EVAL-AD7934CB Analog Devices Inc, EVAL-AD7934CB Datasheet - Page 19

• Manufacturer Part Number: EVAL-AD7934CB
• Manufacturer: Analog Devices Inc

Specifications of EVAL-AD7934CB

• Lead Free Status / Rohs Status: Not Compliant

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.
Figure 22. THD vs. Analog Input Frequency for Various Supply Voltages
Figure 21. THD vs. Source Impedance in Fully Differential Mode
–100
–100
–110
–120
Figure 20. THD vs. Source Impedance in Single-Ended Mode
–40
–45
–50
–55
–60
–65
–70
–75
–80
–85
–90
–60
–65
–70
–75
–80
–85
–90
–95
–50
–60
–70
–80
–90
10
10
0
F
F
F
RANGE = 0 TO V
IN
IN
SAMPLE
V
DD
= 50kHz
= 50kHz
100
= 3V
= 1.5MSPS
V
SINGLE-ENDED MODE
V
DIFFERENTIAL MODE
DD
DD
= 3V
= 5V/3V
200
REF
INPUT FREQUENCY (kHz)
V
SINGLE-ENDED MODE
DD
R
R
= 5V
300
SOURCE
SOURCE
V
DD
100
100
= 5V
400
(Ω)
(Ω)
V
500
V
DD
DD
= 3V
= 5V
600
700
1k
1k
Rev. B | Page 19 of 32
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
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
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.
+1.25V
–1.25V
*ADDITIONAL PINS OMITTED FOR CLARITY.
0V
Figure 23. Single-Ended Mode Connection Diagram
V
REF
IN
mode can be used.
3R
R
R
R
+2.5V
0V
AD7933/AD7934
REF
, or 0 V to 2 × V
V
V
IN
IN
0
3
AD7933/
AD7934*
V
REFOUT
0.47µF
REF
.