The excitation LEDs are biased by an LED driver IC
(MAX1916, Maxim Integrated) that provides the required
current, and a microcontroller (MSP430, Texas Instruments)
that generates a periodic pulse signal to modulate the LED
current for ambient noise rejection. A transimpedance
amplifier is used to convert the output current of the
photodetector into a voltage. Then, a low pass filter acts as a
demodulator by removing the high frequency harmonics in the
output signal. The output of the analog front-end circuit is a
slowly varying signal, the amplitude of which varies with the
absorption of light into the sample flowing in the channel. The
internal analog-to-digital converter of the microcontroller is
used to digitize this output signal. After digitization, the raw
data is sent through a serial link to a PC for processing in
MATLAB. Alongside these parts, there is a power
management unit which provides the voltage references using
a low-dropout (LDO) voltage regulator (TLV70230, Texas
Instruments). Since a common voltage source (battery) powers
all the circuits in this platform, the LDO has a high PSRR to
avoid any noise or disturbance caused by LED pulsation on the
output signal.