ADAS1000BSTZ datasheet(36/80 Pages) AD

36 / 80 page

ADAS1000/ADAS1000-1/ADAS1000-2

Data Sheet

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/ADAS1000-1/ADAS1000-2. 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 67. 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

for RIN, all active electrodes are used to derive the right leg drive,

resulting in a 2 kΩ effective input resistor. Achieving a typical

dc gain of 40 dB thus requires 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 proves necessary, while in others, lag compensation

is more appropriate. The summing junction of the RLD amplifier is

brought out to a package pin (RLD_SJ) to facilitate compensation.

The short circuit current capability of the RLD amplifier

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 output of the amplifier low. This type

of fault is flagged in the header word (see Table 54), 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/ ADAS1000-1/ADAS1000-2, 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 can be useful in the event that

the right leg electrode cannot be reattached, some precautions

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/

ADAS1000-1/ADAS1000-2. Any additional resistance in the

ECG paths certainly interferes with respiration measurement

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 67) and assuming a 330 kΩ patient protection resistor.

Figure 67. Right Leg Drive—Possible External Component Configuration

ELECTRODE LA

ELECTRODE LL

ELECTRODE RA

ELECTRODE V1

ELECTRODE V2

–

SW2

CM_IN OR

CM BUFFER OUT

SW3

SW6

SW5

SW4

SW1

EXTERNALLY SUPPLIED COMPONENTS

TO SET RLD LOOP GAIN

2nF

40k

Ω

RIN*

Ω

RFB*

100kΩ

VCM_REF

(1.3V)

RLD_OUT

RLD_SJ

ADAS1000

10k

Ω

10k

Ω

10k

Ω

10k

Ω

10k

Ω

10k

Ω

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.

Rev. B | Page 36 of 80

ADAS1000BSTZ Datasheet(PDF) 36 Page - Analog Devices

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