01/29/2015 at 13:52 •
This circuit consists of 4 stages to acquire the signal from a differential pair electrode. The first stage implements an INA126 instrumentation amplifier that has low noise, offset voltage, quiescent current, input bias current and offset drift. This is a precision amplifier ideal for acquisition of low voltage signals such as those in surface EMG. The potentiometer connected to the Rg terminals controls the gain of the amplifier via the equation
The two electrodes on the muscle are fed into the differential input of the amplifier while the reference electrode is used as the ground plane for the signal filtering.
An important factor that has to be taken into consideration when designing electronics that come into contact with the body is to isolate the power supply circuitry from the signal acquisition circuitry. This is to prevent current flow through the person in the event of a fault and is a requirement of medical 60601-1 design standards. For this project I will implement galvanic isolation through a 3 kVDC isolated DC/DC converter RBM-0505S connected to all the op-amps and in a later stage of the circuit a digital isolator ISO7220xD after the microcontroller.
In the second stage of the circuit a second order active low pass filter (or Sallen-Key filter) is used with a cutoff frequency of approx. 450 Hz. To calculate the cutoff frequency for a second order RC circuit, the following equation applies:
Using the values of 10.7 kΩ and 33 nF, the cutoff frequency is 450.7 Hz which is more than close enough for these purposes. In terms of the resistor values, too low of a resistance will require a larger current to create the required voltage drop at the input and too high a resistance will incur non ideal errors due to bias current causing a voltage offset at the output. Resistance values around 10's and 100's of kΩ's are typically implemented in similar circuits and have a satisfactory balance of non-ideal factors, although more will be apparent once the circuit is built. The feedback loop for the amplifier is set to unity gain since amplification is being performed by the INA126.
The next stage implements a Sallen-Key high pass filter with a cutoff frequency of 15 Hz. Again, resistor values have been chosen in the 10's of kΩ's range and the amplifier is set to unity gain.
The final stage of signal acquisition is a summing amplifier which offsets the AC EMG signal up 2.5 V so that it is in the 0 - 5 V range. This is purely as a means for making analog to digital conversion easier, there are other ways to go about this issue but this is what I chose to go with for now.