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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?

Note that without an external compenzation network the 1/ curve approaches the AOL curve at with a rate of closure (ROC) >20dB/dec, indicating instability (the proof of this criterion is not discussed here).

The fix for this issue, shown in figure 8, is to add a series Rc and Cc in the local feedback of the RLD amplifier (Zc in figure 9). Zc then becomes the dominant feedback path between 20k-30kHz. The result for the simulation in figure 7 is represented by the 1/ (feedback) curve of figure 9.

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Figure 8: Test circuit for compensation network.. (Click on image to enlarge.)

Figure 9: AOL, 1/, and Zc . (Click on image to enlarge.)

Figure 10 shows the full circuit of the right leg drive with compensation. The new, compensated 1/ plots (based on variations in Rc and Cc) are shown in figure 11. The overall 1/ intersects the AOL curve with a ROC that is 20dB/dec and a loop gain with >45 phase margin (figure 12).

Figure 10: Compensated right leg drive. (Click on image to enlarge.)

Figure 11: AOL and 1/ for different values of Cc . (Click on image to enlarge.)

Figure 12: Loop gain and phase for figure 10. (Click on image to enlarge.)

In summary, SPICE can be a useful tool to quickly help analyze and optimize the performance and stability of the RLD front-end circuitry. Keep in mind that the simulation can only be as good as the models, so it is important to model key specifications, such as noise; AOL; open loop Zout; and CMRR vs. frequency, correctly and up front before analysis and design.

References
1) Brown, John and Joseph Carr, Introduction to Biomedical Equipment Technology, Prentice Hall Inc., New Jersey 1981, 1993.
2) Dubin, Dale, Rapid Interpretation of EKG's, Cover Publishing Company, Fort Myers 2000.
3) Green, Timothy, Operational Amplifier Stability, Part 2 of 15: Op Amp Networks, SPICE Analysis, http://www.analogzone.com/acqt0214.pdf

About the author
Matthew William Hann, Precision Analog Applications Manager at TI, has more than a decade of product expertise which includes temperature sensors, difference amplifiers, instrumentation amplifiers, programmable gain amplifiers, power amplifiers, and TI's line of ECG AFE devices. Through his role as an applications engineer, Matt has developed a focused expertise on the design of analog front ends for medical applications such as ECG, EEG, EMG, blood glucose monitoring, and pulse oximetry. Matt received his BSEE from the University of Arizona, Tucson.

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