TIDM-BIOSIGNMONITOR: query regarding reference design

Gaurav Gusain

Hello I want to make a GSR circuit and i have take reference from TI design and have found a circuit diagram shown below

But i don't get this circuit properly so I designed a basic circuit simply by using ohm's Law :

In this circuit, two probes will be connected to the body and I have assumed a body of 1 kilo ohm now.

please tell me that will this circuit work or it is feasible to work with this circuit?

over 5 years ago

0 Wei.Jetim Zhao over 5 years ago

TI__Genius 13215 points

Hi,

I will assign our team member to check with your problem and reply you soon.

0 JD Crutchfield over 5 years ago

TI__Mastermind 26420 points

Hey Gaurav,

Based on the TIDM-BIOSIGNMONITOR User's guide, section 3.1.2, the human skin resistance can very between 20 M ohm to 50 ohms. This circuit was designed to be able to measure this with good resolution and keep the output between the MSP430's ADC input range.

I'm not sure which circuit would be best, so I would recommend trying both but I believe the reference design circuit should be robust.

Thanks,
JD

0 Prashant Dandriyal over 5 years ago in reply to JD Crutchfield

Intellectual 790 points

Please help me by explaining the role of the part"TP-025-SMT" as can be seen in the circuit diagram shown above.

0 JD Crutchfield over 5 years ago in reply to Prashant Dandriyal

TI__Mastermind 26420 points

Hey Prashant,

TP-025 should mean "Test Point #25". Basically, it's just a test point on the board that can be probed to debugging purposes. It's not an actually component.

Thanks,
JD

0 HOANG TRUONG over 5 years ago in reply to JD Crutchfield

Prodigy 45 points

Hi JD,

What is the equation to convert from the ADC read to the GSR value in micro-Siemens?

Thank you.

Hoang

0 Prashant Dandriyal over 5 years ago in reply to HOANG TRUONG

Intellectual 790 points

Even I am stuck on that... I was thinking of calibrating the ADC output voltage readings with a reference table for Human Skin Hydration values and finally making a look up table out of it. No results till now.

0 JD Crutchfield over 5 years ago in reply to Prashant Dandriyal

TI__Mastermind 26420 points

Hey Hoang,

Unfortunantly, I don't know what the equation to convert to micro-siemens is. Seems like we would need to know the injected voltage, which the TI design said should be <500mV but I didn't see it specified when scanning through.

Prashant,
I think your approach is the most practical.

Regards,
JD

0 HOANG TRUONG over 5 years ago in reply to JD Crutchfield

Prodigy 45 points

Hi JD & Prashant,

I solved this problem, found the correct equation to calculate GSR.

Ignore C34, this is a non-inverting op-amp and V1 = V3 * (1+R10/(R_body+R9)) where V3 is voltage at pin 3 of the opamp (0.487 V theoretically), V1 is output voltage at pin 1.

We can convert ADC read at GSR_ANALOG_OUTPUT to V1 and find R_body respectively. Skin conductance in Siemens is 1/R_body.

I think the reference design here is incorrect because R10 is too large (1M). V1 will be always at max and ADC read will be saturated.

I changed it to a 100k-resistor instead; and validated that my modified circuit works well with the measurement of differently fixed resistors and skin conductance on different body parts.

-- Hoang

0 JD Crutchfield over 5 years ago in reply to HOANG TRUONG

TI__Mastermind 26420 points

Hey Hoang,

Thanks for sharing your findings!!

Thanks,
JD

0 Prashant Dandriyal over 5 years ago in reply to JD Crutchfield

Intellectual 790 points

thank You

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TIDM-BIOSIGNMONITOR: query regarding reference design