4.40 MOTHER BOARD RR-115 ID 5010-115
RIELE BERLIN Photometer 5010 / V4 4.40.5 22.09.2003
Amplifier for photocurrent and ADC
The photocurrent of the photodiode is converted with shortcircuit application into a voltage signal with the aid
of the amplifier IC11 (AD 711). Potentiometer P4 sets the amplification and potentiometer P3 sets the offset
voltage of the amplifier stage.
An voltage / frequency converter IC13 (AD654) is connected to the output of the amplifier.
The microcontroller can set the amplification of the amplifier stage by the multiplexer IC12 (ADG428). The
eight different settings of amplification are called V0 up to V7. The setting V0 is equivalent to the lowest
amplification.
The micocontroller converts the frequency signal of the voltage / frequency converter to a digital result in a
range between 0 and ~ 600.000. 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.
• dependent on the used cuvette adapter
• must be positiv and <> 0.
• is set with potentiometer P3.
The following diagramm shows the ADC-counter values versus the photocurrent.
ADC
photo current
~ 600.000
0
offset
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