Vol 57 No 2—May 2023

StudentZone—
ADALM2000 Activity:
Heartbeat Measurement
Circuit
Doug Mercer, Consulting Fellow, and
Antoniu Miclaus, System Applications Engineer

Objective To have a relevant output, the input signal is passed through multiple circuits:
The objective of this lab activity is to learn how to use a chain of amplifiers for gain X Preamplifier: the output signal from the heartbeat measurement setup is
and filtering, in a practical example that aims to recover heartbeat information. decoupled through the series capacitor and amplified using a negative feed-
The result of the system provides a relevant output that is displayed using the Scopy back resistor (R4)
software tool. X Low-pass filter: RC filter that cuts the high frequencies (noise)
Going through this lab activity, the students will learn how to drive an IR LED and X Voltage follower: buffers the output of the low-pass filter and reproduces its
a phototransistor, design and understand the behavior of a low-pass filter, and voltage with a low impedance output
explore functionalities provided by the operational amplifiers (op amps) in X Inverting amplifier with low-pass filter: amplifies the voltage signal and cuts
different configurations.
the high frequencies (noise).
Combining the previously mentioned electronic devices, the result of the activity will
demonstrate how a real-world application can be implemented with minimum soft-
Materials
ware and hardware equipment. X ADALM2000 Active Learning Module
X Solderless breadboard
Background X Jumper wires
A type of heartbeat measurement device consists of an electronic circuit that
X One OP484 precision rail-to-rail I/O op amp
monitors heartbeat by clipping onto a fingertip. It does this by shining light
through your finger and measuring how much light is absorbed. This goes up X One 100 Ω resistor
and down as blood is pumped through your finger. For the operation of an opti- X One 470 Ω resistor
cal heartbeat detector, an IR LED and a phototransistor are used. The LED emits X One 1 kΩ resistor
light through the finger and is detected by the phototransistor, which acts like
X One 10 kΩ resistor
a variable resistor conducting different amounts of current depending on the
light received. X Two 47 kΩ resistors
X Two 1 µF capacitors
The voltage variations change with the heartbeat and are acquired from the col-
lector of the phototransistor. The small signal obtained is used as input for the X One 47 µF capacitor
circuit, obtaining the behavior of a heartbeat detector. X One infrared LED (QED-123)
X One infrared transistor (QSD-123)

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 C3

Vp

Vp
1 μF

R3 R6

10 kΩ 47 kΩ
R1 R2

Vp
470 Ω 1 kΩ

Vp
Vp
C1 U2 R5
U1 R4 U3
47 μF OP284 100 Ω +1
D1 OP284 47 kΩ OP284
AC 500 n I1
D C2

Vn
Vn
Vn
1 μF –1

Basic Model
of Phototransistor

Figure 1. A heartbeat measurement circuit.

Directions Besides filtering, this stage serves also as an amplifier taking as input the current
On your solderless breadboard, construct the heartbeat measurement circuit (IA1), and generating at the output an inverted voltage (VA1) based on the negative
(designed in LTspice®) as shown in Figure 1. feedback resistance (R3):

The LTspice simulation uses OP284s, which is included in the standard set of VAI = –IAI × R3 (3)
LTspice models. The actual circuit is constructed with the quad OP484FPZ from
the ADALP2000 Analog Parts Kit and powered by ±5 V from the ADALM2000 module
Active Low-Pass Filter
(a total supply voltage of 10 V). Active filters contain active components such as operational amplifiers, within
their circuit design. They draw their power from an external power source and
IR LED use it to boost or amplify the output signal. The active low-pass filter principle
To have a proper current that will not damage the IR LED, a resistor needs to be of operation and frequency response is the same as that of a simple RC low-
added in series to limit the current. Varying the value in between the operating pass filter, the only difference being that it uses an op amp for amplification and
range will change the intensity of the emitted signal of the IR LED. The following gain control.
formula expresses the value of the forward current (IF) through the LED, based This first-order low-pass active filter (A2, R4, C2) consists simply of a passive
on the positive voltage supply 5 V (VP), series resistance (R1), and forward voltage RC filter stage providing a low frequency path to the input of a noninverting opera-
drop on the LED (VF): tional amplifier.
VP – VF The filter aims to cut the high frequencies that correspond to the noise signal.
IF = (1) Taking into account that the heart rate does not exceed a value of 180 beats per
R1
minute (bpm) and the dependency between the bpm and the frequency is:
Phototransistor bpm = frequency (Hz) × 60 (4)
To acquire information from the phototransistor (Q1) when it is in contact with the
The result is that frequencies higher than 3 Hz should get cut. The RC low-pass
IR light, a common-emitter amplifier circuit is designed. This circuit generates an
filter is designed for the mentioned frequency value using the formula:
output that transitions from a high state to a low state when the light in the infra-
red range is detected by the phototransistor. The output is created by connecting 1
a resistor (R2), whose value was determined experimentally, between the voltage Fc2 = (5)
2 � R4C2
supply and the collector pin of the component.
The amplifier is configured as a voltage-follower (buffer), giving it a DC gain of
Preamplifier one, AV = 1.
The input signal from the heartbeat measurement setup is fed into a differentia-
The advantage of this configuration is that the op amps’ high input impedance
tor amplifier circuit (C1, A1, R3). The capacitor blocks any DC content, C1, and R3
prevents excessive loading on the filters’ output while its low output impedance
behaving as a high-pass filter with the cut-off frequency FC1 being determined by
prevents the filters’ cut-off frequency point from being affected by changes in
the following formula:
the impedance of the load. While this configuration provides good stability to the
1 filter, its main disadvantage is that it has no voltage gain above one, AV = 1.
Fcl = (2)
2 � R3C1 However, the power gain is very high as the filter stage output impedance is much
lower than its input impedance.

