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Chapter 5

Design and Development of

ECG amplifier and testing

the developed sensor

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5.1 Design and development of ECG amplifier with the

conventional sensor

5.1.1 Design considerations

Accuracy and precision are very important aspects to be

considered in handling diagnostics or other medical applications. Any

small fluctuation in the waveform generated could carry critical

diagnostic value and hence it is very essential to design any bio-

medical instrument with the highest precision possible. It becomes

very necessary that the designed ECG system must faithfully display

the actual ECG signal, such that any irregularity detected should be

due to an unhealthy cardiac cycle and not from the equipment that was

used. Therefore, there are many special considerations to be taken into

account when designing the ECG amplifier.

1. The input impedance of the pre-amplifier should be high:

The developed sensor uses TiO2 as a dielectric material. Since

the optical band gap of this material is high, around 3.61eV, the

material offers high resistance. The resistance offered by the sensor

was found to be around 3.4kΩ. The input impedance of the per-

amplifier should be high enough to match the sensor resistance. This

is important to achieve the entire sensor signal amplitude to the input

of the pre-amplifier. Op-Amp amplifiers are usually considered for

such requirements. The main advantage of am Op-Amp amplifier is

that it is an AC / DC amplifier. Since the frequency of the ECG signal

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is very low, these amplifiers are ideally suited. LM324 is a low

power quad Op-Amp with a large voltage gain and internal frequency

compensation. They operate from a single power supply over a wide

range of voltages. They are available as a convenient 14 pin DIPs.

2. Gain of the ECG amplifier should be such that the

amplified ECG signal can be displayed:

The amplitude of the ECG signal is in the range of micro volts.

The amplifier should provide a proper gain to the ECG signal over the

low frequency range so that the signal is amplifier without distortion.

The signal needs to be displayed after being amplified and hence, a

good gain is required. The LM386 is a power amplifier designed for

use in low voltage applications. The gain is internally set to 20 to keep

external part count low, but the addition of an external resistor and

capacitor between pins 1 and 8 will increase the gain to any value up

to 200. The IC is available as an 8 pin DIP.

3. Pre-amplifier should be able to accept the low power

signals form the sensors:

The sensor output signal is of very low and very low amplitude.

The pre amplifier required should be able to accept and amplify this

signal. The LM324 is such an amplifier whose important application

includes transducer signal amplification.

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4. The pre-amplifier should be able to amplify low

frequency signals with constant gain:

The LM324 has a constant voltage gain from 0Hz to a few kHz.

Since the ECG signal frequency is very low, LM324 is an ideal

amplifier which can be used as a pre amplifier which can directly be

fed by the sensors

5. The amplifier output should be distortion free:

Since the sensor signal amplitude is very low, it gets easily

affected by noise signals. The main interfering signal is the movement

artifact. The amplifier should be able to reject the common mode

signals and provide a high signal to noise ratio. The circuit should be

able to identify the sensor signal and amplify it.

5.1.2 Block diagram of the amplifier

Block diagram of the amplifier system

The ECG amplifier system block is as shown in figure 5.1. The

preamplifier was designed using IC LM 324 and the ECG amplifier

was designed using IC LM 386. Two output options are provided one

at the output of the pre-amplifier and the other at the output of the

ECG amplifier.

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Figure 5.1: The ECG amplifier system block

Sensors: Since the proposed work is to develop a three lead

ECG system, three sensors, Sensor 1, Sensor 2 and the reference

sensor was used. These developed sensors were connected to the Op-

Amp amplifier inputs.

Pre-amplifier block: This block is constructed to amplify the

low amplitude, low frequency sensor output and to present the same

for further power level enhancement process.

To get the required characteristics of high input impedance and

high CMRR, an Instrumentation Amplifier which Op-Amps is usually

used as the first stage of a bio-potential amplifier. The architectures of

an Instrumentation Amplifier can be either the 3 Op-Amps system or

the current-feedback type [1, 2]

. The three Op-Amps system is easy to

construct while the current feed type has inherent higher CMRR. Both

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types can be constructed using commercial available integrated-circuit

(IC) form.

ECG amplifier block: This block is constructed to enhance the

power levels of the pre-amplified signal. The output of the pre-

amplifier feeds the input of this block. This block uses a low voltage

audio power amplifier in the IC form. The output of this block is the

amplified ECG signal, ready to be transmitted.

5.1.3 Circuit Design

This section deals with the design and development of the

amplifier and the filter required to process the ECG signal for display

and further transmission. The inputs for this section are derived from

three sensors, developed using Titanium dioxide as the base material.

The circuit is designed to amplify the signals and at the same time

eliminate as much noise as possible. The high frequency noise and the

common mode noise both have to be addressed to in designing the

circuit.

The duration of the QRS complex, which gives the highest

frequency is between 0.07 to 0.1 seconds, which gives a frequency of

10 to 14.285Hz. Hence the cut off frequencies for the filter is chosen

to be 15Hz.

