Datasheet

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
( )








−
×+
12
V4.2V3.0
10
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 LOF F, 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-of
f 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