4.40 PCB RR-120 MOTHERBOARD REF
5010-120
Amplifier for photocurrent and ADC
The photocurrent of the photodiode is converted with short circuit application into a voltage signal with the aid
of the amplifier IC5 (AD 711). Potentiometer P3 sets the amplification and potentiometer P4 sets the offset
voltage of the amplifier stage.
A voltage / frequency converter IC9 (AD654) is connected to the output of the amplifier.
The microcontroller can set the amplification of the amplifier stage by the multiplexer IC10 (ADG428). The
eight different settings of amplification are called V0 up to V7. The setting V0 is equivalent to the lowest
amplification.
The microcontroller converts the frequency signal of the voltage / frequency converter to a digital result in a
range between 0 and ~ 600000. This result is called value of the ADC.
The value of the ADC in the case of photocurrent = 0 (darkness) is called offset. The offset is
•
dependent on the setting of the amplifier stage,
•
positive and <> 0,
•
set with potentiometer P4.
The following diagram shows the ADC-counter values versus the photocurrent.
ADC
photo current
~ 600.000
0
offset
RIELE BERLIN
Photometer 5010 V5+
4.40.6
17.03.2009
