanations of the working principles of such sensors, like this:

http://www.pcb.com/techsupport/tech_pres.php

, and others.

Basically, it's not a good idea to connect the (analogue) output of a pressure sensor directly to an ADC input. Use an instrumentation amplifier (like the AD620) close to the sensor.

Or, select a sensor with integrated amplifier electronics.

zzdz2 · ‎2012-11-12

Posted on November 12, 2012 at 15:24

ADC input impedance depends on sampling frequency, it's in datasheet of your device.

I have it in STM32F103 datasheet, section 5.3.18 12-bit ADC characteristics

From table 47:

ts = 0.11us -> 0.4 kOhm max

ts = 3.96us -> 50 kOhm max

lowpowermcu · ‎2012-11-12

Posted on November 12, 2012 at 16:35

Hi knik,

Thank you for your reply.

If I am at sampling time = 3.96us (50 kOhm max), how can I know if I can connect the pressure sensor whose the impedance is in the range [1400W 3000W].

PS: the sensor is mpx2102 and powered @ 3.3V

MCU Lüfter

zzdz2 · ‎2012-11-12

Posted on November 12, 2012 at 17:57

It would mean sensor impedance is in the range 1400 - 3000 ohm, it should be ok to connect directly to ADC input. You can connect it in series with some resistor, e.g. 10k to protect the inputs, 3k+10k is still below 50k max ADC input.