ECG Measurement System

By Nick Rose Under guidance of Dr. Jack Ou

Summer 2012

Built off of Chia-Hung Chen, Shi-Gun Pan, and Peter Kinget’s design for an ECG Measurement System from the Columbia Integrated Systems Lab

Full Circuit

R=10 M

Potentiometer

9 V

Gain= 200

Bandpass Filter Transfer Function

0.01 0.1 1 10 100 1000 10000

-35

-30

-25

-20

-15

-10

-5

0

5

Freq (Hz)

H(

measured) (dB)

Op-Amp

• Purpose: To bias voltage at output of AD624

-10 -8 -6 -4 -2 0 2 4 6 8 10

-8

-6

-4

-2

0

2

4

6

8

10

V3 (V)

V6 (V)

Opto-Coupler

V(1) versus V(2) V(1) versus V(3)

-10 -8 -6 -4 -2 0 2 4 6 8 10

-10

-8

-6

-4

-2

0

2

4

6

8

10

V1 (V)

V3 (V)

V2 (V)

Instrumentation Amplifier

Gain of 200

Gain of 1000

-0.008 -0.006 -0.004 -0.002 0 0.002 0.004 0.006 0.008 0.01

-8

-6

-4

-2

0

2

4

6

8

10

Vin (V)

V9 (V

)

V(3) of op-amp versus:2nd Harmonic Distortion 3rd Harmonic Distortion

-6.5 -6 -5.5 -5 -4.5 -4 -3.5 -3 -2.5 -2 -1.5

-70

-60

-50

-40

-30

-20

-10

0

V3 (V)

H2 dist (dB)

-6.5 -6 -5.5 -5 -4.5 -4 -3.5 -3 -2.5 -2 -1.5

-70

-60

-50

-40

-30

-20

-10

0

V3 (V)

H3 dist (dB)

Gain of 200, V(3) of 741 op-amp = -4.4 V

Adjusted V(3) to -4.4 V to minimize 2nd and 3rd harmonic distortion

Magnitude of input sine wave versus gain (gain includes gain of opto coupler)

0 0.002 0.004 0.006 0.008 0.01 0.012570

580

590

600

610

620

630

640

Input mag (V)

Gain

Power Dissipation

• The entire circuit (AD624, 741, optocoupler)..• Current draw: 114 mA• Voltage: dual power supply (+-9V)

Shielded Versus Unshielded Cables

• Untwisted cable, shielded by aluminum foilAluminum shield is grounded

Unshielded AD624 Output

Shielded AD624 Output

Unshielded ECG Output

Shielded ECG Output

Shield Input of ECG

Unshield Input of ECG

Shielded AD624 Output at DC

Shielded AD624 Output (0 -12.5 Hz)

Shielded AD624 Output at 60 Hz

Unshielded AD624 Output at 60 Hz

Unshielded ECG Output

Shielded ECG Output

Extra Components for stabilizing the Output Voltage of the AD624

• 10 Mohm resistors were used at pin 1 and 2 of AD624 in order to provide bias current transistors in the pre-amplifier stage

• An input offset potentiometer with the middle pin attached to +9 V was used to reduce offset voltage. The voltage at pin 9 was used.

Different Design Iterations

• Breadboard• PCB• Equipment

Future Work: Active Grounding Technique

• Problems observed:– Common mode sensing resistors attached to node

16 and 3 of AD624 can affect of node 9 of AD624.• 25 Kohms, DC voltage at node 9 is disturbed. • 1 Mohm, DC voltage at node 9 is stabilized.

– Large values of Rf makes it difficult to for negative feedback to pin node 2 of 741 op-amp to ground.

– What is the right ratio of Rf/Ra?