PPA Peripheral pulse amplifier


The PPA coupler is designed for the photo-plethysmograph transducers PT1 and PT2. It may also be adapted to work with the Geer Gauge. Such transducers obtain the peripheral pulse signal by emitting light into the skin and picking up the amount of light reflected back. As the blood flows, reflected light is modulated. PSYLAB provides two such transducers, PT1 and PT2 measuring peripheral pulse from the ear or finger.

The amplitude of the resulting wave-form may be quantified to obtain an indication of changes in pulse volume, which may result from vasal constriction, flushing and is also affected by blood pressure. The SAM unit provides an internal cardio-tachometer function producing analog output signal proportional to Inter Beat Interval (inversely proportional to Heart Rate (HR). Pulse signal is found on pin 5 of the SAM analog socket and the cardio-tachometer signal on pin 4.


Gain adjustment. Range setting determines the maximum scale range for the pulse signal. Larger scale means lower gain - i.e. the pulse signal


is amplified less so that the output voltage fits to the range of the A-D converter. Scale setting should be adjusted so that the pulse signal is about half the available display range, so that pulse amplitude may vary in size without exceeding the range, yet is not

too small to easily evaluate. The setting of the scale switch is recorded by PSYLAB software, as well as the actual pulse signal, allowing amplitude comparison to be made between records made at different scale settings.

High pass filter adjustment. The high pass filter has the purpose of ensuring that the pulse signal remains centred between the extremes of the scale range. The transducer picks up very small changes in blood volume, which indicates the wave of blood flowing from the heart, but is also affected by changes in blood pressure. Blood pressure may change for a number of reasons, but the most obvious is as a result of the subject moving: particularly if the hand is raised or lowered large changes in blood pressure occur at the finger.

Such blood pressure changes result in displacement of the oscillating pulse signal away from the centre of the scale range. If the signal reaches the limit of the range (signal clipping), pulse volume information is disrupted. The high pass filter has the effect of continually pulling the signal back toward the


centre. If it is set to a higher frequency setting, e.g. 3Hz, it does this more quickly. At this setting, it would take about one second for the signal to be re-centred if it were fully displaced to one of the extremes.

Unfortunately, there is a disadvantage to setting the high pass filter to a high frequency. The ‘pulling’ effect on the signal occurs to all displacements away from the centre, including the pulse signal. If the shape of the pulse signal is important, the 3Hz setting will significantly distort the pulse wave. For better wave definition, where pulse volume is to be evaluated, the longer high pass setting of 0.1Hz may be recommended.

In practice, a compromise may be needed, weighing experimental design factors such as how likely the subject is to remain still and thus the signal to remain well centred, against the ideal setting for minimum distortion.

If the only purpose of the pulse measure is to obtain Heart Rate, the 3Hz setting may always be used as distortion of the wave-shape will not affect operation of the cardio-tachometer (Interval Timer).

Low pass filter setting. The low pass filter is required for technical reasons in digitizing signals.


When setting up the computer software, a decision is made about the ‘sample rate’ - how often the digitizer will evaluate the pulse signal. It is necessary to ensure that the low pass filter is set to a frequency at most about one third of this sampling frequency. For example, if the sampling frequency is set to 100Hz ( 100 samples per second, rate 100, or 10mS sampling interval), the low pass filter should not be set higher than the 28Hz position. If a sample rate were 45Hz, the 28Hz setting could be used, and if the rate were as low as 20Hz, the 7Hz position would be used. As the pulse wave itself is a low frequency phenomenon, it may be acceptable to use the 7Hz setting all the time, regardless of which of the above sample rates were used.

The low pass filter is a 24dB per octave Bessel characteristic design, optimized for minimum phase distortion.


Low pass filter (anti-aliasing): 24dB per octave

High pass filter:  6dB per octave

Gain:  5 positions

Output Voltage: (adjustable) normally +/- 5 V

Subject Isolation: 6000V

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