CMRR 3.0E | User Manual
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voltage divider and voltage measuring circuit to measure the common mode voltage Vc after 100pF
automatically.
The 100pF also creates a problem with the position of the patient cable. If the cable is allowed to sit on an
earthed plane, the stray capacitance can be enough to provide additional loading and reduce the actual
common mode voltage. This stray capacitance is highly variable and thus can impact the test. For
conservative tests, the cable should be supported off the earth plane, but remain above the earth plane
in order to minimize noise.
A sine wave signal generator is built in CMRR 3.0E. It offers 2 Vrms signal required by EEG standard
(IEC80601-2-26:2019) and the test setup becomes easy and time-saving.
Common mode rejection is also dependent on imbalance impedance test. For this reason, the test also
introduces an imbalance of 10
kΩ//47nF in one lead only. Experience from tests indicates that without this
imbalance, there is usually no visible indication on the EEG, but with the imbalance readings typically
range between 3-7mm (0.3 ~ 0.7mVpp). This suggests that the value of imbalance impedance is critical
for the tests. Although the diagram in the standard shows all switches open, for the purpose of the test,
all switches should be closed except the lead being tested and some tests are in the opposite settings
instead. CMRR 3.0E uses MCU to control the relay switch, which makes it easy to set up various balance
and imbalance impedance.
IEC 80601-2-26:2019 requires a DC offset of ±150mV. CMRR 3.0E uses MCU to c/- 150mV DC
offset and it is easy to set +/- 150mV DC offset with different imbalanced impedance electrodes. The
CMRR 3.0E DC offset is supplied by an internal battery. The lifetime of this battery is estimated to be at
more than 40 hours under continuous use. Therefore, its time in service should be long enough for tests
that only last a few seconds each time. In case there is a need to replace the battery, user just need to
remove the battery cover at the bottom of CMRR 3.0E and replace with a new battery.
5
Panel Function
5.1
Upper Panel
The upper panel of CMRR 3.0E is shown in figure 5-1. Please see below introductions for the functions of
LCD touch screen and knobs:
