Potentiometric Circuits; Hematocrit/Fluid Detector Circuits - Bayer HealthCare RapidPoint 400 Series Service Manual

Potentiometric Circuits

All the ion selective electrodes (ISEs) are potentiometric. Each ISE
contains a membrane that isolates the electrode from the sample and that
selectively passes the desired ion from the sample to the electrode. The
change in potential difference between the ISE output voltage and the
reference electrode output voltage is proportional to the logarithm of the
concentration of the selected ion in the sample.
The eight amplifiers in U1 through U4 are configured as a differential
amplifier, with the signal from the reference electrode subtracted from all
the other signals. REF_VO connects to pin 5 of U4 through R23.
R23 protects U4 from electrostatic discharge.
The feedback voltage at the inverting input of the respective amplifier
completely guards each ISE output. This prevents leakage currents from
corrupting the high impedance signal at the ISE output.

Hematocrit/Fluid Detector Circuits

The Contact board has two hematocrit/fluid detector circuits. These
circuits are conductometric; that is, they measure the electrical
conductance (resistance) of the fluid appearing between the HCT_HS and
HCT_LS terminals. Because both circuits are nearly identical, a
description of only the hematocrit 1 circuit is provided.
The signal labeled DRIVE is a +5V peak-to-peak square wave. R61 and
C43 transforms this voltage into a current source with an amplitude of
approximately 16.67 µA peak-to-peak at the same duty cycle and
frequency as the DRIVE signal. This current is then applied to the sensor,
and a differential voltage results between the HCT_HS and HCT_LS
terminals from the excitation current flowing through the fluid.
The conductance (resistance) of the fluid in the sensor is determined using
a known current and a measured voltage.
When the excitation current goes to its positive state, all of the switches are
toggled. This action effectively disconnects C44 from ground and connects
it to C23 instead. This connection allows the charge stored in C44, which
represents the peak-to-peak voltage excursion at the HCT_HS terminal,
to C23.
Rev. C Electronic System Overview 75