How to determine voltage using difference amplifier?

Question

I made an EMG sensor from a tutorial found on the internet with the amplifiers INA106 and TL072 and another TL072 giving offset. The gain of INA106 is 110 and that of the TL072 is from -15 and -3. INA106 measure potential between (+/- 5mV) electrodes and amplifies it.

I'm using Arduino analog inputs to read this data. I wonder how to determine what is real muscle activity in mV. The Arduino range is 0-5V with a resolution of 1024. So before scale I have a value range from 0-1024 and after scale one of 0-5. But it's in Volts and I can't figure out how to count real muscle activity. Should I use gains in counting? or i just can't find range of INA106 measured values.

Answer 1

Supposing you have an amplifier with a gain of 110.

±5mV * 110 = 550±mV.

Your A/D converter measures inputs between 0V and 5V, so it can't measure negative voltages. You may need a second amplifier or capacitive coupling to offset the measurement by 550mV to make a 0V to 1.1V range.

The resolution of your A/D is 5V / 1024 counts = 4.88mV per count at the output of the amplifier. This corresponds to a resolution of 4.88mV / 110 = 44.3uV per count at your sensor.

With an input range of 1.1V your measurements will span 0 counts to 1.1V/4.88mV = 225 counts. You are not using the full 5V range of your A/D. Therefore your effective resolution based on the input range is effectivly only log2(225) = 7.8 bits, rather than the full 10 bits.

You can improve your resolution by using more gain on the front end. Based on a ±5mV sensor range and 5V A/D input range you would want your gain to optimmally be around 5V / 10mV = 500.


Note that the INA106 has an input offset voltage of up to 200uV on the front end. The offset voltage may cause up to 200uV / 5mV = 2% of full scale of error added to any measurement. Note that having up to 2% error limits the accuracy of your measurement to only -log2(0.04) = 5.64 bits of accuracy. So you may want to pick a better amplifier with a lower offset.