ADAS1000-4BSTZ datasheet(34/80 Pages) AD

34 / 80 page

ADAS1000-3/ADAS1000-4

Data Sheet

Rev. B | 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 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 64. Common-Mode Generation Block

Table 11. Truth Table for Common-Mode Selection

ECGCTL

Address 0x011

CMREFCTL Address 0x052

PWREN

DRVCM

EXTCM

LACM

LLCM

RACM

On Switch

Description

Powered down, paths disconnected

SW7

Internal VCM_REF = 1.3 V is selected

SW2

Internal CM selection: LA contributes to VCM

SW2, SW3

Internal CM selection: LA and LL contribute to VCM

SW2, SW3,

SW4

Internal CM selection: LA, LL, and RA contribute to VCM (WCT)

SW1

External VCM selected

1 See Table 28.

2 See Table 32.

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/

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