ADAS1000-4BSTZ datasheet(35/80 Pages) AD

35 / 80 page

Data Sheet

ADAS1000-3/ADAS1000-4

Rev. B | Page 35 of 80

WILSON CENTRAL TERMINAL (WCT)

The flexibility of the common-mode selection averaging allows

the user to achieve a Wilson Central Terminal voltage from the

ECG1_LA, ECG2_LL, ECG3_RA electrodes.

RIGHT LEG DRIVE/REFERENCE DRIVE

The right leg drive amplifier or reference amplifier is used as

part of a feedback loop to force the patient’s common-mode

voltage close to the internal 1.3 V reference level (VCM_REF)

of the ADAS1000-3/ADAS1000-4. This centers all the electrode

inputs relative to the input span, providing maximum input

dynamic range. It also helps to reject noise and interference

from external sources such as fluorescent lights or other

patient-connected instruments, and absorbs the dc or ac

lead-off currents injected on the ECG electrodes.

The RLD amplifier can be used in a variety of ways as shown

in Figure 65. Its input can be taken from the CM_OUT signal

using an external resistor. Alternatively, some or all of the

electrode signals can be combined using the internal switches.

The dc gain of the RLD amplifier is set by the ratio of the

external feedback resistor (RFB) to the effective input resistor,

which can be set by an external resistor, or alternatively, a

function of the number of selected electrodes as configured in

the CMREFCTL register (see Table 32). In a typical case, using

the internal resistors for RIN, all active electrodes would be used

to derive the right leg drive, resulting in a 2 kΩ effective input

resistor. Achieving a typical dc gain of 40 dB would thus require

a 200 kΩ feedback resistor.

The dynamics and stability of the RLD loop depend on the

chosen dc gain and the resistance and capacitance of the

patient cabling. In general, loop compensation using

external components is required, and must be determined

experimentally for any given instrument design and cable set.

In some cases, adding lead compensation will prove necessary,

while in others lag compensation may be more appropriate. The

RLD amplifier’s summing junction is brought out to a package

pin (RLD_SJ) to facilitate compensation.

The RLD amplifier’s short circuit current capability exceeds

regulatory limits. A patient protection resistor is required to

achieve compliance.

Within the RLD block, there is lead-off comparator circuitry

that monitors the RLD amplifier output to determine whether

the patient feedback loop is closed. An open-loop condition,

typically the result of the right leg electrode (RLD_OUT)

becoming detached, tends to drive the amplifier’s output

low. This type of fault is flagged in the header word (see

Table 53), allowing the system software to take action by

notifying the user, redirecting the reference drive to another

electrode via the internal switches of the ADAS1000-3/

ADAS1000-4, or both. The detection circuitry is local to the

RLD amplifier and remains functional with a redirected

reference drive. Table 32 provides details on reference drive

redirection.

While reference drive redirection may be useful in the event

that the right leg electrode cannot be reattached, some pre-

cautions must be observed. Most important is the need for a

patient protection resistor. Because this is an external resistor,

it does not follow the redirected reference drive; some provision

for continued patient protection is needed external to the

ADAS1000-3/ADAS1000-4. Any additional resistance in the

ECG paths will certainly interfere with respiration measure-

ment and may also result in an increase in noise and decrease

in CMRR.

The RLD amplifier is designed to stably drive a maximum

capacitance of 5 nF based on the gain configuration (see

Figure 65) and assuming a 330 kΩ patient protection resistor.

Figure 65. Right Leg Drive—Possible External Component Configuration

ELECTRODE LA

ELECTRODE LL

ELECTRODE RA

–

SW2

CM_IN OR

CM BUFFER OUT

SW3

SW6

SW1

EXTERNALLY SUPPLIED COMPONENTS

TO SET RLD LOOP GAIN

2nF

40kΩ

RIN*

4MΩ

RFB*

100kΩ

VCM_REF

(1.3V)

RLD_OUT

RLD_SJ

10kΩ

RLD_INT_REDIRECT

CM_OUT/WCT

*EXTERNAL RESISTOR RIN IS OPTIONAL. IF DRIVING RLD FROM

THE ELECTRODE PATHS, THEN THE SERIES RESISTANCE WILL

CONTRIBUTE TO THE RIN IMPEDANCE. WHERE SW1 TO SW5

ARE CLOSED, RIN = 2kΩ. RFB SHOULD BE CHOSEN

ACCORDINGLY FOR DESIRED RLD LOOP GAIN.

ADAS1000-3/

ADAS1000-4

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