Datasheet

ADS1293
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SNAS602B FEBRUARY 2013REVISED MARCH 2013
Common-Mode (CM) Detector
The Common-Mode Detector averages the voltage of up to six input pins. Its output can be used in a right leg
drive feedback circuit. The selection of the input pins that contribute to the average is configured in the
CMDET_EN register. The Common-Mode Detector is automatically disabled when no input pin is selected.
Figure 21. Common-Mode Detector Circuit
Cable Shield Driving
The Common-Mode Detector also has a programmable capacitive load driving capability of up to 8nF that allows
it to drive a cable shield to reduce the common-mode signal current through a cable. This effectively increases
the bandwidth of the filter formed by the electrode impedance and the cable capacitance, reducing the amount of
common-mode to differential mode crosstalk. As a result, the CMRR of the overall ECG system is improved.
The bandwidth and capacitive load driving capability of the Common-Mode Detector can be configured in the
CMDET_CN register to achieve an optimal tradeoff with power consumption. Table 10 lists the power
consumption corresponding to different configuration scenarios given that all inputs are enabled by setting the
CMDET_EN register = 0x3F.
The lowest current consumption setting can be used when the Common-Mode Detector is only used to drive the
Right Leg Driver, and no cable shield is driven. If a cable shield needs to be driven, the power can be increased
to drive the cable capacitance depending on the number and type of the driven cable shields. Note that the
capacitive driving capability is reduced in the higher bandwidth mode.
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