Help! EMG Circuit

Thread Starter

Ocelot

Joined Jul 2, 2009
4
Hi everyone,

I am trying to design a EMG circuit as a sensor for a robotics project I'm working on. I have came up with a design based on design's I've found on the internet and my limited knowledge of op-amps and analog circuitry. (see attachment, sorry if its blurry - i dont have a scanner and my iphone camera sucks)

I can get the diff. amplifier portion of the circuit working (MAX666CPA, LMC6041IN, INA106) by itself but when i try to connect the rectifier circuit (low pass filter, high pass filter, and TL084) not only does it not work but, even after I disconnect the rectifier circuit, the diff. amplifier portion stops working and only will output a constant ~ -1.17V. :confused: I checked to make sure the power supply circuit is still functioning and is in fact generating 5V with a 2.5V virtual ground. Usually if I just turn it off and let it sit overnight, it will start working again in the morning but that sometimes doesn't even work. :mad:

What the heck is going on? I'm at wits end trying to debug it. I've tried creating a separate 5V power supply circuit with 2.5V virtual ground to power the second portion of the circuit and also have tried using a dual supply (+/-9V) to power the second portion.

When the diff. amp portion of the circuit is working, it produces a signal around the magnitude of 20mV. The point of the rectifier circuit is to filter, rectify, envelope and amplify the signal to around 4V (or at least to a scale thats within the 5V detection range of my MCU).

Does anyone know why this intermittent functionality is happening?

here's the data sheets on the chips in the design:
http://focus.ti.com/lit/ds/symlink/ina106.pdf
http://datasheets.maxim-ic.com/en/ds/MAX663-MAX666.pdf
http://www.national.com/ds/LM/LMC6041.pdf
http://focus.ti.com/lit/ds/symlink/tl081a.pdf

Thank you for your time!
 

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Audioguru

Joined Dec 20, 2007
11,251
The minimum supply voltage for a TL084 is 7V because its input common-mode voltage range is from +3.5V above its negative supply to -3.5V below its positive supply.
You have its + inputs at only +2.5V which is too low.

The TL084 has the "Opamp Phase Inversion" problem when its input voltage is too low which causes its output to suddenly go high. With a 5V supply its max output is +3.8V.
The rectifier opamp has a diode in series with its output which reduces its max output voltage to +3.1V.

Your rectifier circuit has an extra 10k resistor from its input to the negative supply which causes its output to be positive all the time.
I think your rectifier circuit is designed to use a dual-polarity supply so its output can be 0V without a signal.
 

Thread Starter

Ocelot

Joined Jul 2, 2009
4
Thanks for replying Audioguru!

The minimum supply voltage for a TL084 is 7V because its input common-mode voltage range is from +3.5V above its negative supply to -3.5V below its positive supply.
You have its + inputs at only +2.5V which is too low.

The TL084 has the "Opamp Phase Inversion" problem when its input voltage is too low which causes its output to suddenly go high. With a 5V supply its max output is +3.8V.
The rectifier opamp has a diode in series with its output which reduces its max output voltage to +3.1V.
Do you know of any chips that I could replace the TL084 with that would be better suited for this application?

Your rectifier circuit has an extra 10k resistor from its input to the negative supply which causes its output to be positive all the time.
Do you have any suggestions on how to improve the rectifier circuitry? I thought I designed this one so that it essentially takes a running average of the EMG signal since and EMG signal is symetrical. So only taking half of the signal gives me the running average. I then filter the signal so that only the envelope (aka outline) is passed. I do want the signal to always be positive since I'll be passing it to a MCU that has a ADC works in the 0-5V range.

I think your rectifier circuit is designed to use a dual-polarity supply so its output can be 0V without a signal.
I thought a virtual ground makes it so you can use a single supply with a circuit that needs a dual-polarity supply. Can I not do this with the rectifier circuit?
 
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