AD7322 Analog Devices, AD7322 Datasheet - Page 20

• Manufacturer Part Number: AD7322
• Description: Software Selectable True Bipolar Input, 2-Channel, 12-Bit Plus Sign ADC
• Manufacturer: Analog Devices

Specifications of AD7322

• Resolution (bits): 13bit
• # Chan: 2
• Sample Rate: 1MSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Bip,Diff-Uni,SE-Uni
• Ain Range: Bip (Vref),Bip (Vref) x 2,Bip (Vref) x 4,Bip 10V,Bip 2.5V,Bip 5.0V,Uni (Vref) x 4,Uni 10V
• Adc Architecture: SAR
• Pkg Type: SOP

AD7322
DRIVER AMPLIFIER CHOICE
In applications where the harmonic distortion and signal-to-
noise ratio are critical specifications, the analog input of the
AD7322 should be driven from a low impedance source. Large
source impedances significantly affect the ac performance of the
ADC and can necessitate the use of an input buffer amplifier.
When no amplifier is used to drive the analog input, the source
impedance should be limited to low values. The maximum source
impedance depends on the amount of THD that can be tolerated
in the application. The THD increases as the source impedance
increases and performance degrades. Figure 21 and Figure 22
show graphs of the THD vs. the analog input frequency for various
source impedances. Depending on the input range and analog
input configuration selected, the AD7322 can handle source
impedances of up to 4.7 kΩ before the THD starts to degrade.
Due to the programmable nature of the analog inputs on the
AD7322, the choice of op amp used to drive the inputs is a
function of the particular application and depends on the input
configuration and the analog input voltage ranges selected.
The driver amplifier must be able to settle for a full-scale step to
a 13-bit level, 0.0122%, in less than the specified acquisition
–2
–4
–6
–8
–2
–4
–6
–8
8
6
4
2
0
4
2
0
V
V
V
V
CC
REF
CC
REF
Figure 40. Pseudo Input Range with V
Figure 41. Pseudo Input Range with V
= 5V
= 3V
RANGE
= 2.5V
RANGE
= 2.5V
±10V
±10V
±5V RANGE
±5V RANGE
±16.5V V
±16.5V V
RANGE
RANGE
±2.5V
±2.5V
DD
DD
0V TO +10V
0V TO +10V
/V
/V
RANGE
RANGE
SS
SS
RANGE
RANGE
±10V
±10V
±12V V
±12V V
±5V RANGE
±5V RANGE
DD
DD
CC
CC
/V
/V
RANGE
SS
RANGE
SS
= 5 V
= 3 V
±2.5V
±2.5V
0V TO +10V
0V TO +10V
RANGE
RANGE
Rev. A | Page 20 of 36
time of the AD7322. An op amp such as the AD8021 meets this
requirement when operating in single-ended mode. The
AD8021 needs an external compensating NPO type of
capacitor. The AD8022 can also be used in high frequency
applications where a dual version is required.
For lower frequency applications, op amps such as the AD797,
AD845, and the
ended mode configuration.
Differential operation requires that V
neously driven with two signals of equal amplitude that are 180°
out of phase. The common mode must be set up externally to the
AD7322. The common-mode range is determined by the REFIN/
OUT voltage, the V
used to drive the analog inputs. Differential mode with either an
ac input or a dc input provides the best THD performance over a
wide frequency range. Because not all applications have a signal
preconditioned for differential operation, there is often a need to
perform the single-ended-to-differential conversion.
This single-ended-to-differential conversion can be performed
using an op amp pair. Typical connection diagrams for an op
amp pair are shown in Figure 42 and Figure 43. In Figure 42 the
common-mode signal is applied to the noninverting input of
the second amplifier.
Figure 43. Single-Ended-to-Differential Configuration with the
Figure 42. Single-Ended-to-Differential Configuration with the AD845
V
V
COM
V
IN
AD8610
IN
100Ω
CC
442Ω
1.5kΩ
10kΩ
3kΩ
supply voltage, and the particular amplifier
can be used in the AD7322 single-
20kΩ
442Ω
442Ω
442Ω
442Ω
442Ω
1.5kΩ
1.5kΩ
1.5kΩ
AD8021
AD8021
IN
+ and the V
V+
V–
V+
V–
IN
− be simulta-
AD8021