2 StudentZone—ADALM2000 Activity: Heartbeat Measurement Circuit

Final Amplifier with Low-Pass Filter X AC sweep: Connect at the input of the circuit an AC source. Configure the
source to have a magnitude of 500 µV. Observe the output signal in a chosen
The configuration of the final stage represents an AC op amp integrator with
frequency domain (100 mHz to 1 kHz) to determine graphically in which fre-
DC gain control. In simpler words, the circuit has the aim to low-pass filter (R4, C2)
quency range the output signal has the biggest amplification (Figure 3).
the signal from the remaining unnecessary frequencies that are higher than the
maximum frequency of the heartbeat and amplify through the inverting amplifier
the useful signal with a gain (AV ) determined by the ratio between R6 and R5:
R6
AV = – (6)
R5
1
FC3 =
2 � R6C3

Simulation
Figure 3. Output voltage–AC sweep.
Considering the circuit designed in LTspice, two types of simulation are made:
Hardware Setup
X Transient: Connect at the input of the circuit a waveform generation source.
Configure the source to generate a sine with an amplitude of 500 µV, fre- Use the variable positive and negative power supply from the ADALM2000 module
quency of 2 Hz, and 500 mV offset. Observe the output signal amplitude to set to 5 V to power your circuit. Use Scope Channel 1 to monitor the voltage at the
determine graphically the total gain of the circuit (Figure 2). collector node of VOUT.
The circuit implemented on the breadboard should look similar to the one
in Figure 4. The blue LED represents the IR LED, and the gray one represents
the phototransistor.

Procedure
Put the top of your finger between the IR LED (D1) and the phototransistor (Q1). The
emitter and the receiver should be aligned and pointing one to another.
Observe the voltage waveform seen at the output of the third stage op amp (A3).
An example of the output waveform is presented in Figure 5.
Figure 2. Output voltage–transient analysis.

Vp Vn 1+ 1– GND

IR LED

1 μF

470 kΩ

47 kΩ
47 kΩ
Phototransistor OP484

10 kΩ
47 kΩ
100 kΩ
1 kΩ

1 μF

Figure 4. A breadboard heartbeat measurement circuit.

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 Questions:
1. Using the values and formulas provided in the laboratory directions compute
the following parameters:
X Forward current through the IR LED (use the data sheet of the QED-123)
■ Cut-off frequency of the high-pass filter
■ Cut-off frequency of the second stage low-pass filter
■ Cut-off frequency of the third stage low-pass filter
■ Gain of the third stage amplifier

Figure 5. A heartbeat output waveform. X What parameters change if R5 is modified?

In the oscilloscope feature of the Scopy tool, activate the measure feature to read X What parameters change if R6 is modified?
the frequency of the obtained signal. To convert the frequency into bpm, use the You can find the answers at the StudentZone blog.
formula from the laboratory directions.

About the Author

Doug Mercer received his B.S.E.E. degree from Rensselaer Polytechnic Institute (RPI) in 1977. Since joining Analog Devices in 1977, he
has contributed directly or indirectly to more than 30 data converter products and holds 13 patents. He was appointed to the position
of ADI Fellow in 1995. In 2009, he transitioned from full-time work and has continued consulting at ADI as a fellow emeritus contribut-
ing to the Active Learning Program. In 2016, he was named engineer in residence within the ECSE department at RPI.

About the Author

Antoniu Miclaus is a system applications engineer at Analog Devices, where he works on ADI academic programs, as well as embed-
ded software for Circuits from the Lab®, QA automation, and process management. He started working at ADI in February 2017 in
Cluj-Napoca, Romania. He is currently an M.Sc. student in the software engineering master’s program at Babes-Bolyai University and
he has a B.Eng. in electronics and telecommunications from the Technical University of Cluj-Napoca.

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