December
2004, Issue 173
Light-to-Frequency
Conversion (Part 1)
TSL230R-Based
Pulse Oximeter
by
Jeff Bachiochi
LIGHT
ABSORPTION
My
sisters and I rarely got to play with flashlights as
kids. But as dusk fell on Halloween, we were given a
flashlight to guide our way between tricks and treats.
The light beam often found its way into our mouths as
we tried turning our heads into jack-o’-lanterns. Glowing,
red cheeks seemed appropriate on All Hollow’s Eve. When
light passes through your body, your bones, tissues,
and fluids absorb a lot of it. Comparing the intensity
of the light that makes its way out (XOUT) to that which
comes in (XIN) gives you the percentage of light that
has passed through your body. Subtracting this percentage
from 100% gives you the percentage of light absorbed
by the body.
Arteries
and veins carry blood throughout the body. They expand
and contract with each heartbeat. During the systolic
phase, the heart contracts, pushing blood into arteries,
capillaries, and veins. Blood flows back to the heart
from the veins during the diastolic (resting) phase.
As the arterial system expands and contracts, it affects
the level of light absorption. This adds an AC component
to the background absorption (or DC level), as you can
see in Figure 1. The DC and AC levels might change drastically
depending on where on the body the measurements are
taken. Earlobes and fingers used most often because
they are relatively thin and easily accessed.
|
(Click
here to enlarge)
|
Figure
1—Check out how various tissues and bones can absorb
light transmitted through the body. As blood flows
through the circulatory system, it changes density
because of the heart’s pumping pressure. This change
also changes the absorption rate of light, effectively
modulating the light absorption. The total light
absorption is a combination of modulated and constant
absorption, which is similar to a small AC noise
riding atop a DC voltage. |
Using
a photodiode to measure intensity requires fairly high
amplification. Care must be taken to minimize the noise
in the external circuitry necessary to create an analog
signal large enough to be read with an A/D converter.
A light-to-frequency converter can replace most of the
sensitive circuitry as the light-monitoring device for
the sensor application.
