Fresenius Medical Care 4008 E Technical Manual page 383

Analog signal processing (circuit diagram 4+5/7)
Dialysate pressure transducer (sheet 4)
The bridge voltage of the pressure transducer applied to the contact strip X633/R, pin 28c and pin
30c, is amplified by IC 33 (5, 6, 7 and 1, 2, 3) (OP1 and OP2). The zero-point offset can be
adjusted by means of the DIAL_DET_ADJ signal (from output board DAC) via R183. The output
voltage is proportional to the pressure applied. The supply voltage (between 4 V and 5 V,
depending on the solder strap selected) for the pressure transducer is generated by IC 32 (1, 2,
3) and transistor T1.
Temperature (sheet 4)
The circuit provided for measuring the temperature of the dialysate by means of the monitor NTC
(3) constitutes a modified inversion adding stage with IC 32 (5, 6, 7). The nonlinearity of the
monitor NTC (3) is compensated by resistor R55. The conversion from one range of measured
values to the other is performed by transistors T8 and T6. For the hot rinsing procedure, the value
of the amplification as well as of the offset of the circuit is modified. For the test procedure, the
output signal is modified by means of resistor R56. This signal is also used for calibrating the
temperature offset.
Conductivity (sheet 5)
Together with R112, C69, C71, and R113 as well as with R110, R111, and the channel resistor of
transistor T4, IC 34 (5, 6, 7) constitutes an amplitude-controlled 1-kHz Wien bridge generator.
The amplitude of the sinusoidal oscillation is controlled at transistor T4 via comparator IC 34 (1,
2, 3) as well as diode D1, R107 and R108. The cell current at pin X633R/27c is rectified via diodes
D3, D4, D5, and D6. The differential voltage is measured at R114 and amplified by IC 39 (1, 2, 3).
R114 is also used to measure the currents of the oscillator and the control unit, compensated by
R120. The ripple differential voltage and any possible superimposed common-mode AC voltage
is filtered by capacitor C72. The initial range from 0 to 15 mS/cm (actual conductivity) is
suppressed by resistors R121 and R126. The input voltage is amplified by IC 39 (5, 6, 7) (OP2) to
an output voltage ranging from 0 to 10 V. Further filtering is accomplished by capacitor C73.
Diode D2 prevents negative selections of the circuits connected to pin X633L/8b. In order to test
both the lower and the upper CD alarm limit, a voltage (0 to 10 V) is applied to pin X633L/31b, so
that the amplitude of the measuring signal will be modified.
Level sensor (sheet 6)
IC 36 constitutes the integrated fluid level indicator LM 1830. This IC comprises a circuit, which is
used to differentiate between two conditions:
– presence of fluid
– absence of fluid
The circuit is provided with two sensing electrodes, to which an AC voltage generated in IC 36 is
applied. A comparator circuit then compares the resistance of the fluid with an internal resistance
or a reference resistance. With the resistances being different from each other, a frequency is
generated in relation to the difference in the resistances. This causes the output of IC 36, pin 12,
to turn to L level. Resistor R25 constitutes the reference resistance of the fluid. The 6-kHz
oscillator frequency is determined by means of C43. In order to prevent the electrodes from
becoming galvanized, the two capacitors C62 and C60 are used as coupling capacitors.
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Fresenius Medical Care 4008 4/09.03 (TM)
Attention
When operated with de-ionized water (osmosis water), the hemodialysis machi-
ne can constantly remain in the prime program during preparation for hemodia-
lysis, until a specific basic conductivity value has been achieved.