Datasheet

ADAS1000/ADAS1000-1/ADAS1000-2 Data Sheet
Rev. A | Page 34 of 80
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 64 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 gets 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 64. Common-Mode Generation Block
Table 11. Truth Table for Common-Mode Selection
ECGCTL
Address
0x01
1
CMREFCTL Address 0x05
2
PWREN DRVCM EXTCM LACM LLCM RACM V1CM V2CM On Switch Description
0 X X X X X X X Powered down, paths disconnected
1 X 0 0 0 0 0 0 SW7 Internal VCM_REF = 1.3 V is selected
1 0 0 1 0 0 0 0 SW2 Internal CM selection: LA contributes to VCM
1 0 0 1 1 0 0 0 SW2, SW3 Internal CM selection: LA and LL contribute to VCM
1 0 0 1 1 1 0 0 SW2, SW3,
SW4
Internal CM selection: LA, LL, and RA contribute to
VCM (WCT)
.
.
.
.
.
.
.
.
.
.
1 X 1 X X X X X SW1 External VCM selected
1
See Table 26.
2
See Table 30.
ECG1_LA
ECG2_LL
ECG3_RA
ECG4_V1
ECG5_V2
+
–
CM_IN
ADAS1000
SW2
VCM_REF = 1.3V
SW3
SW6
SW5
SW4
SW1
SW7
CM_OUT
VCM
(WHEN SELECTED, IT GETS
SUMMED IN ON EACH ECG CHANNEL)
09660-021