ADAS1000BSTZ datasheet(29/80 Pages) AD

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Data Sheet

ADAS1000/ADAS1000-1/ADAS1000-2

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 inverting

inputs of the PGA to be connected to other electrodes and/or

the Wilson central terminal (WCT) 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 full-scale input range of the

ADC 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. The

ADAS1000 contains flags to indicate whether the ADC data is

out of range, indicating a hard electrode off state. Programmable

overrange and underrange thresholds are shown in the LOFFUTH

and LOFFLTH registers (see Table 39 and Table 40, respectively).

The ADC out of range flag is contained in the header word (see

Table 54).

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

Rev. B | Page 29 of 80

ADAS1000BSTZ Datasheet(PDF) 29 Page - Analog Devices

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