Is there any difference between AD620 vs INA128?
The AD620 is also an equivalent INA thanks to @ Scott and @JRE 

Gain is set with one external resistor (Gain range 1 to 10,000).   The INA138 is 1 + (50 kΩ/RG)
Can you read the datasheet for AD620 to find any difference?


  Low Cost Low Power Instrumentation Amplifier.   
  
  Wide power supply range (±2.3 V to ±18 V)
  Higher performance than 3 op amp IA designs
  Available in 8-lead DIP and SOIC packaging 
  Low power, 1.3 mA max supply current
  
  Excellent dc performance (B grade)
  50 μV max, input offset voltage
  0.6 μV/°C max, input offset drift
  1.0 nA max, input bias current
  100 dB min common-mode rejection ratio (G = 10).
  Low noise
  9 nV/√Hz @ 1 kHz, input voltage noise 0.28 μV p-p noise (0.1 Hz to 10 Hz)
  Excellent ac specifications
  120 kHz bandwidth (G = 100) 15 μs settling time to 0.01%


Make sure your  cables must be balanced to get adequate or same CMRR with STP cables or similar. 

You can easily find these designs and examples in Google images.  Let’s see you try and update your question with a schematic.

Unless you at using batteries or know what you are doing with SMPS, we are cautious about mixing earth grounds.
the reason why medical PS have low leakage

Since EMG and EKG amps expect x mV signals as opposed to head skin signals (alpha, Beta waves etc.) which are in the xx uV range, it is pretty easy to make it work without  hum with a good linear supply or battery operated.  SMPS often add high CM noise that can be shunted to earth ground with 1nF max between floating 0V and E-gnd.

Also read all the wiki on EMG and this about placement of sensors.  https://www.delsys.com/Attachments_pdf/TN101%20-%20EMG%20Sensor%20Placement-web.pdf

Also “skin galvanic response” which a transient from sensor interface motion.

Then add a precision OA Peak detector and buffer to get the envelope DC output to drive lights and motors in a feedback loop for giggles with pressure sensors.

For more fun, put a sticky electrode on each temple and move eyes side to side to generate a triangle wave.