ADAS1000 datasheet(38/80 Pages) AD

38 / 80 page

ADAS1000/ADAS1000-1/ADAS1000-2

Data Sheet

Rev. B | Page 38 of 80

Increasing the AVDD voltage raises the voltage at which the input

amplifiers saturate, allowing the off electrode voltage to rise high

enough to trip the DCLO comparator (fixed upper threshold of

2.4 V). The ADAS1000 operates over a voltage range of 3.15 V

to 5.5 V. If using GAIN 2/GAIN 3 and dc lead-off, an increased

AVDD supply voltage (minimum 3.6 V) allows dc lead-off to

flag correctly at higher gains.

AC Lead-Off Detection

The alternative method of sensing if the electrodes are connected

to the patient is based on injecting ac currents into each channel

and measuring the amplitudes of the resulting voltages. The

system uses a fixed carrier frequency at 2.039 kHz, which is

high enough to be removed by the ADAS1000/ADAS1000-1/

ADAS1000-2 on-chip digital filters without introducing phase

or amplitude artifacts into the ECG signal.

Figure 68. Simplified AC Lead-Off Configuration

The amplitude of the signal is nominally 2 V p-p and is centered

on 1.3 V relative to the chip AGND level. It is ac-coupled into each

electrode. The polarity of the ac lead-off signal can be configured

on a per-electrode basis through Bits[23:18] of the LOFFCTL

some can be driven with reversed polarity to minimize the total

injected ac current. Drive amplitude is also programmable. AC

lead-off detection functions only on the input pins (LA, LL, RA,

V1, V2, and CM_IN) and is not supported for the RLD_OUT pin.

The resulting analog input signal applied to the ECG channels is

I/Q demodulated and amplitude detected. The resulting amplitude

is low pass filtered and sent to the digital threshold detectors.

AC lead-off detection offers user programmable dedicated

upper and lower threshold voltages (see Table 39 and Table 40).

Note that these programmed thresholds voltage vary with the

ECG channel gain. The threshold voltages are not affected by

the current level that is programmed. All active channels use

the same detection thresholds.

A properly connected electrode has a very small signal as the

drive current flows into the right leg (RL), whereas a disconnected

electrode has a larger signal as determined by a capacitive

voltage divider (source and cable capacitance).

If the signal measured is larger than the upper threshold, then

the impedance is high, so a wire is probably off. Selecting the

appropriate threshold setting depends on the particular cable/

electrode/protection scheme, as these parameters are typically

unique for the specific use case. This can take the form of starting

with a high threshold and ratcheting it down until a lead-off is

detected, then increasing the threshold by some safety margin.

This gives simple dynamic thresholding that automatically

compensates for many of the circuit variables.

The lower threshold is added for cases where the only ac lead-off

is in use and for situations where an electrode cable has been off

for a long time. In this case, the dc voltage has saturated to a rail,

or the electrode cable has somehow shorted to a supply. In either

case, there is no ac signal present, yet the electrode may not be

connected. The lower threshold checks for a minimum signal level.

In addition to the lead-off flag, the user can also read back the

resulting voltage measurement available on a per channel basis.

The measured amplitude for each of the individual electrodes is

available in Register 0x31 through Register 0x35 (LOAMxx

registers, see Table 52).

The propagation delay for detecting an ac lead-off event is <10 ms.

Note that the ac lead-off function is disabled when the

calibration DAC is enabled.

ADC Out of Range

When multiple leads are off, the input amplifiers may run into

saturation. This results in the ADC outputting out of range data

with no carrier to the leads off algorithm. The ac lead-off algorithm

then reports little or no ac amplitude. The ADAS1000 contains

flags to indicate if the ADC data is out of range, indicating a hard

electrode off state. There are programmable overrange and under-

range thresholds that can be seen in the LOFFUTH and LOFFLTH

registers (see Table 39 and Table 40, respectively). The ADC out

of range flag is contained in the header word (see Table 54).

SHIELD DRIVER

The shield drive amplifier is a unity gain amplifier. Its purpose

is to drive the shield of the ECG cables. For power consumption

purposes, it can be disabled if not in use. Note that the SHIELD

pin is shared with the respiration pin function, where it can be

muxed to be one of the pins for external capacitor connection.

If the pin is being used for the respiration feature, the shield

function is not available. In this case, if the application requires

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,

AC LO

DAC

2.039kHz

12.5nA TO

100nA rms

11kΩ

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