BSL PRO Signal Processing Breadboard Labs
www.biopac.com
© BIOPAC 2005 5.4.06
This is a PREVIEW of the Signal Processing Breadboard Labs...CLICK TO REQUEST LABS & SCHEMATICS
Lab 1: Square Wave Oscillator
Lab 6: QRS Detection: Band Pass Filter video 3
Lab 2: Instrumentation Amplifier video 1
Lab 7: QRS Detection: Absolute Value Circuit video 4
Lab 3: High Pass Active Filter
Lab 8: QRS Detection: Low Pass Filter and
Overall System Test video 5
Lab 4: Active Gain Block and Low Pass Filter
Lab 5: Notch Filter for 50/60 Hz Rejection video 2
The Signal Processing Labs use the BIOPAC SS39L Signal Processing Breadboard, the SS39L Interface
Cable, SS60L Interface Cable, and the BSLTCI-22 Breadboard Electrode Interface.
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BSL PRO Signal Processing Breadboard Labs
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© BIOPAC 2005
Signal Processing Background
The human body can be studied by viewing the potential difference (Voltage) between strategically placed
surface electrodes. Because surface electrodes are an easy, non-invasive way to obtain information about the
body, they represent a good starting point for examining the types of electronic circuits used in Biomedical
Engineering.
In order to view and record the potential difference between surface electrodes, electronic circuits are used.
Electrode leads, which are essentially wires, connect the electrodes to the circuit. Most signals from
physiological activity have small amplitudes and must be amplified and processed before they can be
viewed in a meaningful way. The good news is that the characteristics of biopotential amplifiers are much
the same as any other amplifier. We will review the basics of amplifiers, with special emphasis on the
biopotentials.
1. Gain. Physiological signals have amplitudes that range from several microvolts to a few millivolts.
To drive display and recording equipment, most biopotential amplifiers have gains of 500 or greater.
It is useful to use the decibel form of gain, which is obtained from linear form by the formula:
Gain( dB) = 20 log10 (Linear Gain)
2. Common-mode rejection (CMR). The human body makes a reasonably good antenna, and will
create electric potentials from electromagnetic radiation present in the atmosphere. A serious
problem is 50/60 Hz radiation - present almost anywhere there is electric power. The problem
become acute when the biopotentials we wish to monitor have useful energy in the 50/60 Hz range.
CMR is the property of canceling any signals that are in common to both inputs, while amplifying
differential signals (a potential difference between the inputs). Both AC and DC CMR are important
for physiological signals. CMR is usually specified for a common-mode voltage change at a certain
frequency. The common-mode rejection ratio (CMRR) is first obtained:
CM
CMRR = AD (VVOUT
)
where
AD is the differential gain of the amplifier
VCM is the common-mode voltage present at both inputs of the amplifier
VOUT is the output voltage result when the common-mode inputs are applied
The logarithmic conversion of CMRR, common-mode rejection is defined as:
CMR(dB) = 20 log10 (CMRR)
3. Frequency response. The bandwidth of a physiological amplifier should accurately amplify all the
frequencies of importance in the signal, while rejecting those signals outside the bandwidth of
interest. The bandwidth is defined as the difference between the low frequency cutoff and high
frequency cutoff. The cutoff is defined at the point where the gain is 0.707 of the midpoint gain of
the response, and is alternatively called the half power point ( 0.707 2 = 0.5) , or –3dB point, since
− 3.01(dB) = 20 log10 (0.707) .
4. Input impedance. A fundamental rule of measurement is to not allow the measuring device influence
the signal under observation. An amplifier should exhibit high input impedance so as to not
measurably attenuate physiological signals being measured. In the case of the ECG, the ECG
electrode itself has low impedance, but skin impedance can range from 100 ohms to 1M ohm.
Amplifier input currents cause potentials across the skin impedance that are amplified by the gain of
the amplifier, causing large DC offsets in the amplifier output.
5. Noise and drift. Unwanted signals that contaminate physiological measurements, noise produced
within amplifier circuitry is generally defined as those signals with components above 0.1Hz, while
drift refers to the changes in baseline below 0.1Hz. Noise can be measured in microvolts peak to
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BSL PRO Signal Processing Breadboard Labs
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© BIOPAC 2005
peak ( μV p − p ) or microvolts root mean square ( μVRMS ). Sources of drift include offset voltage drift
(varying input impedance), and gain drift, usually affected most by temperature.
