BME354 Spring 2014 Exam 1

 

1)      Draw a diagram of the body showing where to place the lead III electrodes (no driven right leg) and how to correctly connect the electrodes to an INA.

 

ANSWER:   Lead III is LA-, LL+

 

Sign and connections are right, award 3pts

If the connections are right and the sign is wrong, award 2 pts

If the connections and sign are wrong award 0 pts

 

 

Someone’s cell phone is ruining my lead II ECG.  A unity gain INA has 100dB of CMRR.  The Lead III SNR at the output of the INA is 60dB with the phone on.   Someone suggests grounding the RA to improve the SNR.  Specify the SNR (in dB) after grounding the RA. 

 

 

ANSWER:

Before grounding:  Assume signal is A at input and the noise is N.  The signal at output is A (unity gain).  The CMRR is 100dB so the noise at the output is N/10^5.  The SNR at the output is

10^3 = A/(N/10^5) 

Solving

A/N=10^-2

 

Grounding the RA reduces the CMRR to zero, since all of the common mode is also multiplied by the difference mode gain.

So the SNR at the output is now 10^-2
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