ECG Amplifier - DAIICT Intranet

ECG PROJECT REPORT
Mentor: R. Sai Krishna (200701168)
Prof. Subhajit Sen P. Bharath Reddy (200701179)
ECG CIRCUIT REPORT Page 1

1) Introduction
Table of Contents
2) The design of the Detection circuit
3) Results of the simulation of Design circuit
4) The Monoshot Circuit Design
5) Results of simulation of Monoshot circuit
6) The Hardware Implementation
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Objective:
Introduction
The objective is to design a sensitive amplifier circuit that can detect ECG (Electrocardiogram)
signals obtained from metal electrodes applied at the left arm (LA), Right arm (RA) and right leg(RL).
Brief description of the circuit:
The circuit basically amplifies the difference between the right-arm (RA) lead and the left-arm
(LA) lead with the right-leg (RL) lead as the ground or reference node (i.e. the RA or LA common mode
should be about the same as RL). The circuit consists of a “difference” amplifier which is essentially a 2stage
instrumentation amplifier (IA) followed by another amplifier/filter stage followed by a filter stage .
The sharp “R wave” pulse needs to be detected using a circuit composed of a comparator and a
monoshot (mono-stable multivibrator) that enables an oscillator to drive a speaker. A distinct beep
sound is to be detected. The mono-shot may be required because the pulse is very narrow with respect
to the period of the waveform typically 1 Hz or a heartbeat rate of 60 per minute.
Specifications:
Total Gain= 1000 to 3000
Signal lower cutoff freq.= 0.05 Hz
Upper cutoff freq. = 150 Hz
CMRR= Should be high (say 1000)
R-wave pulse height = 1 mV
Desired Outputs:
Plots from spice simulation of the design.
Outputs from oscilloscope at different points in the circuit.
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Our design can be split into two parts:-
Detection of R wave.
Producing the beep sound with the signal.
Sub-circuit-1:
The Detection Circuit design
This part of the circuit has an amplification of approximately 12 and has a difference amplifier to remove
the common-mode noise. The opamps used in the circuit are AD741. The high-pass filter with 16nf
capacitor and 100k resistor is used to filter the dc voltage. V1 and V2 are the inputs from the right arm
and left arm respectively. The reference voltage i.e., the right leg is grounded.
Sub-circuit-2:
This part of the circuit is for amplification of the output of sub-circuit-1. The opamp used here is AD741.
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Sub-circuit-3:
The above circuit is a Sallen-Key low pass filter with bandwidth 159Hz. The opamp used here is AD741.
The use of low-pass filter in the circuit is to ensure that the AC noise is also removed if it remains during
the stage of difference amplifier. The bandwidth of the main output signal will be less than 150Hz and
greater than 5Hz. Hence the low pass filter successfully removes the remaining AC noise if at all it is
present in the circuit.
The combined circuit:
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Spice Code:
.subckt opamp in+ in- out
R01 3 0 200k
RO4 3 5 1K
D1 3 4 DMODEL
G1 3 0 in+ in- 1M
CC 5 6 5P
G2 6 0 3 0 1M
R02 6 0 200K
D3 6 7 DMODEL
D4 0 7 DMODEL
G3 0 out 6 0 0.1
R03 out 0 10
D5 out 9 DMODEL
D6 out 9 DMODEL
.MODEL DMODEL D(VREV =
12)
.ends
.subckt First v1 v2 vout
X1 v1 2 5 opamp
X2 v2 3 4 opamp
R1 2 3 1K
R2 2 5 1K
R3 3 4 1K
R4 5 6 24K
R5 4 7 24K
X3 6 7 8 opamp
R6 6 0 100K
R7 7 8 100k
C 8 vout 1u
R vout 0 1Meg
.ends
.subckt Second v1 vout
X1 v1 2 vout opamp
R1 2 0 1k
R2 2 vout 100k
.ends
.subckt Sallen vin vout
X1 3 4 vout opamp
r1 vin 2 10k
r2 2 3 24k
c1 2 vout 15n
c2 3 0 15n
r3 4 0 24k
r4 4 vout 10k
.ends
X5 3 4 5 First
X6 5 6 Second
X7 6 7 Sallen
v1 1 0 pulse(0 0.5m 10m 5m
5m 0 800m)
v2 2 0 pulse(0 -0.5m 10m 5m
5m 0 800m)
v3 1 3 sine(300m 1m 150 0 1
0 10)
v4 2 4 sine(300m 1m 150 0 1
0 10)
.tran 1m 1000m 0.01
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Results of spice simulation:
ECG CIRCUIT REPORT Page 7

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Monoshot circuit for sound
The opamp acts as a comparator and first 555 timer is in monostable mode. The input to the circuit is
the output of the detection circuit. The output of the first 555 timer is sent to the second 555 timer
which is configured to work in astable mode. This works as an oscillator of frequency of appx. 200Hz.
Extension of previous spice code:
.subckt Monoshot vin vout
X1 vin vout 4 5 6 6 8 UA555
vcc 8 0 12
r1 6 8 500k
c1 6 0 1u
r 3 0 10k
.ends
.subckt oscill vin reset vout
X1 vin vout reset 5 vin 7 8 UA555
vcc 8 0 12
c 2 0 15n
r1 vin 7 100k
r2 7 8 100k
r 3 0 10k
.ends
X4 11 7 8 opamp
X5 8 9 monoshot
v6 12 0 pulse(0 12 0 0m 0m 10m 2m)
X6 12 9 10 oscill
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Results of Simulation:
Comparator output:
555 timer output:
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The input and output currents for the 555 timer circuits are controlled by load resistances applied w.r.t
the 555 timer chip characteristics.
Hardware Implementation:
The effecting factors:
The input resistance of the human body is very high.
The offset voltage of the opamp.
The resistors are not ideal which affect the gain and CMRR.
The signals passed from the hands to the circuit are generally very low because of the high
impedance of the body. Hence a small amplification is required before the removal of the common
mode noise by the differential amplifier. Hence a small amplification of 3 was designed before the
difference amplifier of a amplification 4.
The output of the sub-circuit-1 of the detection circuit was not able to remove the dc offset when
the circuit was switched on. The signal got at the output of the high pass filter gave a negative dc offset.
The problem we noticed was the input offset current of the opamp which is around 1nA created a
negative offset of 20mV in presence of 1Mega Ω resistor of the high pass filter. This affected the overall
output as the amplification in next stage was around 100. To solve this problem, we connected a
potentiometer across the offset terminals of AD741 opamp i.e, across the 1 and 5 terminals. At a
particular point the DC offset was nullified.
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Also, initially we used we connected the wires directly to the hands and right leg and tested the
signal. The signal strength was very low and the result was not clearly visible. So, we used the AgCl
(silver chloride) electrodes which are actually used with the ECG machines. Using the electrodes the
signal strength was pretty good and the result was clearly visible.
The output had two frequencies, one, the output of 2 nd 555 timer circuit and the other the input
frequency signal. The output signal frequency was more significant.
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