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Signal chain basics: Analyzing RL drive in ECG front end with SPICE

Posted: 01 Nov 2011 ?? ?Print Version ?Bookmark and Share

Keywords:Electrocardiography? right leg drive? analog electronics interface?

Electrocardiography (ECG) is the science of converting the ionic depolarization of the heart to a measurable electrical signal for analysis. One of the most common challenges in the design of the analog electronics interface to the electrodes/patient is in the optimization of the right leg drive (RLD) for common mode performance and stability. Leveraging SPICE to help in this effort can greatly simplify this process.

In an ECG front end, the RLD amplifier provides a common electrode bias at Vref and feeds back the inverted common mode noise signal (enoise_cm) to reduce the overall noise seen at the inputs of the instrumentation amplifier gain stage. In figure 1, the sources ECGp and ECGn are split to show how the RLD amplifier provides the common reference point for a portion of the ECG signal that is seen at the positive and negative inputs of the instrumentation amplifier (INA).

Figure 1: Here's simplified LEAD I with RLD connections.

The parallel RC combination for the left arm, right arm, and right leg represents the lumped passive electrode connection impedance, which will be represented though the rest of this article as 52k? and 47nF. Assuming enoise couples parasitically into the inputs, the feedback of enoise_cm will reduce the overall noise signal at each input, leaving the residual noise to be filtered externally or rejected by the common mode rejection ratio (CMRR) of the INA.

The variation in common mode rejection can be seen in Figures 2, 3, and 4, which show the common mode test circuit with varying gain of the RLD amplifier. These plots show that the best low-frequency CMRR is achieved with no feedback resistor (i.e. infinite gain); however, in reality, eliminating the DC path and/or setting RF to a high value may not be practical for applications that require linear operation of the RLD amplifier when one of the input amplifier leads is removed.

Figure 2: CMRR vs. RLD gain. (Click on image to enlarge.)

Figure 3: Here's a plot of CMRR vs. frequency and RLD gain (RF). (Click on image to enlarge.)

Figure 4: CMRR RLD vs. no RLD. (Click on image to enlarge.)

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