Low supply voltage electrocardiogram
Signal amplifier
Saeedeh Lotfi Mohammad Abad
Dr. Keivan Maghooli
Department of Biomedical Engineering
Science & Research branch, Islamic Azad University
Tehran, Iran
Email:s_lt80@yahoo.com
Department of Biomedical Engineering
Science & Research branch, Islamic Azad University
Tehran, Iran
Email:k_m_iau@yaho.com
MODERN tendency in patient diagnosis and trea tment
involvesthe use of personalised portable biomedical
instrumentation. In addition to well-known (ECG) and blood
pressure signals, various telemedicine applications require
instruments of improved design, compatible with modern
microcomputers and microcontrollers.
Low voltage and low power are among the most important
requirements for such instrumentation. Present-day
rechargeable or non-rechargeable 3.6V or 3V battery voltages
need adequate biopotential amplifiers. High performance
should be obtained in spite of the low supply voltage
limitation, especially concerning electrode polarisation voltage
and common-mode input voltage tolerance. The most widely
used circuits for bio signal amplifiers are based on the threeoperational-amplifier configuration, or instrumentation
amplifier, followed by an additional AC-coupled stage
(NEUMANN, 1998). Usually, the ‘classical’ amplifier gain is
split between the instrumentation amplifier and the stage after
the high-pass decoupling filter. The first stage gain is set to
low values, because of the electrode polarisation potentials.
Their voltage difference can reach up to about 200mV,
depending on various factors (electrode metal, conductive gel,
patient skin etc.), and appears as an input signal DC
component (NEUMAN, 1995). The main performance
characteristics of ECG amplifiers can summarised as follows:
• frequency band at 3 dB from 0.05 to 100 Hz, with
first- Order high-pass filter
• A tolerance of DC input voltage (of level depending
on the Holter-type ambulatory recorders of
electrocardiogram type of electrode) without input
stage saturation
• overall gain in the range 200–1000 (46–60 dB), with
a maximum input signal of about _5mV without
output stage saturation
•
differential input impedance >5MO in the entire
frequency band
• common mode rejection ratio (CMRR) >60 dB
• For a two-electrode amplifier, the inputs should
tolerate at least 3 mA common mode current per
input, without saturation of the input stage.
Abstract— Portable biomedical instrumentation has become
an important part of diagnostic and treatment
instrumentation, including telemedicine applications. Low
voltage and low-power design tendencies prevail. Modern
battery cell voltages in the range of 3–3.6V require
appropriate circuit solutions. A two-electrode bio potential
amplifier design is presented, with a high common-mode
rejection ratio (CMRR), high input voltage tolerance and
standard first-order high-pass characteristic. Most of these
features are due to a high-gain first stage design. The
circuit makes use of passive components of popular values
and tolerances. Powered by a single 3V source, the
amplifier tolerates _1 V common mode voltage, _50 mA
common mode current and 2 V input DC voltage, and its
worst-case CMRR is 60 dB. The amplifier is intended for
use in various applications, such as Holter-type monitors,
defibrillators, ECG monitors, biotelemetry devices etc.
Keywords-component ECG amplifier, Biopotential amplifier, Low
supply voltage amplifier, AC coupled amplifier
I.
INTRODUCTION
The action potential created by heart wall contraction spreads
electrical currents from the heart throughout the body. The
spreading electrical currents create different potentials at
Different points on the body, which can be sensed by
electrodes on the skin surface using biological transducers
made of metals and salts. This electrical potential is an AC
signal with bandwidth of 0.05 Hz to 100 Hz, sometimes up to
1 kHz. It is generally around 1-mV peak-to-peak in the
presence of much larger external high frequency noise plus
50-/60-Hz interference normal-mode (mixed with the
electrode signal) and common-mode voltages (common to all
electrode signals). The common-mode is comprised of two
parts: (1) 50- or 60-Hz interference and (2) DC electrode
offset potential. Other noise or higher frequencies within the
biophysical bandwidth come from movement artifacts that
change the skin-electrode interface, muscle contraction or
electromyographic spikes, respiration (which may be rhythmic
or sporadic), electromagnetic interference (EMI), and noise
from other electronic devices that couple into the input. A
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The last requirement corresponds to interference level, which
commonly occurs in a hospital room environment, according
to our pre vious experience (DOBREV and DASKALOV, 2002;
DOBREV, 2002). Even with a battery-supplied amplifier, input
common mode currents can often reach 1.5 mA per input.The
idea of setting high gain in the first amplifier stage is well
known. It allows a high CMRR to be obtained easily. The
simplest solution is to add a capacitor in series with the gain
setting resistor of the differential amplifier (MCCLELLAN,
1981; PALLAS-ARENY and WEBSTER, 1993), but its value can
be inconveniently high, depending on the high-pass cutoff
frequency. A version of this circuit, having the same
disadvantage, was patented by CHEE (2002). In addition, as the
first stage is a differential follower, any DC input voltage is
amplified by the second stage. An old solution, using
differential high-pass filters at the inputs, has been
reconsidered by BURKE and GLEESON (2000).The circuit needs
a reference electrode; otherwise the input stage would be
saturated even by very small common mode currents.
