AD7329 Analog Devices, AD7329 Datasheet - Page 23

• Manufacturer Part Number: AD7329
• Description: 1 MSPS , 8-Channel, Software Selectable True Bipolar Input, 12-Bit Plus Sign A/D Converter
• Manufacturer: Analog Devices

Specifications of AD7329

• Resolution (bits): 13bit
• # Chan: 8
• Sample Rate: 1MSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Bip,Diff-Uni,SE-Uni
• Ain Range: Bip (Vref),Bip (Vref) x 2,Bip (Vref) x 4,Bip 10V,Bip 2.5V,Bip 5.0V,Uni (Vref),Uni 10V
• Adc Architecture: SAR
• Pkg Type: SOP

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, within 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 AD8021
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
Rev. A | Page 23 of 40
Table 8. Typical AC Performance Using Different Op Amps
in Single-Ended Mode, ±10 V Input Range
Parameter
SNR (dB)
SNRD (dB)
THD (dB)
Table 9. Typical AC Performance Using Different Op Amps
in Differential Mode, ±10 V Input Range
Parameter
SNR (dB)
SNRD (dB)
THD (dB)
Differential operation requires that V
neously 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
REF
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.
OUT
voltage, the V
No Buffer
74.24
72.42
−77.05
No Buffer
77.16
76.50
−84.91
CC
supply voltage, and the particular amplifier
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
− be simulta-
AD7329
AD8610
73.88
71.98
−76.47
AD8610
76.76
75.89
−83.24
IN
/