Driven Right Leg (DRL) Circuit

Interactive Simulation of ECG Interference Suppression & Patient Protection

- + PATIENT 220V 50/60Hz Parasitic Cap 220V FAULT RA LA RL R(ra) R(la) Common Mode DRL Amp V+ / Vcc V- / GND R_o (e.g. 5 MΩ) Current Limiter
I_leakage ≈ 1 µA
I_safe = 220V / 5MΩ ≈ 44 µA Safe limit maintained by R_o
WITHOUT DRL (Direct Ground): I_fatal = 220V / 1kΩ ≈ 220 mA
LETHAL

Normal Operation

Interference Suppression

In normal conditions, the patient acts as an antenna, picking up 50/60Hz electromagnetic interference from power lines. This creates a tiny displacement current (thin blue line).

The DRL circuit senses the common-mode voltage (the average noise on RA and LA), inverts it, and feeds it back to the Right Leg (RL). This active negative feedback cancels out the interference by continually sinking current to the negative supply (V-) and sourcing current from the positive supply (V+) (thin green line). Notice how the animation splits and reverses direction to mimic Alternating Current (AC)!

Note: The DRL circuit drives the body toward the amplifier's reference/ground potential, not necessarily absolute zero volts.