EVAL-AD7321CB AD [Analog Devices], EVAL-AD7321CB Datasheet - Page 21

• Manufacturer Part Number: EVAL-AD7321CB
• Description: 500 kSPS, 2-Channel, Software-Selectable, True Bipolar Input, 12-Bit Plus Sign ADC
• Manufacturer: AD [Analog Devices]

DRIVER AMPLIFIER CHOICE
In applications where the harmonic distortion and signal-to-
noise ratio are critical specifications, the analog input of the
AD7321 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 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
AD7321 can handle source impedances of up to 5.5 kΩ before
the THD starts to degrade.
Due to the programmable nature of the analog inputs on the
AD7321, 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.
–2
–4
–6
–8
–2
–4
–6
–8
8
6
4
2
0
4
2
0
V
V
V
V
CC
REF
CC
REF
= 5V
= 3V
RANGE
RANGE
= 2.5V
Figure 40. Pseudo Input Range with V
Figure 41. Pseudo Input Range with V
= 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
/V
/V
CC
CC
RANGE
SS
RANGE
SS
±2.5V
±2.5V
0V TO +10V
0V TO +10V
= 5 V
= 3 V
RANGE
RANGE
Figure 21 and
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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
time of the AD7321. 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 AD8610 can be used with
the AD7321 in single-ended mode configuration.
Differential operation requires that V
simultaneously driven with two signals of equal amplitude that
are 180° out of phase. The common mode must be set up
externally to the AD7321. The common-mode range is
determined by the REFIN/OUT voltage, the V
and the particular amplifier 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 AD8021
Figure 42. Single-Ended-to-Differential Configuration with the AD845
V
V
COM
V
IN
IN
100Ω
442Ω
1.5kΩ
10kΩ
3kΩ
20kΩ
442Ω
442Ω
442Ω
442Ω
442Ω
1.5kΩ
1.5kΩ
1.5kΩ
AD8021
AD8021
AD845
AD845
IN
+ and V
V+
V–
V+
V–
CC
IN
− be
supply voltage,
AD7321