ADAS1000BSTZ datasheet(39/80 Pages) AD

39 / 80 page

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

1kΩ

100Ω

RESPDAC_LA

RESPDAC_RA

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

±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

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