ADS1294 datasheet(60/88 Pages) TI

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Wcta

8:1MUX

IN4N

IN4P

IN3N

IN3P

IN2N

IN2P

WCT

IN1N

IN1P

Wctb

8:1MUX

Wctc

8:1MUX

ToChannel

PGAs

ADS1294/6/8

30k

80pF

AVSS

ADS1294, ADS1294R

ADS1296, ADS1296R

ADS1298, ADS1298R

SBAS459H

– JANUARY 2010 – REVISED OCTOBER 2011

www.ti.com

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 30k

Ω 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 500M

Ω).

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

ADS1294 Datasheet(PDF) 60 Page - Texas Instruments

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