According to the sampling theorem, good sampling of the signal is

possible only if the bandwidth of the circuit is two times the highest

frequency to be handled. Hence, the cut off for the antialiasing filter is

chosen to be 30Hz.

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The first stage of the circuit is the preamplifier stage which is a

very crucial stage which has to produce high gain and should provide

a good Common Mode Rejection. The input level shifter circuit shifts

the signal level required for the digitizing of the signal at a later stage

[3, 4]. The buffer amplifier ensures that the resistive loading is avoided

and also provides high input impedance.

The voltage difference between the two electrodes serves as the

signal input that is amplified through the Op-Amp circuits. These

signals are then differentially amplified and passed through a low pass

filter whose cut-off frequency is around 33Hz. The next stage is a

second order, Sallen-Key, low pass filter which is used for anti-

aliasing.

Pre-amplifier Designing:

To nullify any wandering DC effects, a simple RC high pass

filter with a very low cut off frequency has been implemented. The

cut-off frequency has been chosen to be 1.6Hz and the capacitor value

is chosen to be 10μF.

Since f = 1/(2πRC), R = 9.9522k Ω, which has been taken to be 10kΩ.

To reject the noise frequency at 50Hz from the power mains,

the acquired signal requires low pass filtering below 50Hz. The cut-

off frequency has been selected to 33Hz to reject the power line

frequency.

Let R = 100kΩ, f = 1/(2πRC) and f = 33Hz, hence C = 47nF = 47kpF

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The two signals are added and passed through an anti-aliasing

Low Pass Filter which also provides a gain of 100.

Gain A = -R2/R1 and the required gain is 100

Chose R1 = 100kΩ, then R2 = 10MΩ

f = 1/(2πR2C1) and f = 33Hz

Then C1 = 483pF and hence chosen to be 470pF

(a)

(b)

Figure 5.2: (a) Low pass filter and (b) simulation result of the filter

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This signal is passed through a Second order Sallen-key, unity

gain low pass filter to get a narrow bandwidth of about 15.31 Hz.

F = 1/√(2πR1C1R2C2); when R1 = R2 = R and C1 = C2 = C;

then f = 1/(2πRC)

Let R = 100kΩ, then C = 106.15nF. Hence C is chosen to be 104kpF

(a)

(b)

Figure 5.3: (a) Second order Sallen-key, unity gain low pass filter, (b)

simulation result

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The output of this filter is fed to two stages of passive filters, as

shown in figure 5.3, for step wise elimination of noise. These two

stages of passive LPFs give a second order response with a roll-off

rate of -40 dB/ decade. The two step attenuation of the signal is

compensated by providing an equivalent gain by the non-inventing

amplifier.

F = 1/(2πRC); f = 15.31Hz,

Let R = 100kΩ, then C = 106.15nF. Hence C is chosen to be 104kpF

(a)

(b)

Figure 5.4: (a) Two stages of passive filters (b) simulation result

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The final output is ready to be connected to the CRO, and is fed

to the ECG amplifier. The same output is fed to an LED through a

transistor switch to indicate the pulsing of the heart.

(a)

(b)

Figure 5.5: (a) Final stage of the amplifier system, (b) simulation result

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ECG amplifier designing:

The LM386 is a power amplifier designed for use in low

voltage consumer applications. The gain is internally set to 20 to keep

external part count low, but the addition of an external resistor and

capacitor between pins 1 and 8 will increase the gain to any value up

to 200. The capacitor also maintains the DC bias levels in the gain

adjustment circuit. The unused input is grounded to keep the DC

offset voltages low [5]

.

Also, when using LM386 with higher gains, it is necessary to

bypass the unused inputs, preventing degradation of gain and possible

instabilities. The capacitor connected between pins 2 and 6, provides

power supply decoupling. The RC circuit between the output point

and ground acts as a high frequency load to provide stability. The

potentiometer at the input pin provides adjustable input level

attenuation.

Figure 5.6: ECG amplifier

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5.1.4 Circuit diagram of the amplifier system

Figure 5.7: Circuit diagram of the ECG amplifier system

5.2 Testing the fabricated sensor with the developed amplifier

The circuit diagram was translated on to a printed circuit board.

The components were procured and soldered on to the board as shown

in Figure 5.7.

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Figure 5.8: ECG amplifier circuit on a PCB

The developed sensors were connected to the circuit with the

help of crocodile clips as shown in Figure 5.8. The sensors were

placed on the subject with help of adhesive tape.

Figure 5.9: ECG sensors placed on the subject with the help of adhesive

tapes. Conventional sensors placed on the subject but not connected to the

system can also be seen

The output of the circuit was connected to a digital oscilloscope

and tested for signal acquisition as shown in Figure 5.9.

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Figure 5.10: ECG signal display on the CRO from the developed sensors

and amplified by the developed amplifier system

The developed sensor was connected to the designed amplifier

circuit and connected to a digital oscilloscope. The ECG signals were

obtained on the CRO. The obtained signals were compared with the

signals from conventional, commercially available pre-gelled sensors.

