Connecting outputs from an EMG circuit to the inputs of a SCC-68 Connector Block

[Jazlan]

Hello everyone. Currently i have just started to work on an electronics class project where I am constructing an EMG circuit to measure EMG signals from two forearm muscles while hand gripping a dynamometer, namely the flexor digitorum superficialis and the extensor carpi radialis longus muscles. The circuit im going to construct consists of an instumentation amplifier (INA 128), sallen key 2nd order high pass filter (20 Hz cut off frequency), sallen key 2nd order low pass filter (500Hz cut off frequency) and lastly a 4N25 optocoupler. The output wires of the EMG circuit will be screwed into the inputs on the SCC-68 connector block and the SCC-68 connector block is connected to a National Instruments PCI-6221 Low Cost M- Series Multifunction Data Aquisition device and computer running labview 8.5. I would like to ask:

1) Is it safe for me to directly connect the outputs of the EMG circuit (which is attached to a human) to the inputs of the SCC-68 connector block? There is a 4N25 optocoupler at the output of the EMG circuit in order to isolate between the EMG circuit and the Data Aquisition. If there is a safety issue im planning to transmit the EMG signals wirelessly using xbee tranceivers and an arduino microcontroller.

2) Im interested in finding the EMG power spectral density at different levels of hand grip strength. Is there any suitable hand dynamometer that can be used together with the SCC-68 connector block, PCI-6221 data aquisition card and labview 8.5? I did come a across the Vernier Hand Dynamometer but the Vernier Hand Dynamometer works together with its own DAQ, Vernier Sensor DAQ.

3) Do you guys have any suggestions on how I should implement this project?

Thanks! I would really appreciate any feedback.

[Coq_Rouge]

Safety is a very wide deffinition. It will depend on your teacher goal. Is the goal to incorporate some safety. Or should the system comply with IEC 60601-1 requirements. The first option may be easy. And the last option quite hard. Also safety must be evaluated for the whole system. An optocoupler may add some safety to the system. But this safety measure may be pointless, if say the EMG amplifier is powered by a very hazardous power supply. Your selection of optocoupler is also somewhat dodgy. This kind of optocouples are highly unlinear regarding the transfer function. And mostly used for digital signals. The HCNR200/201 used correct will be an far better option. Another circuit may be the ISO124 or ISO124 circuit. But these circuits are somewhat expensive. Using a xbee tranceivers and an arduino microcontroller. may be a good alternative. But a microcontroller ADC is 10 bit only. And in this case this will be somewhat flimsy. This can be solved by using an external ADC. 

Also what kind of samplerate do you plan to use. I prefer to use a samplerate at least 10 times higher than required. Then looking only at band width and Nyquist frequency. As an example: In your case with a 500 Hz band width. The sample frequency must be 1 KHz. But here I would have used 10 or 20 KHz. Then I would have applied all the needed digital signal processing. And perhaps in the end used an average technique to downsample the signal to 1 KHz sample rate. 

[Jazlan]

Thanks allot for your comments CoqRouge! I really appreciate your suggestions. For this project we are only required to incorporate safety features to prevent any danger to our classmates who are the test subjects, our teacher didnt make it a requirement for us to comply with IEC 60601-1 standards. I will used a single 9V battery to power the EMG amplifer which has 2 channels, one channel to measure EMG from the Extensor Carpi Radialis longus and the other channel for the Flexor Digitorum Superficialis. Im more worried about the power supply from the computer.

Thank you for pointing out the how select the sampling frequency. I have a better understanding now.

[Newton_de_Faria]

As for the hand dynamomemeter from Vernier, you will need a "cable adapter" - vernier has a analog protoboard adapter - http://www.vernier.com/probes/probes.html?bta-elv&template=standard.html. With that you can use the 5V supply from the DAQ board through you SCB-68 connector block) to power it and connect the output signal directly to an analog input channel. The dynamometer manual shows you what pins are what and what calibration coeficients to apply.

[Jazlan]

Thankyou NewtondeFaria! The +5V supply from the DAQ board is power the hand dynamometer right? The vernier hand dynamometer has a BTA connector at its end, so i should connect the this BTA connector from the hand dynamometer to the analog proto board connector. The output signal from the analog proto board connector can be screwed into the the screw terminals of the SCC-68 which route the input signal to the DAQ.. The next step would be to callibrate the hand dynamometer.. I found this article to be helpful too: http://www.vernier.com/til/1952.html

Please do correct me if i got it wrong.

[mc.man]

Jazlan,

if you don't mind, can you share with me how to create INA128 in Multisim

tq

best regards,

mc