6. Electrode polarization. Electrodes made of metal, and used with a electrolyte, such as the standard
ECG Silver/Silver chloride electrode, form small potentials resulting from ion electron exchange
between the electrode and the electrolyte (as in a battery). The challenge for the amplifier designer
is to amplify the weak physiological signals in the presence of these polarized dc signals.
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ECG Background
The generation of electrical activity in the heart is characterized by mechanical events. During the period of
diastole, the heart rests between beats, and assumes its maximum size while filling with blood that has been
oxygenated by the lungs and venous blood from the body. Mechanical activity in the heart is called systole
and is initiated by contraction of muscles surrounding the atria by electrical stimulation. The stimulations of
the sinoatrial node (SA node), a bundle of nerves located in the right atrium, start the heartbeat and set the
frequency of cardiac rhythm. This rhythm can be modified by nerve fibers external to the heart that function
to control the hearts response to increases or decreases in the body’s demand for blood. Contractions of
muscles comprising the atria are stimulated by impulses generated by the SA node.
Impulses from the SA node are conducted along
nerve fibers in the atrium to depolarize the
atriovetricular node (AV node). AV node
stimulation causes contraction of the muscles
comprising the ventricles via the bundle of His and
the Purkinje conducting system. The depolarization
and repolarization of the SA node is followed by the
depolarization and repolarization of the AV node –
this is the electrical control system that initiates the
muscle contractions necessary to maintain the
heart’s pumping action. This nerve system generates
the external action potentials known as
electriocardiogram (ECG), which can be recorded
by electrodes at the surface of the body.
ECG waveform and heart function (from Biophysical
Measurements, P. Strong, Tektronix Inc.)
It is important to understand the basic functions of the heart as shown by the ECG waveform. The QRS
spike is associated with the rapid depolarization of ventricular muscle immediately preceding its contraction.
The P wave is the result of atrial depolarization and the T wave is caused by ventricular muscle
repolarization.
Monitoring this electrical generator, enclosed in a torso, is the function of electrocardiography. By attaching
electrodes to certain places on the body, the small electrical potentials on the surface are sensed. The
potentials can then be amplified, conditioned and displayed to give a representation of the heart’s electrical
activity. Assuming we will be using the basic frontal plane cardiac vector – a standard placement of
electrodes – we can construct a simple ECG monitor to record the potentials.
Circuit requirements:
The ECG QRS spike can range from 400 uV to 2.5 mV peak, and will require a voltage gain of 100
to 1000. ECG bandwidth has been standardized to make interpretation of the results uniform. Two
filters with 3dB cutoffs are used – a high pass filter at 0.05Hz and a low pass filter at 100Hz. A
60Hz (or 50Hz) notch filter is used to attenuate nominal mains interference.
Although the waveform of the ECG is considered as a low frequency AC signal, there can be
significant DC offsets between the electrodes on the body (0 to ±20mV DC). With a voltage gain at
+1000, a DC offset of 20mV will cause an amplifier to try to produce a signal at 20.0V, higher than
typical power supplies permit. We must prevent DC offsets from swamping the AC signal of
interest, and we do this with a high pass filter.
Signals present on the electrodes higher than 100Hz contribute noise to the ECG that must be
reduced to present an accurate view of the ECG. Potentials generated from muscle activity are
undesirable in the ECG and are partially reduced with the low pass filter (it is required that a person
being monitored for ECG remain relaxed and motionless.)
There are numerous other potentials that are inadvertently amplified when using electrodes to
monitor ECG. The mains power produces very high levels of EMI interference, which must be
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BSL PRO Signal Processing Breadboard Labs
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© BIOPAC 2005
carefully eliminated. The use of shielded electrodes, coupled into a differential amplifier, can reduce
EMI mains interference dramatically. Other sources of EMI include radio stations, cell or portable
phones, microwave sources, computers, and automotive ignition. We will not attempt to squelch all
these sources of interference, although you may experience them in your circuits.
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BSL PRO Signal Processing Breadboard Labs
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Breadboard Setup
The SS39L Breadboard is designed to rapidly prototype the ECG circuitry without the use of soldering
equipment.
