Datasheet

Data Sheet ADAS1000/ADAS1000-1/ADAS1000-2
Rev. A | Page 37 of 80
a shield drive, an external amplifier connected to the CM_OUT
pin can be used.
RESPIRATION (ADAS1000 MODEL ONLY)
The respiration measurement is performed by driving a high
frequency (programmable from 46.5 kHz to 64 kHz) differential
current into two electrodes; the resulting impedance variation
caused by breathing causes the differential voltage to vary at the
respiration rate. The signal is ac-coupled onto the patient. The
acquired signal is AM, with a carrier at the driving frequency
and a shallow modulation envelope at the respiration frequency.
The modulation depth is greatly reduced by the resistance of the
customer-supplied RFI and ESIS protection filters, in addition
to the impedance of the cable and the electrode to skin interface
(see Table 12). The goal is to measure small ohm variation to sub
ohm resolution in the presence of large series resistance. The
circuit itself consists of a respiration DAC that drives the ac-
coupled current at a programmable frequency onto the chosen
pair of electrodes. The resulting variation in voltage is amplified,
filtered, and synchronously demodulated in the digital domain;
what results is a digital signal that represents the total thoracic
or respiration impedance, including cable and electrode contri-
butions. While it is heavily low-pass filtered on-chip, the user is
required to further process it to extract the envelope and perform
the peak detection needed to establish breathing (or lack thereof).
Respiration measurement is available on one of the leads (Lead I,
Lead II, or Lead III) or on an external path via a pair of dedicated
pins (EXT_RESP_LA, EXT_RESP_RA, or EXT_RESP_LL).
Only one lead measurement can be made at one time. The
respiration measurement path is not suited for use as additional
ECG measurements because the internal configuration and
demodulation do not align with an ECG measurement; however,
the EXT_RESP_LA, EXT_RESP_RA, or EXT_RESP_LL paths
can be multiplexed into one of the ECG ADC paths, if required,
as discussed in the Extend Switch On Respiration Paths section.
Internal Respiration Capacitors
The internal respiration function uses an internal RC network
(5 kΩ/100 pF), and this circuit is capable of 200 mΩ resolution
(with up to 5 kΩ total path and cable impedance). The current
is ac-coupled onto the same pins that the measurement is sensed
back on. Figure 66 shows the measurement on Lead I, but,
similarly, the measurement can be configured to measure on
either Lead II or Lead III. The internal capacitor mode requires
no external capacitors and produces currents of ~64 µA p-p
amplitude when configured for maximum amplitude setting
(±1V) through the RESPCTRL register (see Table 28).
Table 12. Maximum Allowable Cable and Thoracic Loading
Cable Resistance Cable Capacitance
R < 1 kΩ
C < 1200 pF
1 kΩ < R < 2.5 kΩ C < 400 pF
2.5 kΩ < R < 5 kΩ C < 200 pF
Rthoracic < 2 kΩ
Figure 66. Simplified Respiration Block Diagram
LA CABLE
ECG1_LA
EXT_RESP_LA
ECG2_LL
EXT_RESP_LL
ECG3_RA
EXT_RESP_RA
ADAS1000
OVERSAMPLED
5kΩ
5kΩ
100pF
100pF
RESPIRATION DAC
DRIVE +
±1V
±1V
CABLE AND ELECTRODE
IMPEDANCE < 5kΩ
LL CABLE
RA CABLE
FILTER
FILTER
FILTER
RESPIRATION
MEASURE
RESPIRATION DAC
DRIVE–
HPF
IN-AMP AND
ANTI-ALIASING
MAGNITUDE
AND
PHASE
SAR
ADC
09660-023
46.5kHz TO
64kHz
46.5kHz TO
64kHz
LPF