Datasheet

AD7938/AD7939 Data Sheet
Rev. C | Page 20 of 36
R
SOURCE
(Ω)
THD (dB)
–40
–45
–50
–55
–80
–75
–70
–65
–60
–90
–85
10 100 1k
03715-018
F
IN
= 50kHz
V
DD
= 5V
V
DD
= 3V
Figure 21. 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
03715-019
F
IN
= 50kHz
V
DD
= 5V
V
DD
= 3V
Figure 22. THD vs. Source Impedance in Fully Differential Mode
Figure 23 shows a graph of the THD vs. the analog input
frequency 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 40020
0 300 600500 700
03715-020
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 23. THD vs. Analog Input Frequency for Various Supply Voltages
ANALOG INPUTS
The AD7938/AD7939 have software-selectable analog input
configurations. The user can choose eight single-ended inputs,
four fully differential pairs, four pseudo differential pairs, or
seven pseudo differential inputs. The analog input
configuration is chosen by setting the MODE0/MODE1 bits in
the internal control register (see Table 10).
Single-Ended Mode
The AD7938/AD7939 can have eight 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 op amp suitable for this function is
the AD8021. The analog input range of the AD7938/AD7939
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 24 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 AD7938/AD7939. 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 AD7938/AD7939 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
+2.5V
0V
V
IN
0
V
IN
7
V
REFOUT
AD7938/
AD7939*
R
*ADDITIONAL PINS OMITTED FOR CLARITY.
03715-031
Figure 24. Single-Ended Mode Connection Diagram
Differential Mode
The AD7938/AD7939 can have four 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 25 defines the fully differential analog
input of the AD7938/AD7939.