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AD7327 Datasheet(PDF) 21 Page - Analog Devices

Part No. AD7327
Description  500 kSPS, 8-Channel, Software-Selectable, True Bipolar Input, 12-Bit Plus Sign ADC
Download  36 Pages
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD7327 Datasheet(HTML) 21 Page - Analog Devices

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AD7327
Rev. 0 | Page 21 of 36
8
–8
6
4
2
0
–2
–4
–6
±2.5V
RANGE
±10V
RANGE
±10V
RANGE
±5V RANGE
±2.5V
RANGE
±5V RANGE
0V TO +10V
RANGE
0V TO +10V
RANGE
±16.5V VDD/VSS
±12V VDD/VSS
VCC =5V
VREF =2.5V
Figure 40. Pseudo Input Range with VCC = 5 V
±2.5V
RANGE
±10V
RANGE
±10V
RANGE
±5V RANGE
±2.5V
RANGE
±5V RANGE
0V TO +10V
RANGE
0V TO +10V
RANGE
±16.5V VDD/VSS
±12V VDD/VSS
4
–8
2
0
–2
–4
–6
VCC =3V
VREF =2.5V
Figure 41. Pseudo Input Range with VCC = 3 V
DRIVER AMPLIFIER CHOICE
In applications where the harmonic distortion and signal-to-
noise ratio are critical specifications, the analog input of the
AD7327 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 AD7327 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
AD7327, 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
time of the AD7327. 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 AD7327 in single-ended mode configuration.
Differential operation requires that VIN+ and VIN− be simulta-
neously driven with two signals of equal amplitude that are 180°
out of phase. The common mode must be set up externally to the
AD7327. The common-mode range is determined by the REFIN/
OUT voltage, the VCC supply voltage, 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.
VIN
V+
V–
3kΩ
1.5kΩ
1.5kΩ
1.5kΩ
1.5kΩ
10kΩ
20kΩ
VCOM
AD845
AD845
Figure 42. Single-Ended-to-Differential Configuration with the AD845
VIN
V+
V–
442Ω
442Ω
442Ω
442Ω
442Ω
100Ω
AD8021
AD8021
442Ω
Figure 43. Single-Ended-to-Differential Configuration with the AD8021


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