Datasheet

ADAS1000-3/ADAS1000-4 Data Sheet
Rev. A | Page 34 of 76
RESPIRATION (ADAS1000-4 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 13). 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 contributions. 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.
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 63 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
(±1 V) through the RESPCTRL register (see Table 29).
Table 13. 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
R
THORACIC
< 2 kΩ
Figure 63. Simplified Respiration Block Diagram
LA CABLE
ECG1_LA
EXT_RESP_LA
ECG2_LL
EXT_RESP_LL
ECG3_RA
EXT_RESP_RA
ADAS1000-4
OVERSAMPLED
5kΩ
5kΩ
100pF
100pF
±1V
±1V
RESPIRATION DAC
DRIVE +
CABLE AND ELECTRODE
IMPEDANCE < 5kΩ
LL CABLE
RA CABLE
FILTER
FILTER
FILTER
LPF
RESPIRATION
MEASURE
RESPIRATION DAC
DRIVE–
HPF
IN-AMP AND
ANTI-ALIASING
MAGNITUDE
AND
PHASE
SAR
ADC
46.5kHz TO
64kHz
46.5kHz TO
64kHz
10997-023