Datasheet

ADAS1000-3/ADAS1000-4 Data Sheet
Rev. A | Page 42 of 76
VOLTAGE REFERENCE
The ADAS1000-3/ADAS1000-4 have a high performance,
low noise, on-chip 1.8 V reference for use in the ADC and
DAC circuits. The REFOUT of one device is intended to
drive the REFIN of the same device. The internal reference
is not intended to drive significant external current; for
optimum performance in gang operation with multiple
devices, each device should use its own internal reference.
An external 1.8 V reference can be used to provide the
required VREF. In such cases, there is an internal buffer pro-
vided for use with external reference. The REFIN pin is a
dynamic load with an average input current of approximately
100 μA per enabled channel, including respiration. When
the internal reference is used, the REFOUT pin requires
decoupling with a10 μF capacitor with low ESR (0.2 Ω
maximum) in parallel with 0.01 μF capacitor to REFGND,
these capacitors should be placed as close to the device pins
as possible and on the same side of the PCB as the device.
GANG MODE OPERATION
Increasing the number of ECG channels enables the user to
measure an increased number of patient electrodes. Typically
a 12-lead system would require nine electrodes (and one right
leg drive reference electrode), but a derived arrangement is
possible by using just eight electrodes (and one right leg drive
reference electrode). As such, mating a 5-electrode ADAS1000,
ADAS1000-1, or ADAS1000-2 with either a ADAS1000-3 or
ADAS1000-4 device delivers the required eight electrodes.
The approach used is a master slave arrangement, where one
device is designated as master, and any others are designated
as slaves. It is important that multiple devices operate well
together; with this in mind, the pertinent inputs/outputs to
interface between master and slave devices have been made
available.
Note that when using multiple devices, the user must collect
the ECG data directly from each device. If using a traditional
12-lead arrangement where the Vx leads are measured
relative to WCT, the user should configure the master device
in lead mode with the slave device configured for electrode
mode. The LSB size for electrode and lead data differs (see
Table 42 for details).
In gang mode, all devices must be operated in the same
power mode (either high performance or low power) and
the same data rate.
Master/Slave
Any of the ADAS1000, ADAS1000-1, ADAS1000-3, or
ADAS1000-4 can be configured as a master or slave, while
the ADAS1000-2 can only be configured as a slave. A device
is selected as a master or slave using Bit 5, master, in the
ECGCTL register (see Table 27). Gang mode is enabled by
setting Bit 4, gang, in the same register. When a device is
configured as a master, the SYNC_GANG pin is automati-
cally set as an output.
When a device is configured as a slave (ADAS1000-2), the
SYNC_GANG and CLK_IO pins are set as inputs.
Synchronizing Devices
The ganged devices need to share a common clock to ensure
that conversions are synchronized. One approach is to drive
the slave CLK_IO pins from the master CLK_IO pin. Alter-
natively, an external 8.192 MHz clock can be used to drive
the CLK_IO pins of all devices. The CLK_IO powers up high
impedance until configured in gang mode.
In addition, the SYNC_GANG pin is used to synchronize
the start of the ADC conversion across multiple devices. The
SYNC_GANG pin is automatically driven by the master and
is an input to all the slaves. SYNC_GANG is in high
impedance until enabled via gang mode.
When connecting devices in gang mode, the SYNC_GANG
output is triggered once when the master device starts to
convert. Therefore, to ensure that the slave device(s) receive
this synchronization signal, configure the slave device first
for operation and enable conversions, followed by issuing the
conversion signal to the ECGCTL register in the master
device.
Figure 69. Master/Slave Connections in Gang Mode, Using Multiple
Devices
Calibration
The calibration DAC signal from one device (master) can be
output on the CAL_DAC_IO pin and used as the calibration
input for other devices (slaves) when used in the gang mode
of operation. This ensures that they are all being calibrated
using the same signal which results in better matching across
channels. This does not happen automatically in gang mode
but, rather, must be configured via Table 35.
MASTER
SYNC_GANG
CM_OUT
CAL_DAC_IO
CLK_IOCLK_IO
SYNC_GANG
CM_IN
CAL_DAC_IO
SYNC_GANG
CM_IN
SLAVE 0
SLAVE 1
CAL_DAC_IO
CLK_IO
10997-029