Datasheet

ADAS1000-3/ADAS1000-4 Data Sheet
Rev. A | Page 36 of 76
Figure 65. Respiration Using External Capacitor and External Amplifiers
If required, further improvements in respiration performance
may be possible with the use of an instrumentation amplifier
and op amp external to the ADAS1000-4. The instrumentation
amplifier must have sufficiently low noise performance to meet
the target performance levels. This mode uses the external
capacitor mode configuration and is shown in Figure 65. Bit 14
of the RESPCTL register (Table 29) allows the user to bypass
the on-chip amplifier when using an external instrumentation
amplifier.
Respiration Carrier
In applications where an external signal generator is used to
develop a respiration carrier signal, that external signal source
can be synchronized to the internal carrier using the signal
available on GPIO3 when Bit 7, RESPEXTSEL, is enabled in
the respiration control register (see Table 29).
EVALUATING RESPIRATION PERFORMANCE
ECG simulators offer a convenient means of studying the
ADAS1000-3/ADAS1000-4’s performance. While many
simulators offer a variable-resistance respiration capability,
care must be taken when using this feature.
Some simulators use electrically-programmable resistors, often
referred to as digiPOTs, to create the time-varying resistance
to be measured by the respiration function. The capacitances
at the digitPOT's terminals are often unequal and code-
dependent, and these unbalanced capacitances can give rise
to unexpectedly large or small results on different leads for
the same programmed resistance variation. Best results are
obtained with a purpose-built fixture that carefully balances
the capacitance presented to each ECG electrode.
PACING ARTIFACT DETECTION FUNCTION
(ADAS1000-4 ONLY)
The pacing artifact validation function qualifies potential
pacing artifacts and measures the width and amplitude of valid
pulses. These parameters are stored in and available from any of
the pace data registers (Address 0x1A, Address 0x3A to Address
0x3C). This function runs in parallel with the ECG channels.
Digital detection is performed using a state machine operating
on the 128 kHz 16-bit data from the ECG decimation chain.
The main ECG signals are further decimated before appearing
in the 2 kHz output stream so that detected pace signals are not
perfectly time-aligned with fully-filtered ECG data. This time
difference is deterministic and may be compensated for.
The pacing artifact validation function can detect and
measure pacing artifacts with widths from 100 μs to 2 ms
and with amplitudes of <400 μV to >1000 m V. Its filters are
designed to reject heartbeat, noise, and minute ventilation
pulses. The flowchart for the pace detection algorithm is
shown in Figure 66.
The ADAS1000-4 pace algorithm can operate with the ac lead-
off and respiration impedance measurement circuitry enabled.
Once a valid pace has been detected in the assigned leads, the
pace-detected flags appear in the header word (see Table 52) at
the start of the packet of ECG words. These bits indicate that a
pace was qualified. Further information on height and width of
pace is available by reading the contents of Address 0x1A (Register
PACEDATA, see Table 43). This word can be included in the
ECG data packet/frame as dictated by the frame control register
(see Table 36
). The data available in the PACEDATA register
is limited to seven bits total for width and height information;
therefore, if more resolution is required on the pace height
and width, this is available by issuing read commands of the
PACExDATA registers (Address 0x3A to Address 0x3C) as
shown in Table 51.
LA CABLE
50kHz TO
56kHz
46.5kHz TO
64kHz
OVERSAMPLED
1kΩ
10kΩ
10kΩ
10kΩ
10kΩ
1kΩ
100Ω
100Ω
RESPDAC_LA
RESPDAC_RA
1nF TO 10nF
1nF TO 10nF
RESPIRATION DAC
DRIVE + ve
CABLE AND ELECTRODE
IMPEDANCE < 1kΩ
RA CABLE
RESPIRATION
MEASURE
RESPIRATION DAC
DRIVE – ve
HPF
IN-AMP AND
ANTI-ALIASING
MAGNITUDE
AND
PHASE
SAR
ADC
EXT_RESP_LA
EXT_RESP_RA
REFOUT = 1.8V
0.
9
V
GAIN
1/2 OF AD8606
1/2 OF AD8606
ADAS1000-4
10997-025
±1V
±1V
LPF