Datasheet

Wcta
8:1MUX
WCT1[2:0]
IN4N
IN4P
IN3N
IN3P
IN2N
IN2P
WCT
IN1N
IN1P
Wctb
8:1MUX
WCT2[5:3]
Wctc
8:1MUX
WCT2[2:0]
ToChannel
PGAs
ADS1294/6/8
30kW 30kW 30kW
80pF
AVSS
ADS1294, ADS1294R
ADS1296, ADS1296R
ADS1298, ADS1298R
www.ti.com SBAS459I –JANUARY 2010–REVISED JANUARY 2012
WILSON CENTRAL TERMINAL (WCT) AND CHEST LEADS
In the standard 12-lead ECG, WCT voltage is defined as the average of Right Arm (RA), Left Arm (LA), and Left
Leg (LL) electrodes. This voltage is used as the reference voltage for the measurement of the chest leads. The
ADS129x has three integrated low-noise amplifiers that generate the WCT voltage. Figure 53 shows the block
diagram of the implementation.
Figure 53. WCT Voltage
The devices provide flexibility to choose any one of the eight signals (IN1P to IN4N) to be routed to each of the
amplifiers to generate the average. Having this flexibility allows the RA, LA, and LL electrodes to be connected to
any input of the first four channels depending on the lead configuration.
Each of the three amplifiers in the WCT circuitry can be powered down individually with register settings. By
powering up two amplifiers, the average of any two electrodes can be generated at the WCT pin. Powering up
one amplifier provides the buffered electrode voltage at the WCT pin. Note that the WCT amplifiers have limited
drive strength and thus should be buffered if used to drive a low-impedance load.
See Table 5 for performance when using any 1, 2, or 3 of the WCT buffers.
As can be seen in Table 5, the overall noise reduces when more than one WCT amplifier is powered up. This
noise reduction is a result of the fact that noise is averaged by the passive summing network at the output of the
amplifiers. Powering down individual buffers gives negligible power savings because a significant portion of the
circuitry is shared between the three amplifiers. The bandwidth of the WCT node is limited by the RC network.
The internal summing network consists of three 30kΩ resistors and a 80pF capacitor. It is recommended that an
external 100pF capacitor be added for optimal performance. The effective bandwidth depends on the number of
amplifiers that are powered up, as shown in Table 5.
The WCT node should be only be used to drive very high input impedances (typically greater than 500MΩ).
Typical application would be to connect this WCT signal to the negative inputs of a ADS129x to be used as a
reference signal for the chest leads.
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Product Folder Link(s): ADS1294 ADS1294R ADS1296 ADS1296R ADS1298 ADS1298R