EVAL-AD7324CB AD [Analog Devices], EVAL-AD7324CB Datasheet - Page 21

• Manufacturer Part Number: EVAL-AD7324CB
• Description: 4-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
AD7324 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
AD7324 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
AD7324, 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
= 2.5V
RANGE
= 2.5V
Figure 41. Pseudo Input Range with V
±10V
Figure 40. Pseudo Input Range with V
±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
RANGE
SS
RANGE
SS
CC
CC
±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 AD7324. 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 AD7324 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 AD7324. 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
IN
+ and V
V+
V–
V+
V–
CC
IN
− be
supply voltage,
AD7324