ELECTRONIC DESIGN / DECEMBER 15, 1997より引用
W. STEPHEN WOODWARD
Venable Hall, CB3290, University of North Carolina,
Chapel Hill, NC 27599-3290; e-mail: woodward@net.chem.unc.edu.
Plenty of noninvasive methods exist for electronically sensing the
human heartbeat. The job can be
done acoustically (stethoscope or
Doppler), mechanically (sphygmo
manometer), electrically (EKG), and
optically. One handy optical technique presented here exploits the
fact that tiny subcutaneous blood
vessels (capillaries) in any patch of
skin (fingertip, ear lobe, etc.) furnished with a good blood supply, alternately expand and contract in time
with the heartbeat. An ordinary infrared LED/phototransistor pair can
sense this rhythmic change as small
but detectable variations in skin contrast (Fig. 1, upper half).
When gently held against the skin
(too much pressure will flatten the
surface capillaries and suppress the
pulsation effect), some of the radiation from D1 reflects back into Q1.
Q1’s photocurrent produces an ac signal across Q2 and Q3 of ≈500-µV p-p
for every 1% change in skin reflectance. This logarithmic relation
ship is constant over many orders of
magnitude of photocurrent. Consequently, reliable circuit operation is
possible despite wide variations in
skin contrast and light level. A1 and
the surrounding discrete components
comprise a high-gain adaptive filter
that rejects ambient optical and electrical noise (mostly 60-Hz pickup) and
presents a cleaned-up signal to comparator A2 so that it can extract a
digital pulse-rate signal.
A2’s TTL/CMOS-compatible output is suitable for direct input to a digital period-measurement circuit, and
for such applications, that’s all that’s
needed. But for some simple heartrate display situations, an analog representation of pulse rate is convenient.
So the lower half of Figure 1 illustrates an unusual, “zero-ripple,” frequency-to-voltage converter (FVC)
uniquely optimized for human pulserate measurement.
Most FVCs are characterized by an
unavoidable trade-off between response time and output ripple. Usually, in order to have an acceptable
output ripple of the order of a few percent, the settling time of the converter
must be at least ten periods of the lowest expected input frequency. Normal
human hearts usually beat at rates
within the 4:1 range of 50 to 200 beats
per minute (bpm) ≈0.83 Hz to ≈3.3 Hz.
A conventional frequency-to-voltage
converter (FVC) would therefore
need an unpleasantly long ( >10 second) output settling time. Some converters avoid this limitation but tend
to be complex (ELECTRONIC DESIGN,
Feb. 21, 1994, p. 115).
The relatively simple “instant-settling” FVC in Figure 1 employs a period-to-rate approximation trick that
works well in this application. To understand the idea, consider S1, which
is arranged to alternately switch C7
between ground and A3’s summing
point. When a rising edge from A2 at
S1 pin 9 connects C7 to A3 and C2, the
resulting transfer of charge into C2
causes A3’s output to slew positive until clamped by Q6. Adjusting R3 trims
the clamp voltage and thereby sets
full-scale calibration for the circuit.
C2 immediately begins to discharge
back toward zero, but it doesn’t do so
linearly. Instead, R6, R7, and Q7 synthesize a composite exponential curve
V3 (Fig. 2)that, from 285 (210 bpm) to
1250 ms (48 bpm), is a good approximation (within 5%) of the reciprocal
relationship between pulse period and
pulse rate. Thus, each time the digital
signal from A2 returns high, V3 will
equal the reciprocal of the time
elapsed since the previous transition
and, therefore, the actual instantaneous heart rate. S2, S3, and C1 then
transfer V3 to sample/hold A4 for continuous output as V4 = 10 mV/bpm. If
C1 = C4, only one charge transfer is
needed for instant convergence.
However, normal hearts have significant beat-to-beat aperiodicity that
sometimes causes V3 to jump around
quite a bit. If desired, choose C1 < C4
to provide a degree of signal averaging and thus smooth this effect out. Q8
provides Vbe and temperature compensation for the Q6 and Q7 breakpoint voltages. Thus, these transistors
should be thermally coupled for good
tracking. Notice that with an appropriate change of time constants, this
circuit also has general potential as an
optical tachometer for other difficult
low-contrast applications.
