Datasheet

Data Sheet ADAS1000/ADAS1000-1/ADAS1000-2
Rev. A | Page 39 of 80
Figure 68. 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. 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 68. Bit 14
of the RESPCTL register (Table 28) 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 28).
EVALUATING RESPIRATION PERFORMANCE
ECG simulators offer a convenient means of studying the
ADAS1000’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.
EXTEND SWITCH ON RESPIRATION PATHS
There is additional multiplexing on the external respiration
inputs to allow them to serve as additional electrode inputs
to the existing five ECG ADC channels. This approach allows
a user to configure eight electrode inputs; however, it is not
intended as a true 8-channel/12-lead solution. Time overheads
will be required to reconfigure the multiplexer arrangement
using the serial interface in addition to filter the latency as
described in Table 14.
The user has full control over the SW1/SW2/SW3 configuration
as outlined in Table 48.
Figure 69. Alternative Use of the Respiration Paths
LA CABLE
ADAS1000
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
0.9V
GAIN
1/2 OF
AD8606
1/2 OF AD8606
09660-025
±1V
±1V
LPF
TO ECG1_LA CHANNEL
TO ECG2_LL CHANNEL
TO ECG3_RA CHANNEL
TO ECG4_V1 CHANNEL
TO ECG5_V2 CHANNEL
EXT_RESP_RA
EXT_RESP_LL
EXT_RESP_LA
SW1a
SW1b
SW1c
SW1d
SW1e
SW2a
SW2b
SW2c
SW2d
SW2e
SW3a
SW3b
SW3c
SW3d
SW3e
TO RESPIRATION CIRCUITRY
09660-032