NOISE AND NOISE REDUCTION QUESTIONS AND ANSWERS

Question 1

What are ‘noise’ signals in the context of clinical physiology?

Answer 1

• unwanted signals

Question 2

Why are noise signals troublesome to our recordings?

Answer 2

• The CMN is in the same range as the signal of interest BUT the amplitudes are larger
• if we amplify the noise in the same way as the ECG/medical device output would be saturated by the noise
• occurs as fuzzy lines on the ECG

Question 3

What is a common-mode signal?

Answer 3

• EM waves in the environment/body that have the same frequency and amplitude as the signal of interest
• common mode gain = VOUT/VIN of common mode signal

Question 4

How are ‘pseudo-differential’ signals created in an amplifier?

(impact of electrode/skin preparation)

Answer 4

-They keep the impedance between skin and electrode as low and as even as possible

Question 5

Define Common Mode Rejection Ratio:

Answer 5

DIFFERENTIAL GAIN/ COMMON MODE GAIN

CMRR(dB) = 20 X Log(CMRR)

-in clinical practice it is usually 80dB

Question 6

Explain how the Driven Right Leg system is applied to the amplifier as a noise reduction technique

Answer 6

• Right leg electrode is used as a reference
• common mode signal from RL electrode is inverted and fed back into negative terminal of amplifier
• noise signal should be cancelled out but in practice there’s still a small potential difference which is further Eliminated by the CMRR

Question 7

What practical methods that can be used to minimise/remove noise?

Answer 7

• Skin Prep (reduces impedance)
• Keep Leads parallel
• turn off nearby switches on mains
• driven right leg