Datasheet

Data Sheet AD7329
Rev. B | Page 17 of 40
THEORY OF OPERATION
CIRCUIT INFORMATION
The AD7329 is a fast, 8-channel, 12-bit plus sign, bipolar input,
serial ADC. The AD7329 can accept bipolar input ranges that
include ±10 V, ±5 V, and ±2.5 V; it can also accept a 0 V to +10 V
unipolar input range. A different analog input range can be
programmed on each analog input channel via the on-chip
registers. The AD7329 has a high speed serial interface that
can operate at throughput rates up to 1 MSPS.
The AD7329 requires V
DD
and V
SS
dual supplies for the high
voltage analog input structures. These supplies must be equal to
or greater than the analog input range. See Tabl e 6 for the
requirements of these supplies for each analog input range. The
AD7329 requires a low voltage 2.7 V to 5.25 V V
CC
supply to
power the ADC core.
Table 6. Reference and Supply Requirements for Each
Analog Input Range
Selected
Analog
Input
Range (V)
Reference
Voltage (V)
Full-Scale
Input
Range (V) V
CC
(V)
Minimum
V
DD
/V
SS
(V)
1
±10 2.5 ±10 3/5 ±10
3.0 ±12 3/5 ±12
±5
2.5
±5
3/5
±5
3.0 ±6 3/5 ±6
±2.5 2.5 ±2.5 3/5 ±5
3.0
±3
3/5
±5
0 to +10 2.5 0 to +10 3/5 +10/AGND
3.0 0 to +12 3/5 +12/AGND
1
Guaranteed performance for V
DD
= 12 V to 16.5 V and V
SS
= −12 V to −16.5 V.
The performance specifications are guaranteed for V
DD
= 12 V
to 16.5 V and V
SS
= −12 V to −16.5 V. With V
DD
and V
SS
supplies
outside this range, the AD7329 is fully functional but performance
is not guaranteed. When the AD7329 is configured with the
minimum V
DD
and V
SS
supplies for a chosen analog input range,
the throughput rate should be decreased from the maximum
throughput range (see the Typical Performance Characteristics
section). Figure 18 and Figure 19 show the change in INL and
DNL as the V
DD
and V
SS
voltages are varied. When operating at
the maximum throughput rate, as the V
DD
and V
SS
supply voltages
are reduced, the INL and DNL error increases. However, as the
throughput rate is reduced with the minimum V
DD
and V
SS
supplies, the INL and DNL error is reduced.
Figure 23 shows the change in THD as the V
DD
and V
SS
supplies
are reduced. At the maximum throughput rate, the THD degrades
significantly as V
DD
and V
SS
are reduced. It is therefore necessary to
reduce the throughput rate when using minimum V
DD
and V
SS
supplies so that there is less degradation of THD and the specified
performance can be maintained. The degradation is due to an
increase in the on resistance of the input multiplexer when the
V
DD
and V
SS
supplies are reduced.
The analog inputs can be configured as eight single-ended inputs,
four true differential input pairs, four pseudo differential inputs, or
seven pseudo differential inputs. Selection can be made by program-
ming the mode bits, Mode 0 and Mode 1, in the control register.
The serial clock input accesses data from the part and provides
the clock source for the successive approximation ADC. The
AD7329 has an on-chip 2.5 V reference. However, the AD7329
can also work with an external reference. On power-up, the
external reference operation is the default option. If the internal
reference is the preferred option, the user must write to the
reference bit in the control register to select the internal
reference operation.
The AD7329 also features power-down options to allow power
savings between conversions. The power-down modes are
selected by programming the on-chip control register as
described in the Modes of Operation section.
CONVERTER OPERATION
The AD7329 is a successive approximation analog-to-digital
converter built around two capacitive DACs. Figure 24 and
Figure 25 show simplified schematics of the ADC in single-
ended mode during the acquisition and conversion phases,
respectively. Figure 26 and Figure 27 show simplified
schematics of the ADC in differential mode during acquisition
and conversion phases, respectively. In both examples, the
MUX
OUT
+ pin is connected to the ADC
IN
+ pin, and the
MUX
OUT
− pin is connected to the ADC
IN
− pin. The ADC is
composed of control logic, a SAR, and capacitive DACs. In
Figure 24 (the acquisition phase), SW2 is closed and SW1 is in
Position A, the comparator is held in a balanced condition, and
the sampling capacitor array acquires the signal on the input.
CAPACITIVE
DAC
CONTROL
LOGIC
COMPARATOR
AGND
SW2
SW1
A
B
C
S
V
IN
0
05402-023
Figure 24. ADC Configuration During Acquisition Phase, Single-Ended Mode
When the ADC starts a conversion (Figure 25), SW2 opens and
SW1 moves to Position B, causing the comparator to become
unbalanced. The control logic and the charge redistribution
DAC are used to add and subtract fixed amounts of charge from
the capacitive DAC to bring the comparator back into a balanced
condition. When the comparator is rebalanced, the conversion
is complete. The control logic generates the ADC output code.
CAPACITIVE
DAC
CONTROL
LOGIC
COMPARATOR
AGND
SW2
SW1
A
B
C
S
V
IN
0
05402-024
Figure 25. ADC Configuration During Conversion Phase, Single-Ended Mode