Fresenius Medical Care 4008 E Technical Manual page 580

Conductivity Measurement
The portion of the circuit with IC 20,21,30 as well as the transistors T12 and T13 are used for CD
measurement and are identical with the circuits on LP 633. The measured value is read in at the
analog input of the controller on AN4.
Both the level and the conductivity circuits are supplied via line COND_C1 from the oscillator on
LP 633.
This means that these circuits can also be tested during detuning in the T1-test.
Reference Voltage Monitoring
D 12 is an additional reference voltage diode of 1.235 Volt which is connected with the analog
input AN2 of the controller. U_REF and REF_ADC are compared with each other. A deviation of
the voltage of REF_ADC from the nominal value by > ±2.5% is considered to be an error.
In addition, the hardware monitors the reference value for ± 5 % via comparators IC 29a and 92b
and the voltage divider, consisting of the resistors R 99, 100, 101 and 149.
After switching on, the comparators are T1 tested.
The transistors T 10b and 10a are alternatively switched to conducting via the signals TEST_H
and TEST_L, and the upper or lower part of the voltage divider is bridged with R97 or R98. This
is equivalent to a U_REF change of ± 10 %. The respective open collector output of each
comparator goes to L level and the REF_FAIL signal is read in on P1.5 of IC 9.
The reading in of the REF_ADC voltage and the testing of the comparator stage is performed only
once after switching on (T1-test !!). The output of the comparator stage is continuously monitored
(refer to T0 monitoring „2.2.4.3 Reference Voltage Monitoring").
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Fresenius Medical Care 4008 4/09.03 (TM)