Close examination of the SS39L Breadboard
reveals that there are five individual boards
mounted on the plastic base. The top bus strip
has a row of two power bus rows. Next is a
strip that contains a 5x64x2 connection array,
designed to easily accept 0.3” Dual In Line
(DIP) integrated circuits (IC). Beneath the
5x64x2 strip is another power bus strip
identical to the top strip. The BIOPAC
lessons do not use or refer to the bottom two
boards.
The power busses are not labeled on the
SS39L, so for safety and reliability, add the
following labels:
•
•
•
•
Top Row: ‘-5’
Row above row A: ’G1’
Row below row J: ‘G2’
Next Row: ‘+5’
The power strips must be wired so that the top bus is connected to -5.0V (Green terminal), the bus above
row A is GND (Black terminal), the bus below row J is also GND (Black terminal), and the last bus is
connected to +5.0V (Red terminal).
Needle nosed pliers are recommended for inserting wires into the breadboard.
1. Connect a Brown 1.0” jumper from the Black terminal (GND) to the bus above Row A (G1).
Connect a Red 2.0” jumper from G1 to G2.
2. Connect a Yellow 4.0” jumper from the Red terminal (+5V) to the bus +5.
3. Connect a Brown 1.0” jumper from the Green terminal (-5V) to the bus -5.
4. Each bus is actually divided into two half busses. Bridge all the busses by adding four 0.3” Orange
jumpers as shown below.
Next, place all the integrated circuits and wire them to the power busses, adding the decoupling capacitors.
1. Place the LMC6484A with pin 1 in row F, column 55
2. Place an LM324 with pin 1 in row F, column 35
3. Place an LM324 with pin 1 in row F, column 24
4. Place an LM324 with pin 1 in row F, column 13
5. Connect four 0.01uF capacitors with one end in pin 4
of each IC, the other end to G2 bus.
6. Connect four 0.01uF capacitors with one end in pin 11
of each IC, the other end to G1 bus.
7. Check all capacitors for short circuits.
8. Place the BSLTCI-22 Breadboard Electrode Adapter
with pin 1 in row A, column 48.
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BSL PRO Signal Processing Breadboard Labs
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© BIOPAC 2005
Power Verification
BME ECG Board Layout with Integrated Circuits
The following steps will apply +5V and -5V isolated power to the breadboard from the MP35. The MP35
power is limited to about 100ma. If the MP35 power is overloaded by the breadboard circuitry due to
improper wiring, the MP35 may not perform acquisitions properly.
1. If inserted, remove the DB9 end of the SS39L cable from the MP35, and turn off the MP35.
2. Plug in the SS39L Cable Black (GND) banana plug into the Black terminal jack.
3. Plug in the SS39L Cable Red (+5) banana plug into the Red terminal jack.
4. Plug in the SS39L Cable Green (-5) banana plug into the Green terminal jack.
5. Plug in the SS39L Cable GND Reference pin into the breadboard GND bus G1.
6. Plug in the SS39L Cable SIGNAL pin into the breadboard GND bus G1.
7. Plug in the SS39L Cable DB9 plug into CH1 of the MP35.
8. Plug in the SS60L Cable DB9 plug into CH2 of the MP35.
9. Plug in the SS60L Cable SIGNAL pin into the breadboard GND bus G1.
10. Plug in the SS60L Cable GND pin into the breadboard GND bus G1.
11. Turn on power to the MP35. Verify 5V(±5%) and -5V(±5%) power buses with a DVM.
•
If at any time during the experiments the measured power busses drop below their rated
values, turn off the MP35 and find the cause of the circuit overload in your breadboard
wiring.
12. Start BSL PRO. Make a sample reading of the MP35 (Start). Verify that the reading is ~0.00V DC.
If there is a problem with power that causes the MP35 startup routine to fail, the Busy light on the
MP35 front panel will flash an error code sequence, and will not allow a recording to occur. The
Busy light normally comes on for 1 or 2 seconds, blinks once for USB renumeration, stays on for
10-20 seconds for self-calibration, then turns off. The Busy light then monitors USB data transfers.
This is a PREVIEW of the Signal Processing Breadboard Labs...CLICK TO REQUEST FULL LABS & SCHEMATICS
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