Bootstrapped input stages also suffer from saturation by
relatively low input voltages (THAKOR and WEBSTER, 1980).
SPINELLI and MAYOSKY (2000) proposed the use of opto
couplersin photo ltaic mode and an integrator, included in
anegative feedback loop, for input DC voltage compensation
and high-pass filtering. The optocoupler transfer
characteristics are non-linear, and there is a wide variation
between specimens of the current-to-current transfer ratio
(about twice). This leads to low accuracy of the high-pass
cutoff frequency. In a similar design, JORGOVANOVIC et al.
(2001) used differential-to-differential amplifiers instead of
optocouplers.The circuit is unacceptable for low-power
systems, as these types of amplifier, designed for highfrequency operation, consume large amounts of current
(20mA or more).These and other inconveniences of existing
solutions stimulatedus to try and develop a low-voltage, lowpower, two electrode amplifier,satisfying the above
requirements.
.
parallel. Therefore the ratio of the currents in R2 and R3 is
IR2 /IR3=R3/R2. The current in R1 is the sum of the
currents in R2 and R3: IR1=IR2+IR3= (1+R3/R2) IR3.
(a)
(b)
Fig. 1 Basic amplifier circuit concept. (a) Simplified and (b) detailed
Circuits
As mentioned above, the resistors R3 and C form a first
order low-pass filter, and the AC component on C
decreases with 6 dB Oct^-1 and becomes practically zero
for the operating frequency band. The A1 and A2
amplifiers take one-half of the differential input AC signal
each. The input DC component is filtered by C and appears
at the A3 and A4 outputs. The second stage is a unity gain
four input adder/subtractor stage. It implements (1), where
Ad is as follows:
Ad = 1 +R1/ (R2IIR3)
with R3>>R2, Ad = 1 +R1/R2
Another solution for the second stage could be by two
differential channel analogue-to-digital converters (ADCs),
producing a digitised V out, ready for microcomputer
processing. When ± 5 supply voltage is available, it is
possible to obtain Vout by two difference amplifiers in a
microchip, such as INA2134, for example.The first stage has
unity common mode voltage gain. The second stage has unity
differential mode voltage gain. The minimum CMRR can be
calculated as
CMRR = (Ad1_4/Acm1_4) *Ad5/Acm5 = (Ad/1) *1/ (4 δ / (1
(2)
+ R4/2R4)) = Ad *1.5/4 δ
Where δ is the tolerance of the R4 resistors used?
