Long time lurker, first time asker. I've been working through a biosensor schematic from a BME class to build an EKG (I'm a physician, aware of risks of these devices etc) (typical voltages being detected in the range of <100mV). After passing through a series of amplifiers and filters, it connects to an Arduino via a optocoupler for electrical safety.
In the original schematic I'm working with, there are 3 generic red LEDs in series with a 220 Ohm resistor with the input side of the optocoupler (CNY17-3) producing about 10mA of bias current through the optocoupler. With this set up, the output waves look crisp with good amplitude. I tried to ditch the LEDs and just up the resistor value to maintain 10mA (in this case, supply voltage of 9V battery w/ resistors with values between 600-1k ohms), but the output waveforms I'm getting have much lower amplitudes compared to with LEDs.
CNY has a CTR of 100% at 10mA, forward voltage of 1.2 at that current as well (max is 60mA).https://docs.broadcom.com/docs/AV02-0772EN datasheet
Any thoughts on if this is a special effect of LEDs (if so, any way to use a different component to replicate this) or whether I'm choosing inappropriate resistor values?