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Help with Skin Resistance Output

pkg9991

Jun 7, 2013
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sias.JPG

Hello friends,

I'm using the attached circuit for getting the galvanic skin resistance. The bridge circuit is working fine and I'm able to obtain resistance output as required (points Out_1 and Out_2 - varying between 0.8M - 1.5M ohm) but the op-amp circuit is not functioning. I'm not getting the required voltage levels (not shows any voltage levels at all at pin 14 of IC) which I then need to interface to ATmega16 microcontroller by internal ADC of uC to get this reading on LCD. Is something wrong with the circuit. Some help would be really appreciated guys. I'm stuck.

Values for various components are:
R1 = R2 = R3 = 1M ohm
R4 = R5 = R6 = R7 = 1K ohm
IC used is LM324
 

Harald Kapp

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1) there is an error in your schematic: Out1 is connected to the junction R2/R3. It shouldn't.
2) The bridge resistances are 1 Meg Ohm. The inpu resistance of the amplifier is 2kOhm. Your bridge is toatally overloaded. If you measure out1-out2 with the amplifier attached, you should see ~0V. If you want to use this circuit, use much higher resistances for teh amplifier.
I recommend using an instrumentation amplifier as a much bettter circuit for this measurement.
 

pkg9991

Jun 7, 2013
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sias.JPG

Thanks for the reply.
I'm uploading the corrected schematic. This is what I'm exactly using to make the circuit.

What resistance value do you suggest I should use, to make this circuit work?
As I'm a newbie, so I might confuse myself now switching to instrumentation amplifier :(
 

Harald Kapp

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Use >= 1Meg. Better yet, use an instrumentation amplifier (see link in my previous post). Use an integrtaed instrumentation amplifier to minimize your headaches :D.
 

CDRIVE

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Just a suggestion but it would make things easier for everyone, including Harald, if you populate your schematics with values. ;)

Chris
 

KrisBlueNZ

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I don't see why you need such a complicated circuit. A bridge, feeding a differential amplifier...
All you need is a source of current and a buffer or gain stage. Just one resistor, one set of contacts, and one op-amp. (A couple of protection diodes would be a good idea too.)

A simple circuit will not give a linear relationship between skin resistance and output voltage, but neither will your bridge circuit. I would suggest measuring skin conductivity, instead of resistance. In this case you can get a fairly linear relationship between skin conductivity and output voltage if a low voltage range is used across the current shunt resistor, with gain being provided by the op-amp.

The circuit for this is so simple it's scarcely worth drawing up, but...

attachment.php


Current from the positive supply rail flows through the skin and causes a voltage drop across RS. With the value given, and a supply voltage of 10V, a skin resistance of 10 kilohms gives about 1V across RS (actually it's 0.90909V; if you want better linearity, reduce RS and RG in proportion). The op-amp multiplies this voltage by a factor calculated as RF / RG, which in this case is 5, to produce an output voltage proportional to the skin current, which is nearly proportional to skin conductivity (see previous parenthetical comment).

The output voltage therefore is proportional to skin current, which is roughly proportional to skin conductivity, the reciprocal of skin resistance. With no skin touching the contacts, the output will be roughly 0V.

The two diodes are there to protect the op-amp from electrostatic discharge. Extremely unlikely to be a problem with such a low value of shunt resistor; I guess I'm just being thorough.

Any device intended for direct connection to the body must be insulated to the applicable safety standards. I recommend using a fully battery-powered circuit, that cannot be connected to a person while the charger is plugged into it. There may be other legal requirements; it's your obligation, not mine, to determine them and ensure your device complies with them.
 

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CDRIVE

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Kris, I think he used a bridge input to cope with all the noise that the human body hosts. His circuit operates differentially and provides a degree of common mode rejection.

Chris
 

KrisBlueNZ

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I don't think it will provide any better common-mode rejection than mine. It doesn't use a differential input, even though there is a differential amplifier there - the inverting input of the differential amplifier is just biased at half the supply voltage, via R2 and R3. So it doesn't have any common-mode rejection.

I think my circuit will probably perform better because there is a more direct connection between the body and the supply rail, so there will be less noise voltage between them, assuming that the circuit is fully isolated (which it needs to be, for safety reasons). The direct connection will force the circuit's supply voltage to more closely follow the common-mode signal coming from the skin being measured. One would need to test it to be sure, though.
 

Harald Kapp

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I think I can agree with Kris. The bridge isn't really operated in a bridge fashion. The second leg of the bridge only serves to bias the OpAmps other input. Generally reducing the circuit's resistance, as Kris did, will also reduce the sensitivity against noise. A little bit of filtering wouldn't hurt either.
 

KrisBlueNZ

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Agreed, Harald. Perhaps the best approach would be to reduce the shunt resistance and increase the op-amp gain, then slash the bandwidth by adding a capacitor across RF and increasing RF and RG. This would make the op-amp's input offset voltage more critical, but the bandwidth unimportant. He could use a Microchip MCP616 - 150 uV input offset and very low power consumption.

To the OP. I just noticed that you want to drive the ADC on a micro from the output. I can give you more specifics if you answer these questions: 1. What is the supply voltage you'll be using for the op-amp? 2. What is the ADC voltage range? 3. Do you already have an LM324 in the design or can I specify a different device?
 

CDRIVE

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Listen mates I beg to .. to.. uh ... agree! A total misspeak on my part. Not my first; won't be my last. Let me know if we ever have a contest on member misspeaks. I'll work extra hard to win it! :p

Chris
 

KrisBlueNZ

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No worries Chris. We all make mistakes, and we hate it when it happens :)
 

CDRIVE

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No worries Chris. We all make mistakes, and we hate it when it happens :)

Somehow my eyes saw pads in both legs of the bridge, which as Harald rightly pointed out, is not operated as a bridge at all. So double whammy for me! :eek:
I wish I could say that I posted that after a happy hour. :D

Chris
 
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