If Ad=1000 and δ =1%, the theoretically computed minimum
CMRR (assuming ideal operational amplifiers) is 91.5 dB,
taking opposite signs for the resistor tolerances. With Ad=200,
CMRR becomes 77.5 dB. Taking into account real operational
amplifiers (with CMRRmin=75 dB) and with Ad=200, the real
minimum CMRR is 60.3 dB. A very important parameter is
the operational amplifiers’ input offset voltage, especially
concerning A3 and A4. The A1 and A2 offsets do not
contribute to error, as they are added to the input signal DC
component, which is cancelled by the capacitor C. The
II: AMPLIFIER CIRCUIT CONCEPTS
The simplified amplifier circuit is shown in (Fig. 1a). The
general principle is that the input signal is buffered (two
buffers marked ‘1’) and AC decoupled by the capacitor C and
the resistors R3. The second stage consists of two differential
amplifiers Ad. Each of them amplifies half of the differential
input signal. By summing, the output signal is obtained as
V out = Ad (V a _ V b + V c _ V d)
(1)
= Ad*. [s2R3C/(1 + s2R3C )]*(V inP _ VinN)
b and c, d are the two differential amplifier inputs and Ad
is the gain. The high-pass cutoff frequency is defined by
the time constant 2R3C.The detailed circuit is shown in
(Fig. 1b). The input stage consists of operational amplifiers
A1, A2, A3 and A4. A1 and A2 are the main gain stages,
andA3 andA4 are unity gain buffers. As the non-inverting
input voltages of A3 and A4 are equal to their respective
output voltages, resistors R2 and R3 are virtually in
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maximum output voltage error due to the operational
amplifiers’ input offset voltage is
V oo max = (VioA3 max + VioA4 max) *(1 +R1/R2) +
3VioA5 max ~ 2AdVioA3; 4 max
Here V iomax are the maximum offset voltages of the
corresponding operational amplifiers. When selecting
operational amplifiers, the following should be respected: A3,
A4 and A5 to be low input offset voltage and high CMRR
types; A1 and A2 to be of high open-loop gain, high CMRR
and high gain-bandwidth product.
In the signal frequency band, Zd also has an inductive
component (3)
LD = 4R3C /gm=4*1.6 MΩ *1 µF *(R5IIR6) ~ 205 kH
The simulated Ad, differential Zd and common mode Zcm
input impedances for this amplifier (circuit of Fig. 2) are
shown in( Fig. 3).The frequency band is 0.05–100 Hz, as is
usual for ECG amplifiers. The circuit tolerates up to 50 mA
common mode currents and up to about 2V DC differential
signal. The current consumption is 150 mA (0.45 mW) at 3V
supply voltage.
III.PRACTICALAMPLIFIER CIRCUITS
The two-electrode amplifier design was implemented in a
practical circuit shown in (Fig. 2). It is powered by a single 3V
supply voltage. Several operational amplifiers types can be
used, e.g. MCP607, OPA2336 or similar. Because of the input
common mode voltage range, the signal ground is set to one
third of the supply voltage (U4B). The diodes D1–D4 prevent
latch up of the circuit. The inputs are RF noise-protected by
Fig3 Simulated gain, differential Zd and common mode Zcm input
impedances of practical amplifier circuit. (uu) Ad, dB; (ss) ZdO; (, ,) Zcm, O
A sample recording of an ECG signal acquired using a
commercial electrocardiograph* and the proposed ECG amplifier is shown in( Fig. 4). This type of three-channel
electrocardiograph was selected owing to its abilities to record
one lead I ECG synchronously with two ‘experimental inputs’,
where external units can be connected. The trace in (Fig4a)
was obtained by the electrocardiograph own amplifier (lead I)
and, in (Fig. 4b), the signal from the proposed amplifier output
is displayed. Standard stick-on disposable ECG electrodes
were used, two for the ECG channel and two for the tested
amplifier, at 5 cm distance from each other on the arms, plus a
third one for the ECG unit, which required a reference
electrode. The two signals were identical, except for a small
difference in channel sensitivities. Low amplitude
electromyogram signals can be observed in both traces.The
measured CMRR was 60 dB, using 1Vpp 50 Hz common
mode voltage. The measurements were extended for the
frequency range of 3–129 Hz, yielding the same value. In
addition, this value includes common mode input voltage and
input current simultaneously, owing to the low common mode
input impedance (21 KΩ ). The common mode input current
was 48 µA pp.
Eliminating the two current sources at the amplifier inputs
produced CMRR=66 dB. Obviously, the price for the common
mode input impedance reduction (which prevents saturation
by a high level of common mode noise) was the loss of 6 dB
CMRR, mainly owing to non-ideal resistor matching in the
current sources.
