Datasheet

Data Sheet ADAS1000-3/ADAS1000-4
Rev. A | Page 27 of 76
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 54). 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 to 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 ADAS1000-3/ADAS1000-4 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 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 54. Simplified Schematic of a Single ECG Channel
ADC
f
S
14
TO COMMON-MODE AMPLIFIER
FOR DRIVEN LEG AND
SHIELD DRIVER
SHIELD DRIVER
AVDD
VREF
ELECTRODE
PREAMP
G = 1, 1.5, 2, 3
FILTER
DIFF AMP
BUFFER
G = 1.4
PATIENT
CABLE
EXTERNAL
RFI AND DEFIB
PROTECTION
VCM
+
–
ELECTRODE
ELECTRODE
EXTERNAL
RFI AND DEFIB
PROTECTION
ADAS1000-3/
ADAS1000-4
10997-014