AD7367-5ARUZ AD [Analog Devices], AD7367-5ARUZ Datasheet - Page 23

• Manufacturer Part Number: AD7367-5ARUZ
• Description: 1 MSPS, 8-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
AD7329 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
AD7329 can handle source impedances of up to 4 kΩ before the
THD starts to degrade.
Due to the programmable nature of the analog inputs on the
AD7329, 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 AD7329. An op amp such as the
requirement when operating in single-ended mode. The
needs an external compensating NPO type of capacitor. The
AD8022
a dual version is required. For lower frequency applications, op
amps such as the AD797, AD845, and
the AD7329 in single-ended mode configuration.
can also be used in high frequency applications where
AD8610
AD8021
can be used with
meets this
AD8021
Rev. 0 | Page 23 of 40
Table 8. Typical AC Performance
Using Different Op Amps in Single-Ended Mode
±10 V
Table 9. Typical AC Performance
Using Different Op Amps in Differential Mode
±10 V
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 AD7329. The common-mode range is
determined by the REF
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 45 and Figure 46. In Figure 45,
the common-mode signal is applied to the noninverting input
of the second amplifier.
SNR (dB)
SNRD (dB)
THD (dB)
SNR (dB)
SNRD (dB)
THD (dB)
IN
No
Buffer
74.24
72.42
−77.05
No
Buffer
77.16
76.50
−84.91
/REF
OUT
voltage, the V
AD845
74.03
74.88
−75.95
AD845
76.81
76.02
−83.74
IN
+ and V
AD8021
73.78
72.11
−77.04
AD8021
76.95
76.78
−90.55
IN
CC
− be
supply voltage,
AD7329
AD8610
73.88
71.98
−76.47
AD8610
76.76
75.89
−83.24