MCU ADC

MCU, Firmware, etc.
AAR
Posts: 6
Joined: Fri Nov 14, 2014 12:29 pm

MCU ADC

Postby AAR » Wed Dec 24, 2014 10:51 am

Hello,

What is the voltage reference used for the ADC when using each of the sensors when converting them to digital data?

Thank you,
AAR

BITalino
Site Admin
Posts: 567
Joined: Tue Aug 27, 2013 3:47 pm

Re: MCU ADC

Postby BITalino » Wed Dec 24, 2014 4:26 pm

Hi,

The ADC should be using Vcc = 3.3V as a reference, however some of the sensors are differential and are pre-conditioned for an output voltage centred around Vcc/2 = 1.65V. For information on how to convert the ADC codes or voltage output of each sensor to the correct physical units, please refer to:
http://forum.bitalino.com/viewtopic.php?f=12&t=128

Best regards,
The BITalino Team

AAR
Posts: 6
Joined: Fri Nov 14, 2014 12:29 pm

Re: MCU ADC

Postby AAR » Wed Dec 24, 2014 5:52 pm

Hello,

But if the reference is 3.3V that means that the lowest unit of measurement is 3.3/1023 = around 3.2 mV. From what I see from that post is that the expected output should be -1.5 mV to 1,5 mV for the ECG for example. How can the variations in the signal be detected?

BITalino
Site Admin
Posts: 567
Joined: Tue Aug 27, 2013 3:47 pm

Re: MCU ADC

Postby BITalino » Wed Dec 24, 2014 7:17 pm

Hi,

Effectively each ADC step corresponds to 3.2mV, but the sensors have onboard pre-amplification and signal conditioning circuitry . The EMG and ECG sensors are differential and bipolar, in which the reference voltage is the Vcc/2 = 1.65V midpoint voltage.

The amplification gain depends on the sensor; the EMG has 1000, whereas the ECG has 1100. The sensor always outputs a signal between 0-3.3V, but you have to take into account the gain and midpoint voltage for the conversion.

Best regards,
The BITalino Team

AAR
Posts: 6
Joined: Fri Nov 14, 2014 12:29 pm

Re: MCU ADC

Postby AAR » Fri Jan 09, 2015 10:28 am

So now the ADC reference should be 1.65V ? Wouldn't that only give a range of 0 to 1.65? While the measured range is -1.5 to 1.5? So wouldn't we need a voltage range -1.65 to 1,65 V to get the full range of values?

BITalino
Site Admin
Posts: 567
Joined: Tue Aug 27, 2013 3:47 pm

Re: MCU ADC

Postby BITalino » Fri Jan 09, 2015 11:00 am

Hi,

The 1.65V are only the reference voltage for the OpAmp on the sensor. The ADC reference is always 3.3V.

Note also that the measurement range depends on the gain, which for the EMG is 1000 and for the ECG is 1100.

Best regards,
The BITalino Team


Return to “Microcontroller”