Datasheet

Data Sheet ADAS1000-3/ADAS1000-4
Rev. A | Page 31 of 76
COMMON-MODE SELECTION AND AVERAGING
The common-mode signal can be derived from any combina-
tion of one or more electrode channel inputs, the fixed internal
common-mode voltage reference, VCM_REF, or an external
source connected to the CM_IN pin. One use of the latter
arrangement is in gang mode where the master device creates
the Wilson central terminal for the slave device(s). The fixed
reference option is useful when measuring the calibration DAC
test tone signals or while attaching electrodes to the patient,
where it allows a usable signal to be obtained from just two
electrodes.
The flexible common-mode generation allows complete
user control over the contributing channels. It is similar to,
but independent of, circuitry that creates the right leg drive
(RLD) signal. Figure 61 shows a simplified version of the
common-mode block. If the physical connection to each
electrode is buffered, these buffers are omitted for clarity.
There are several restrictions on the use of the switches:
• If SW1 is closed, SW7 must be open.
• If SW1 is open, at least one electrode switch (SW2 to SW7)
must be closed.
• SW7 can be closed only when SW2 to SW6 are open, so
that the 1.3 V VCM_REF is summed in only when all ECG
channels are disconnected.
The CM_OUT output is not intended to supply current or drive
resistive loads, and its accuracy is degraded if it is used to drive
anything other than the slave ADAS1000-2 devices. An external
buffer is required if there is any loading on the CM_OUT pin.
Figure 61. Common-Mode Generation Block
Table 12. Truth Table for Common-Mode Selection
ECGCTL
Address 0x01
1
CMREFCTL Address 0x05
2
PWREN DRVCM EXTCM LACM LLCM RACM On Switch Description
0 X X X X X Powered down, paths disconnected
1
X
0
0
0
0
SW7
Internal VCM_REF = 1.3 V is selected
1 0 0 1 0 0 SW2 Internal CM selection: LA contributes to VCM
1 0 0 1 1 0 SW2, SW3 Internal CM selection: LA and LL contribute to VCM
1 0 0 1 1 1 SW2, SW3,
SW4
Internal CM selection: LA, LL, and RA contribute to VCM (WCT)
. . . . . . . .
1 X 1 X X X SW1 External VCM selected
1
See Table 27.
2
See Table 31.
ECG1_LA
ECG2_LL
ECG3_RA
+
–
CM_IN
SW2
VCM_REF = 1.3V
SW3
SW4
SW1
SW7
CM_OUT
VCM
(WHEN SELECTED, VCM_REF IS
SUMMED IN ON EACH EC CHANNEL)
ADAS1000-3/
ADAS1000-4
10997-021