ADAS1000 datasheet(29/80 Pages) AD

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Data Sheet

ADAS1000/ADAS1000-1/ADAS1000-2

Rev. A | Page 29 of 80

ECG CHANNEL

The ECG channel consists of a programmable gain, low noise,

differential preamplifier; a fixed gain anti-aliasing filter; buffers;

and an ADC (see Figure 56). Each electrode input is routed to

its PGA noninverting input. Internal switches allow the PGA’s

inverting inputs to be connected to other electrodes and/or

the Wilson central terminal to provide differential analog

processing (analog lead mode), to a computed average of some

or all electrodes, or the internal 1.3 V common-mode reference

(VCM_REF). The latter two modes support digital lead mode

(leads computed on-chip) and electrode mode (leads calculated

off-chip). In all cases, the internal reference level is removed

from the final lead data.

The ADAS1000/ADAS1000-1/ADAS1000-2 implementation

uses a dc-coupled approach, which requires that the front end

be biased to operate within the limited dynamic range imposed

by the relatively low supply voltage. The right leg drive loop

performs this function by forcing the electrical average of all

selected electrodes to the internal 1.3 V level, VCM_REF,

maximizing each channel’s available signal range.

All ECG channel amplifiers use chopping to minimize 1/f noise

contributions in the ECG band. The chopping frequency of

~250 kHz is well above the bandwidth of any signals of interest.

The 2-pole anti-aliasing filter has ~65 kHz bandwidth to support

digital pace detection while still providing greater than 80 dB of

attenuation at the ADC’s sample rate. The ADC itself is a 14-bit,

2 MHz SAR converter; 1024 × oversampling helps achieve the

required system performance. The ADC’s full-scale input range

is 2 × VREF, or 3.6 V, although the analog portion of the ECG

channel limits the useful signal swing to about 2.8 V.

Figure 56. Simplified Schematic of a Single ECG Channel

ADC

TO COMMON-MODE AMPLIFIER

FOR DRIVEN LEG AND

SHIELD DRIVER

AVDD

VREF

ELECTRODE

PREAMP

G = 1, 1.5, 2, 3

FILTER

DIFF AMP

BUFFER

G = 1.4

PATIENT

CABLE

ADAS1000

EXTERNAL

RFI AND DEFIB

PROTECTION

VCM

–

ELECTRODE

EXTERNAL

RFI AND DEFIB

PROTECTION

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