Datasheet

ADAS1000-3/ADAS1000-4 Data Sheet
Rev. A | Page 32 of 76
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 62. 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 31). In a typical case, using
the internal resistors for R
IN
, 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 52), 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 31 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 62) and assuming a 330 kΩ patient protection resistor.
Figure 62. 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
CZ
2nF
40k
Ω
R
IN
*
4MΩ
RFB*
100kΩ
RZ
VCM_REF
(1.3V)
RLD_OUT
RLD_SJ
10kΩ
10kΩ
10kΩ
10kΩ
RLD_INT_REDIRECT
CM_OUT/WCT
*EXTERNAL RESISTOR R
IN
IS OPTIONAL. IF DRIVING RLD FROM
THE ELECTRODE PATHS, THEN THE SERIES RESISTANCE WILL
CONTRIBUTE TO THE R
IN
IMPEDANCE. WHERE SW1 TO SW5
ARE CLOSED, R
IN
= 2kΩ. RFB SHOULD BE CHOSEN
ACCORDINGLY FOR DESIRED RLD LOOP GAIN.
ADAS1000-3/
ADAS1000-4
10997-022