Datasheet

AD7264 Data Sheet
Rev. B | Page 20 of 28
MODES OF OPERATION
The AD7264 allows the user to choose between two modes of
operation: pin driven mode and control register mode.
PIN DRIVEN MODE
Pin driven mode allows the user to select the gain of the PGA,
the power-down mode, internal or external reference, and to
initiate a calibration of the offset for both ADC A and ADC B.
These functions are implemented by setting the logic levels on
the gain pins (G3 to G0), the power-down pins (PD2 to PD0),
the REFSEL pin, and the CAL pin, respectively.
The logic state of the G3 to G0 pins determines which mode of
operation is selected. Pin driven mode is selected if at least one
of the gain pins is set to a logic high state. Alternatively, if all
four gain pins are connected to a logic low, the control register
mode of operation is selected.
GAIN SELECTION
The on-board PGA allows the user to select from 14 program-
mable gain stages: ×1, ×2, ×3, ×4, ×6, ×8, ×12, ×16, ×24, ×32,
×48, ×64, ×96, and ×128. The PGA accepts fully differential
analog signals and provides three key functions, which include
selecting gains for small amplitude input signals, driving the
ADCs switched capacitive load, and buffering the source from
the switching effects of the SAR ADCs. The AD7264 offers the
user great flexibility in user interface, offering gain selection via
the control register or by driving the gain pins to the desired
logic state. The AD7264 has four gain pins, G3, G2, G1 and G0,
as shown in Figure 3 and Figure 4. Each gain setting is selected
by setting up the appropriate logic state on each of the four gain
pins, as outlined in Table 6. If all four gain pins are connected to
a logic low level, the part is put in control register mode, and
the gain settings are selected via the control register.
Table 6. Gain Selection
G3 G2 G1 G0 Gain
0 0 0 0 Software control
via control register
0 0 0 1 1
0 0 1 0 2
0 0 1 1 3
0 1 0 0 4
0 1 0 1 6
0 1 1 0 8
0 1 1 1 12
1 0 0 0 16
1 0 0 1 24
1 0 1 0 32
1 0 1 1 48
1
1
0
0
64
1 1 0 1 96
1 1 1 0 128
POWER-DOWN MODES
The AD7264 offers the user several of power-down options to
enable individual device components to be powered down
independently. These options can be chosen to optimize power
dissipation for different application requirements. The power-
down modes can be selected by either programming the device
via the control register or by driving the PD pins to the
appropriate logic levels. By setting the PD pins to a logic low
level when in pin driven mode, all four comparators and both
ADCs can be powered down. The PD2 and PD0 pins must be
set to logic high and the PD1 pin set to logic low level to power
up all circuitry on the AD7264. The PD pin configurations for
the various power-down options are outlined in Table 7.
Table 7. Power-Down Modes
PD2 PD1 PD0
Comparator A,
Comparator B
Comparator C,
Comparator D
ADC A,
ADC B
0 0 0 Off Off Off
0 0 1 Off Off On
0 1 0 Off On Off
0 1 1 On Off Off
1 0 0 On On Off
1
0
1
On
On
On
1
1
1
1
1
1
Off Off Off
1
PD2 = PD1 = PD0 = 1; resets the AD7264 when in pin driven mode only.
The AV
CC
and V
DRIVE
supplies must continue to be supplied to
the AD7264 when the comparators are powered up but the
ADCs are powered down. External diodes can be used from the
C
A
_C
B
V
CC
and/or C
C
_C
D
V
CC
to both the AV
CC
and the V
DRIVE
supplies to ensure that they retain a supply at all times.
The AD7264 can be reset in pin driven mode only by setting the
PD pins to a logic high state. When the device is reset, all the
registers are cleared and the four comparators and the two
ADCs are left powered down.
In the normal mode of operation with the ADCs and compara-
tors powered on, the C
A
_C
B
V
CC
/C
C
_C
D
V
CC
supplies and the
AV
CC
supply can be at different voltage levels, as indicated in
Table 1. When the comparators on the AD7264 are in power-
down mode and the C
A
_C
B
V
CC
/C
C
_C
D
V
CC
supplies are at a
potential 0.3 V greater than or less than the AV
CC
supply, the
supplies consume more current than would be the case if both
sets of supplies were at the same potential. This configuration
does not damage the AD7264 but results in additional current
flowing in any or all of the AD7264 supply pins. This is due to
ESD protection diodes within the device. In applications where
power consumption in power-down mode is critical, it is
recommended that the C
A
_C
B
V
CC
/C
C
_C
D
V
CC
supply and the
AV
CC
supply be held at the same potential.