
Data Sheet
AD7327
Rev. B | 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
PSEU
D
O
I
N
PU
T
VO
LT
AG
E
RANG
E
(
V
)
0V TO +10V
RANGE
±16.5V VDD/VSS
±12V VDD/VSS
05401-
039
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
05401-
040
VCC = 3V
VREF = 2.5V
PSEU
D
O
I
N
PU
T
VO
LT
AG
E
RANG
E
(
V
)
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
show graphs of the THD vs. the analog input frequency for various
source impedances. Depending on the input range and analog
input configuration selected, t
he AD7327 can handle source
impedances of up to 5.5 k 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
requirement when operating in single-ended mode. Th
e AD8021needs 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
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
the common-mode signal is applied to the noninverting input
of the second amplifier.
VIN
V+
V–
3k
1.5k
1.5k
1.5k
1.5k
10k
20k
VCOM
05401-
029
AD845
Figure 42. Single-Ended-to-Differential Configuration with th
e AD845VIN
V+
V–
442
442
442
442
442
100
AD8021
442
05401-
030
Figure 43. Single-Ended-to-Differential Configuration with th
e AD8021