Fig. 2 Practical amplifier circuit
The RC networks R7, C4. Its value was derived from the
following Consideration. With
R7 *C4= (R1 I I R2 II R3) C2 ~ R2C2, the high frequency zero in
the amplifier transfer function is cancelled
Ad(s) = V out /(V InP – V inN) = (1/ 1 + sC4R7)* (sC32R3/ 1
+sC32R3) * (1 + R1/ R2IIR3) *1+sC2 (R1IIR2IIR3) / 1 +
sC2R1
(4)
Inserting C5 capacitors ensures the circuit stability. The input
impedances are implemented by two bidirectional modified
Howland voltage controlled current sources (VCCSs),
described in DOBREV and DASKALOV (2002). The VCCS
transconductance can be chosen in the range of 1/20–
1/100 kΩ . Thus a high VCCS output minimum resistance is
ensured for a given resistor tolerance and signal frequency
band. The corresponding input impedance (3) differential and
common mode resistive components including the input RF
filters are
Rd =2(1 /gm+R7) =2(R5IIR6 +R7) ~84 kΩ
Rcm= (1/gm+R7)/2= (R5IIR6+R7)/2~ 21 kΩ
800
The biopotential amplifier. (A) Circuit diagram of the 3-OP
IA. To avoid the performance degradation of the IA, a lownoise low-dropout linear regulator (G914D) was used to
stabilize the supply voltage
Fig. 4 Lead I electrocardiogram of volunteer taken simultaneously by (a)
commercial electrocardiograph and (b) the amplifier of (Fig. 3)
(B) Circuit of the power supply.
The following advantages of this circuit should be pointed out:
(i) The overall gain is ensured by the first stage; thus a high
CMRR is obtained without the use of high-precision resistors
in the second stage
(ii) Additional input buffers are avoided by connecting the low
frequency determining RC network to the inverting inputs of
op amp pair which amplifies the input signal
(iii) Implementing different common mode and differential
mode input impedances achieves two goals:
– improved tolerance to input common mode currents, thus
avoiding saturation even with low supply voltage;
– Low resistive differential impedance component, helping to
minimize and equalize electrode polarization potentials
difference
(iv)Low supply current and power consumption:
150 µA 0.45mW
(v) Acceptable input common mode currents (<50 mA) and
input DC differential voltage (2 V)
This OP was used to split the 3.3 V output of the G914D into
± 1.65 V and provides a low- impedance system ground. The
frequency response is plotted in Fig. C. The pass band is from
0.09 to 800 Hz.
(C) Frequency response of the biopotential amplifier. The measured pass-band
is from 0.09 to 800 Hz (-3 dB).
REFERENCES
[1] Tietze U and Schenk Ch: Measurement circuits.In Electronic Circuits
Design and Application.1990; 767-778.
[2] Neuman MR: Biopotential amplifiers.In Webster JG, editor.Medical
instrumentation application and design. John Wiley & Sons: New York, 1998;
233- 286.
[3] Hamstra GH, Peter A and Grimbergen CA: Lowpower, low-noise
instrumentation amplifier for physiological signals. Med Biol Eng Comput,
1984; 22: 272-274.
[4] Dobrev D: Two-electrode low supply voltage electrocardiogram signal
amplifier. Med Biol Eng Comput, 2004; 42: 272-276.
[5] Amer MB: A design study of a bioelectric amplifier and improvement of
its parameters. J Med Eng Technol, 1999; 23: 15-19.
[6] Spinelli EM, Martinez NH and Mayosky MA: A single supply biopotential
amplifier. Med Eng Phys, 2001; 23: 235-238.
[7].Jefferson CB: Special-purpose OP amps.In Operational amplifiers for
technicians. Breton publishers: 1983; 281-285.
[8] BURKE, M. J., and GLEESON, D. T. (2000): ‘A micropower dryelectrode
ECG preamplifier’, IEEE Trans. Biomed. Eng., 47,
pp. 155–162
[9.] CHEE, J. (2002): ‘Low-frequency high gain amplifier with high DCoffset
voltage tolerance’. US patent, US6396343 B2
IV: RESULTS AND CONCLUSIONS
A high input impedance, high common mode rejection ratio,
fixed gain (* 100) amplifier is proposed for recording
biopotential signals. Practical application of this amplifier for
ECG recording has shown that it has great potential for
recording other biomedical signals. Hence, this amplifier can
be used as a building block at the front end of most biomedical
systems. After some researches, we can use A threeoperational-amplifier (3- OP) instrumentation amplifier (IA)
was designed with fixed gain to provide the characteristics of
high input impedance and high CMRR instead of above
circuit. That is
(A)
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