ECG circuit Pre amplifier
The resistors and capacitors form a low pass filter to attenuate differential-mode and common-mode noise. The filter cutoff frequencies are


\begin{equation}
f_{C_D} = \frac{1}{2 \pi \left( 2(10 k \Omega + 15 k \Omega) \right)\left(10 pF + \frac{2.2pF}{2} \right) } = 286.7657 \text{ kHz}
\end{equation}

for differential-mode and

\begin{equation}
f_{C_D} = \frac{1}{2 \pi (25 k \Omega)(2.2pF)} = 289.3726 \text{ kHz}
\end{equation}

for common-mode. This results in improved noise immunity, which is essential in low voltage measurements. The diodes limit the measured voltage to \$ \pm 5V \$ for safety, to insure the voltage inputs stay within the AD8220's specifications.


It creates a low pass filter along with 866k resistor.


I personally use the free cross-platform LTspice as a simulator.