ADAS1000BSTZ datasheet(36/80 Pages) AD

36 / 80 page

ADAS1000/ADAS1000-1/ADAS1000-2

Data Sheet

Rev. A | Page 36 of 80

CALIBRATION DAC

Within the ADAS1000/ADAS1000-1, there are a number of

calibration features.

The 10-bit calibration DAC can be used to correct channel gain

errors (to ensure channel matching) or to provide several test

tones. The options are as follows:

•

DC voltage output (range: 0.3 V to 2.7 V). The DAC

transfer function for dc voltage output is

(

)









−

code

•

1 mV p-p sine wave of 10 Hz or 150 Hz

•

1 mV 1 Hz square wave

Internal switching allows the calibration DAC signals to be

routed to the input of each ECG channel (see Figure 63).

Alternatively, it can be driven out from the CAL_DAC_IO

pin, enabling measurement and correction for external error

sources in the entire ECG signal chain and/or for use as an

input to the ADAS1000-2 companion chip calibration input.

To ensure a successful update of the calibration DAC (see

Table 34), the host controller must issue four additional SCLK

cycles after writing the new calibration DAC register word.

GAIN CALIBRATION

The gain for each ECG channel can be adjusted to correct for

gain mismatches between channels. Factory trimmed gain

correction coefficients are stored in nonvolatile memory

on-chip for GAIN 0, GAIN 1, and GAIN 2; there is no factory

calibration for GAIN 3. The default gain values can be

overwritten by user gain correction coefficients, which are

stored in volatile memory and available by addressing the

appropriate gain control registers (see Table 49). The gain

calibration applies to the ECG data available on the standard

interface and applies to all data rates.

LEAD-OFF DETECTION

An ECG system must be able to detect if an electrode is no

longer connected to the patient. The ADAS1000/ADAS1000-1/

ADAS1000-2 support two methods of lead-off detection: either

ac or dc lead-off detection. The two systems are independent

and can be used singly or together under the control of the

serial interface (see Table 27).

For both ac and dc lead-off detection, the upper and lower

threshold voltages are programmable via Table 37 and Table 38.

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.

DC lead-off detection uses fixed gain-independent upper

and lower threshold voltages. AC lead-off detection offers

user-programmable thresholds; because the detection is

performed digitally, it may be necessary to adjust the

thresholds, depending on the selected ECG channel gain. In

either case, all active channels use the same detection

thresholds.

A lead-off event sets a flag in the frame header word (see

Table 52). Identification of which electrode is off is available

as part of the data frame or as a register read from the lead-

off status register (Register LOFF, see Table 45). In the case of

ac lead-off, information about the amplitude of the lead-off

signal(s) can be read back through the serial interface (see

Table 50).

DC Lead-Off Detection

This method injects a small programmable dc current into each

input electrode. When an electrode is properly connected, the

current flows into the right leg (RLD_OUT) and produces a

minimal voltage shift. If an electrode is off, the current charges

that pin’s capacitance, causing the voltage at the pin to float

positive and create a large voltage change that is detected by the

comparators in each channel.

The dc lead-off detection current can be programmed via the

serial interface. Typical currents range from 10 nA to 70 nA in

10 nA steps.

The propagation delay for detecting a dc lead-off event depends

on the cable capacitance and the programmed current. It is

approximately

Delay = Voltage × Cable Capacitance/Programmed Current

For example:

Delay = 1.2 V × (200 pF/70 nA) = 3.43 ms

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 slightly

above 2 kHz, 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.

The polarity of the ac lead-off signal can be configured on a

per-electrode basis. All electrodes can be driven in phase, or

some can be driven with reversed polarity to minimize the total

injected ac current. Drive amplitude is also programmable. 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.

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

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