I you want to have a try be my guest. Then you have the hardware ready I can send you a test program in Labview 8.2 or higher. The software is working 100%.
Here is a test program. It will measure skin conductance, and give some insight in the method. This is not the full application. But now is the time to warm up the solder iron. I will post a how to use document first in the coming week
The test is in 8.0
Edit: It is a small bug in the program. Then I put the whole thing in a LLB the path to the configuration file get messed up. The application will work, but the configuration file will not be read or written. I will not fix it now, but in later versions. But you are welcome to modify
Edit Friday, June 26, 2009. The bug is now fixed. Made some minor changes This program is now moved to my posting 26.jun.2009
Message was edited by: Coqrouge
You are using transimpedance amplifier to sense the current flow and the input is directly connected to M electrode.
This circuit could be overdriven by 60 (50) Hz surrounding noise.
TI uses DC input current cancellation source or one can use a capacitor in series with the M electrode.
Capacitor value could be in a range such that its time constant with R14 would give 8.33 ms assuring suppression of 60 Hz and elimination of all DC currents.
Can you explain the function of the R electrode?
I have included a PDF file hope it explain your question. If I not have mentioned it, the nice thing about this system is that much of the 50/60Hz from the mains is removed. And also that the system only measure skin conductance related to the M electrode. Since we are using Phase-sensitive detection. The same techniques that is used in Lock-in amplifiers. We do not need assuring suppression of 60 Hz and elimination of all DC currents. Beside any element causing phase is not wanted since it will cause measurement error. Phase-sensitive detection is done in the software
I was just wondering if anybody has managed to implement the circuit yet. I would like to hear if it is some problems like getting components etc. Some of the components are perhaps to exotic. If you have problems just try to match the components characteristic as good as possible. I think a LT1057/LT1058 can be used in all places
This diagram explains functions of the electrodes. Phase detectors rely on linear transfer function. On the schematic the amplifier U3A my go into non-linear region when pushed against its rails by a DC or 50/60 Hz leakage from mains. Maybe that is assumed to never be the case.
I have study the system even under surgical procedures, streamed output to disk at 20Khz. Besides diathermy that knocks out the system as long it is active. I have not observed the problems you are talking about. And mains power frequency is not a problem, even under surgical procedures. As long as constant voltage regulator are able to maintain a stable signal, noise from the mains is not a problem. So nothing is assumed But then the constant voltage regulator fails, the whole system is influenced. If you do not believe what I am saying you are free to build your own system and test it. I actually encourage everybody to do it. I will of course give you all the help you need.
This looks like a great bioengineering project especially for the BE student. I have enjoyed following the threads. As a physician I thought I would note my search with MerckMedicus, with access to hundreds of medical textbooks. Unfortunately I did not find any correlation between skin conductance and a well defined pathology. Never the less, I am intriged by the interesting discussion and will be following these threads on a daily basis.
My understanding is that skin conductance is widely used for psychophysiological research...it picks up changes in sweat gland activity that is in response to stress, arousal, discomfort, etc. and is often used in conjunction with other physiological parameters like heart rate, EMG, etc. As you say, I'm not sure that it is associated with any disorder (other than chronic fibbing!) and I've never heard it used in a diagnostic application. Maybe it would be a way to quantify disorders related to over/under-active sweat glands...
Although an important function of the skin is to protect the organism it covers. For example, by keeping bacteria, parasites, and noxious chemicals out and keeping vital fluids in. it also has a role in thermoregulatory activities. The thermoregulatory contribution is produced by dilation of blood vessels in the skin and increased sweating. Both of which result in decreased skin and body temperature. Blood vessel dilation increases blood flow in surface areas to enhance cooling. The increased sweat on the skin surface produces cooling through evaporation. Humans are remarkable for their relatively hairless skin and high density of innervated thermoregulatory sweat glands. The density of sweat glands in human skin varies from around 50-200 glands/cm2 on the trunk and limbs. to around 200-400 glands/ cm2 on the face and the dorsal sides of the hands and feet, with 600 to perhaps more than 2000 glands/cm2 on the palms of the hands and soles of the feet An interesting aspect of sweating is that it is not merely themaregulatory (coaling). This fact forms the basis for many behavioral studies. Sweating or sweat gland activity, is reflected as changes in skin potential (SP) and skin conductance (SC) in a variety of situations, including those that are emotionally arousing. For example, eccrine glands of the palms and fingers of the hand respond only weakly at certain levels of heat and strongly to psychological and sensory stimuli. You may have noticed wet or "clammy" palms in situations that were fear or anxiety provoking, but that were otherwise not very warm. The sweating to psychological stimuli has sometimes been termed "arousal" sweating, and some researchers believe it has adaptive value. It has suggested that the sweating of palms and soles may be an adaptive response that persisted over the course of evolution, because it aids in grasping objects. Darrow (1933, 1936) suggested that palmar sweating served to increase grip strength and tactile acuity, and drew the analogy of a laborer spitting on his hands before grasping a pickaxe or shovel. It has been hypothesized that arousal sweating in any part of the body toughens the skin and protects it from mechanical injury. Observation by Edelberg (1961). Who found that palmar skin was difficult to cut during profuse sweating. When sweating is blocked. The skin is more susceptible to mechanical injury. Hence, these interpretations of arousal sweating suggest that it has survival value for the organism, as do other SNS responses in emergency situations.