Datasheet S6846 (Hamamatsu) - 4

Block diagram and internal functions (S6986, S6846, S4282-51, S7136) Light modulation photo IC S4282-51, S6809, S6846, S6986, S7136/-10, S10053/-01DT Block diagram and internal functions
Constant
voltage Vcc Buffer
Comparator PD
Preamp
Vref Signal
processing
circuit Timing
generator Oscillator Vout
Output
circuit LED
Driver Cathode
(LED) GND Truth table
Input Output level Light on low Light off high
KPICC0002EA (a) O scillator and timing signal generator
The oscillator produces a reference oscillation output by charging and discharging the built-in capacitor with constant current. The
oscillation output is fed to the timing signal generator, which then creates LED drive pulses and various timing pulses for digital signal
processing.
(b) L ED driver circuit
KPICC0002EA
This circuit drives an external LED using the LED drive pulses created by the timing signal generator. The duty cycle is 1/16.
(c) P hotodiode and preamplifier circuit
The photodiode is formed on the same monolithic chip. A photocurrent generated in the photodiode is converted to a voltage by a
preamplifier circuit. The preamplifier circuit uses an AC amplifier to expand the dynamic range versus DC or low-frequency background
light, without impairing signal detection sensitivity.
(d) C apacitive coupling, buffer amplifier and reference voltage generator
Capacitive coupling removes low-frequency noise and also cancels the DC offset in the preamplifier. The buffer amplifier boosts the
signal up to the comparator level, and the reference voltage generator produces a comparator level voltage.
(e) C omparator circuit
The comparator circuit has a hysteresis function to prevent chattering caused by small fluctuations in the input light.
(f) S ignal processing circuit
The signal processing circuit consists of a gate circuit and digital integrator circuit. The gate circuit discriminates input pulses during
synchronous detection, to prevent operational errors caused by asynchronous background light. Background light which is synchronized
with the signal detection timing cannot be eliminated by the gate circuit, but is canceled out by the digital integrator circuit at the
latter stage.
(g) Output circuit
This circuit serves as an output buffer for the signal processing circuit and outputs the signal to an external circuit. 4