Datasheet

AD7328 Data Sheet
Rev. C | Page 20 of 36
04852-039
8
–8
6
4
2
0
–2
–4
–
6
±2.5V
RANG
E
±10V
RANGE
±
10V
RAN
GE
±
5V RANGE
±
2.
5
V
RAN
GE
±5V RANGE
0
V
TO
+10
V
RAN
GE
0V TO +10V
RANG
E
±
16.
5
V V
D
D
/V
S
S
±12V V
DD
/V
S
S
V
C
C
= 5
V
V
REF
= 2.5V
Figure 40. Pseudo Differential Input Range with V
CC
= 5 V
04852-040
±2.5V
RANGE
±10V
RANGE
±10V
RANGE
±5V RANGE
±2.5V
RANGE
±5V RANGE
0V TO +10V
RANGE
0V TO +10V
RANGE
±16.5V V
DD
/V
SS
±12V V
DD
/V
SS
4
–8
2
0
–2
–4
–6
V
CC
= 3V
V
REF
= 2.5V
Figure 41. Pseudo Differential Input Range with V
CC
= 3 V
DRIVER AMPLIFIER CHOICE
In applications where the harmonic distortion and signal-to-
noise ratio are critical specifications, the analog input of the
AD7328 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 AD7328 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
AD7328, 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 AD7328. 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 AD7328 in single-ended mode configuration.
Differential operation requires that V
IN
+ and V
IN
− 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 AD7328. The common-mode range is determined by the
REFIN/OUT voltage, the V
CC
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 a 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.
04852-029
V
IN
V+
V–
2kΩ
1.5kΩ
1.5kΩ
1.5kΩ
1.5kΩ
10kΩ
1.5kΩ
Figure 42. Single-Ended-to-Differential Configuration with the AD845
V
IN
V+
V–
442Ω
442Ω
442Ω
442Ω
442Ω
100Ω
AD8021
AD8021
442Ω
04852-030
Figure 43. Single-Ended-to-Differential Configuration with the AD8021