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I am in the process of designing an ECG circuit to detect heartbeat abnormalities. I have two question regarding the AD8220 datasheet:

  1. What is the function of the 2.2pF, 10pF capacitor, 10k, 15k resistors, and the diodes prior to the AD8220?
  2. What is the function of the capacitor on the OP2177 right leg drive circuit?

AD8220 ECG configuration

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    \$\begingroup\$ If your assignment is to acquire the signals and not reinvent everything with basic components, then you could just use an integrated analog front-end like ADS1198 or ADS1194 and only worry about data processing. \$\endgroup\$ – filo Dec 29 '17 at 19:10
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    \$\begingroup\$ Unrelated to your queries: The +/- 5V DC supply to the front-end is a critical component that must be rated for human contact (exceptional ground isolation). The +2.5V DC supply is not so critical, and should probably be derived from the ADC reference voltage. \$\endgroup\$ – glen_geek Dec 29 '17 at 20:10
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  1. 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.

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

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

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    \$\begingroup\$ 7 digits is reduced to 1 significant figure when you include inductance of lead length and RLC shift down in breakpoint. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Dec 29 '17 at 22:59
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The common mode 2.2pF caps and 10pF differential cap reduce the impedance of AM signals 0.5~1.5MHz in 100k to 10k impedance to reduce chance of demodulation in front end. Otherwise the LPF response of T=68pF*866k=59us for f-3dB= 2.7kHz to lower CM impedance of body using the CM signal Right Leg Drive feedback.

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  • \$\begingroup\$ Why the Right-Leg-Drive point? Wouldn't a more centrally-located attachment point be better - I think the medical term is sagittal. \$\endgroup\$ – glen_geek Dec 29 '17 at 20:18
  • \$\begingroup\$ For EKG point C is most commonly used as a remote reference point, not near heart. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Dec 29 '17 at 22:58

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