Datasheet

AD7933/AD7934
Rev. B | Page 19 of 32
R
SOURCE
(Ω)
THD (dB)
–
40
–45
–50
–55
–80
–75
–70
–65
–60
–90
–85
10 100 1k
03713-018
F
IN
= 50kHz
V
DD
=5V
V
DD
=3V
Figure 20. THD vs. Source Impedance in Single-Ended Mode
R
SOURCE
(Ω)
THD (dB)
–
60
–80
–75
–70
–65
–100
–85
–90
–95
10 100 1k
03713-019
F
IN
= 50kHz
V
DD
=5V
V
DD
=3V
Figure 21. THD vs. Source Impedance in Fully Differential Mode
Figure 22 shows a graph of the THD vs. the analog input fre-
quency for various supplies, while sampling at 1.5 MHz with an
SCLK of 25.5 MHz. In this case, the source impedance is 10 Ω.
INPUT FREQUENCY (kHz)
THD (dB)
–
50
–60
–70
–80
–110
–100
–90
–120
0 100 400200 300 600500 700
03713-020
F
SAMPLE
= 1.5MSPS
RANGE = 0 TO V
REF
V
DD
=3V
SINGLE-ENDED MODE
V
DD
=5V/3V
DIFFERENTIAL MODE
V
DD
=5V
SINGLE-ENDED MODE
Figure 22. THD vs. Analog Input Frequency for Various Supply Voltages
ANALOG INPUTS
The AD7933/AD7934 have software selectable analog input
configurations. Users can choose from among the following
configurations: four single-ended inputs, two fully differential
pairs, or two pseudo differential pairs. The analog input
configuration is chosen by setting the MODE0/MODE1 bits in
the internal control register (see
Table 10).
Single-Ended Mode
The AD7933/AD7934 can have four single-ended analog input
channels by setting the MODE0 and MODE1 bits in the control
register to 0. In applications where the signal source has a high
impedance, it is recommended to buffer the analog input before
applying it to the ADC. An amplifier suitable for this function is
the AD8021. The analog input range of the AD7933/AD7934
can be programmed to be either 0 V to V
REF
, or 0 V to 2 × V
REF
.
If the analog input signal to be sampled is bipolar, the internal
reference of the ADC can be used to externally bias up this
signal to make it the correct format for the ADC.
Figure 23 shows a typical connection diagram when operating
the ADC in single-ended mode. This diagram shows a bipolar
signal of amplitude ±1.25 V being preconditioned before it is
applied to the AD7933/AD7934. In cases where the analog
input amplitude is ±2.5 V, the 3R resistor can be replaced with a
resistor of value R. The resultant voltage on the analog input of
the AD7933/AD7934 is a signal ranging from 0 V to 5 V. In this
case, the 2 × V
REF
mode can be used.
0.47µF
+1.25V
V
IN
R
R
R
3R
0V
–
1.25V
+2.5
V
0V
V
IN
0
V
IN
3
V
REFOUT
AD7933/
AD7934*
*ADDITIONAL PINS OMITTED FOR CLARITY.
03713-031
Figure 23. Single-Ended Mode Connection Diagram
Differential Mode
The AD7933/AD7934 can have two differential analog input
pairs by setting the MODE0 and MODE1 bits in the control
register to 0 and 1, respectively.
Differential signals have some benefits over single-ended
signals, including noise immunity based on the device’s
common-mode rejection and improvements in distortion
performance.
Figure 24 defines the fully differential analog
input of the AD7933/AD7934.