The signals obtained from the developed sensors were on par with that

obtained from the commercial sensors. The signals were validated in

Shanthi Hospital and Research center, Bangalore, by Doctor Sanjay

Gururaj, Director, SHRC.

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5.3 Skin Impedance measurements

Skin resistance measurements, Variation of resistance with

different skin conditions:

Skin resistance measurements without and with sensors were made as

shown in Figure 5.11

Figure 5.11: Skin resistance measurements

The skin resistance of the volunteer was found to be 5.26MΩ

without any sensor. When the conventional sensors were attached to

the skin of the same person, the resistance was found to be 2.28MΩ as

shown in Table 5.1. The reduction in the resistance can be attributed

to the gel used between the skin and the conventional sensor material.

When the developed dry sensor was attached to the skin of the same

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person, a still lower resistance was examined, around 1.68MΩ,

indicating a better signal achievement from the developed sensor.

Skin Resistance in MΩ

Normal

Skin

Hairy

Skin

Moist

Skin

Without Sensors 5.26 8.2 8.85

With Conventional Sensors

2.28 5.4 9.2

With Developed Dry Sensors

1.68 0.794 7.54

Table 5.1: Skin resistance measurements

Skin impedance measurements were carried out using the

following experimental set up as shown in Figure 5.12.

Figure 5.12: Experimental set up to measure skin impedance

A small current of 1mA is given as input from the current

source. Three sensors were connected to the forearm as shown. The

input frequency is varied from 10Hz to 10 KHz and the corresponding

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output voltages are noted down. The impedance is calculated for these

different frequency values.

Skin Impedance in MΩ

Frequency in kHz

Normal Skin

Hairy Skin

Moist Skin

0.01 5.26 8.2 8.85

0.1 2.68 3.98 4.12

1 0.5 0.5 0.5

10 0.5 0.5 0.5

100 0.5 0.5 0.5

Table5.2: Skin impedance measurements without sensors on the skin

Figure 5.13: Skin impedance measurements without sensors on the skin

Without any sensors attached to the skin, the impedance variations of

the skin are shown in Figure 5.14. It is observed that the impedance is

the maximum for moist skin and it decreases with frequency.

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Skin impedance measurements were done for various skin types

with conventional Silver / Silver Chloride sensors placed on the skin.

Table 5.3 and Figure 5.14 give the details of the measurements.

Frequency in

kHz

Normal

Skin

Hairy

Skin

Moist

Skin

0.01 2.28 5.4 9.2

0.1 1.09 3.001 3.89

1 0.52 0.53 0.5

10 0.52 0.53 0.5

100 0.52 0.53 0.5

Table 5.3: Skin impedance measurements with conventional Ag/AgCl sensors

Figure 5.14: Skin impedance measurements with conventional Ag/AgCl sensors

on the skin

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With conventional sensors, there is a clear reduction in the skin

impedance for normal skin, but the impedance increases with moist

skin.

Skin impedance measurements were done with the developed dry

sensors on the skin. The details are shown in Table 5.4 and Figure

5.15.

Frequency

in kHz

Normal

Skin

Hairy

Skin

Moist

Skin

0.01 1.68 0.79 7.54

0.1 0.85 0.36 3.21

1 0.5 0.5 0.5

10 0.5 0.5 0.5

100 0.5 0.5 0.5

Table 5.4 Skin impedance measurements with the developed dry sensors on the

skin.

Figure 5.15: Skin impedance measurements with the developed dry sensors

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With the dry developed sensors, the skin impedance is

considerably reduced, but shows a large value for moist skin. But the

large moist skin impedance of the moist skin with the developed

sensor is lesser compared to the moist skin impedance with

conventional sensors and without sensors.

In conclusion, the developed sensors tend to provide low skin

impedance compared to the conventional sensors indicating better

performance and avoidance of skin preparation.

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References:

[1] Chia-Nan Chien, Fu-Shan Jaw, “Miniature Ultra-Low-

Power Bio-potential Amplifier for Potable Applications”,

Biomedical Engineering Applications - Basis &

communications vol. 17 no. 2 April 2005, 108-110

[2] Baghini, M. S., Lal Rakesh, Sharma, D. K, “A Low-

Power and Compact Analog CMOS Processing Chip for

Portable ECG Recorders”, Proceedings of the Asian

Solid-State Circuits Conference, Hsinchu, Taiwan, 1-3

November 2005, 473-476

[3] D Rowlands, D. A. James, C Vanegas, “Design and

fabrication of an ECG amplifier on silicon using standard

CMOS process”. Sensors (Peterboroug (2003), Volume:

2, Publisher: IEEE, Pages: 1348-1352

[4] G.M. Patil et. al, “Embedded Microcontroller based

Digital Tele-monitoring system for ECG”, J. instrum.

Soc. India, 37(2), 134-149

[5] IC LM386 data sheets