EDA Sensor - Need help for a project

Connection, ECG, EMG, EDA, etc.
Posts: 1
Joined: Sat Nov 18, 2017 1:36 pm

EDA Sensor - Need help for a project

Postby Eiffel » Wed Dec 13, 2017 1:08 pm

Hello there,
I'm here 'cause i'm taking a part in a project using the Bitalino's EDA sensor.
The purpose of this project is the mesurement of the skin's EDA and show it on an LCD screen.
For that matter, i'm using an Arduino Leonardo ETH, linked with the sensor. The main Bitalino device is not used.

I've 3 questions :
1) In a multiple topics found in this site ( http://forum.bitalino.com/viewtopic.php?f=12&t=100 ; http://forum.bitalino.com/viewtopic.php?t=98 ), i've found that the "Ref" pin should linked with the Arduino, using a couple of resistors (100kOhm each). I'm asking why should i have to use these 2 resistors ? Why the resistors' value have to be 100kOhm ?

2) I'm using (without knowing why) the electrical assembly i've told before. Using the formula on the datasheet, i've found the EDA value = 1/(1-(eda_out/2^n)). What is the average mesure that an human should have ? What is the standard ? In the "Fig. 2. Typical raw EDA data (acquired with BITalino)" in the EDA's sensor datasheet (link here http://bitalino.com/datasheets/EDA_Sensor_Datasheet.pdf ), we can see the mean value around 6 uS, is that correct ?

3) What are the uncertainty of the measurement ? What is the percentage/proportion of validity or reliability ? (I don't know how to say that in english, I hope to be understood :roll: )

A pic of the circuit is available at the attachements section.

Thanks for your help,
The circuit i'm using
IMG_1343.JPG (160.76 KiB) Viewed 145 times

Posts: 2
Joined: Wed Jun 28, 2017 8:59 am

Re: EDA Sensor - Need help for a project

Postby fsilva » Tue Dec 19, 2017 12:33 pm

Hello Eiffel,

Regarding your questions:
1) Those resistors are used to create a voltage level equal to Vcc/2 (middle point between Vcc and Gnd) on Vref pin. They don't need to be exactly 100 kOhm; they may have any value lower than this as long as both resistors are equal (but not too low to prevent excessive current across the voltage divider). If the voltage in Vref is not Vcc/2, there will be a voltage offset in the sensor output which changes the sensor transfer function from the one specified in the datasheet.

2) The EDA signal varies greatly between different people and also with the same person over time. The sensor range is between 0 and 25 uS, so any value within this range can be considered "normal".

3) The measurement error is defined by the precision of the components used (particularly the resistors). We can say that this error is below 1%, but the biomedical relevance on the EDA signal is not in its absolute value, but in its changing over time (for example, as a response to a certain stimulus).

Best regards,
Filipe Silva

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