Datasheet

AD7294 Data Sheet
Rev. H | Page 20 of 48
THEORY OF OPERATION
ADC OVERVIEW
The AD7294 provides the user with a 9-channel multiplexer, an
on-chip track-and-hold, and a successive approximation ADC
based around a capacitive DAC. The analog input range for the
part can be selected as a 0 V to V
REF
input or a 2 × V
REF
input,
configured with either single-ended or differential analog inputs.
The AD7294 has an on-chip 2.5 V reference that can be disabled
when an external reference is preferred. If the internal ADC
reference is to be used elsewhere in a system, the output must
first be buffered.
The various monitored and uncommitted input signals are multi-
plexed into the ADC. The AD7294 has four uncommitted
analog input channels, V
IN
0 to V
IN
3. These four channels allow
single-ended, differential, and pseudo differential mode
measurements of various system signals.
ADC TRANSFER FUNCTIONS
The designed code transitions occur at successive integer LSB
values (1 LSB, 2 LSB, and so on). In single-ended mode, the
LSB size is V
REF
/4096 when the 0 V to V
REF
range is used and
2 × V
REF
/4096 when the 0 V to 2 × V
REF
range is used. The ideal
transfer characteristic for the ADC when outputting straight
binary coding is shown in Figure 35.
000...000
111...111
1LSB = V
REF
/4096
1LSB
V
REF
– 1LSB
ANALOG INPUT
ADC CODE
0V
000...001
000...010
111...110
111...000
011...111
NOTE
1. V
REF
IS EITHER V
REF
OR 2 × V
REF
.
05747-016
Figure 35. Single-Ended Transfer Characteristic
In differential mode, the LSB size is 2 × V
REF
/4096 when the 0 V
to V
REF
range is used, and 4 × V
REF
/4096 when the 0 V to 2 × V
REF
range is used. The ideal transfer characteristic for the ADC when
outputting twos complement coding is shown in Figure 36 (with
the 2 × V
REF
range).
100...000
011...111
1LSB = 2 × V
REF
/4096
+V
REF
– 1LSB–V
REF
+ 1LSB V
REF
– 1LSB
ANALOG INPUT
ADC CODE
100...001
100...010
011...110
000...001
000...000
111...111
05747-017
Figure 36. Differential Transfer Characteristic with V
REF
± V
REF
Input Range
For V
IN
0 to V
IN
3 in single-ended mode, the output code is
straight binary, where
V
IN
= 0 V, D
OUT
= x000, V
IN
= V
REF
− 1 LSB, and D
OUT
= xFFF
In differential mode, the code is twos complement, where
V
IN+
− V
IN−
= 0 V, and D
OUT
= x00
V
IN+
− V
IN−
= V
REF
− 1 LSB, and D
OUT
= x7FF
V
IN+
− V
IN−
= −V
REF
, and D
OUT
= x800
Channel 5 and Channel 6 (current sensor inputs) are twos
complement, where
V
IN+
− V
IN−
= 0 m V, and D
OUT
= x000
V
IN+
− V
IN−
= V
REF
/12.5 − 1 LSB, D
OUT
= x7FF
V
IN+
− V
IN−
= −V
REF
/12.5, D
OUT
= x800
Channel 7 to Channel 9 (temperature sensor inputs) are twos
complement with the LSB equal to 0.25°C, where
T
IN
= 0°C, and D
OUT
= x000
T
IN
= +255.75°C, and D
OUT
= x7FF
T
IN
= −256°C, and D
OUT
= x800
ANALOG INPUTS
The AD7294 has a total of four analog inputs. Depending on
the configuration register setup, they can be configured as two
single-ended inputs, two pseudo differential channels, or two
fully differential channels. See the Register Setting section for
further details.
Single-Ended Mode
The AD7294 can have four single-ended analog input channels.
In applications where the signal source has high impedance, it is
recommended to buffer the analog input before applying it to the
ADC. The analog input range can be programmed to be either
0 V to V
REF
or 0 V to 2 × V
REF
. In 2 × V
REF
mode, the input is
effectively divided by 2 before the conversion takes place. Note
that the voltage with respect to GND on the ADC analog input
pins cannot exceed AV
DD
.
If the analog input signal to be sampled is bipolar, the internal
reference of the ADC can be used to externally bias up this