EVAL-AD7933CB AD [Analog Devices], EVAL-AD7933CB Datasheet - Page 20

• Manufacturer Part Number: EVAL-AD7933CB
• Description: 4-Channel, 1.5 MSPS, 12-Bit and 10-Bit Parallel ADCs with a Sequencer
• Manufacturer: AD [Analog Devices]

AD7933/AD7934
Figure 25 shows a graph of the THD versus the analog input
frequency for various supplies, while sampling at 1.5 MHz with
an SCLK of 20 MHz. In this case, the source impedance is 10 Ω.
ANALOG INPUTS
The AD7933/AD7934 have software selectable analog input
configurations. Users can choose either four single-ended
inputs, two fully differential pairs, or two pseudo-differential
pairs. The analog input configuration is chosen with Bits
MODE0/MODE1 in the internal control register (see
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 both 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. The analog input range is either 0
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 of the correct format for the ADC.
Figure 26
the ADC in single-ended mode.
Figure 24. THD vs. Analog Input Frequency for Various Source Impedances
Figure 25. THD vs. Analog Input Frequency for Various Supply Voltages
REF
or 0 to 2 × V
shows a typical connection diagram when operating
REF
.
Table 9
).
Rev. PrG | Page 20 of 32
Differential Mode
The AD7933/AD7934 can have two differential analog input
pairs by setting Bits MODE0 and MODE1 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. F
input of the AD7933/AD7934.
The amplitude of the differential signal is the difference
between the signals applied to the V
differential pair (i.e., V
simultaneously driven by two signals, each of amplitude V
that are 180° out of phase. The amplitude of the differential
signal is therefore −V
This is regardless of the common mode (CM). The common
mode is the average of the two signals, i.e. (V
therefore the voltage that the two inputs are centered on. This
results in the span of each input being CM ± V
voltage has to be set up externally and its range varies with V
As the value of V
decreases. When driving the inputs with an amplifier, the actual
common-mode range is determined by the amplifier’s output
voltage swing.
Figure 28 and Figure 29 show how the common-mode range
typically varies with V
The common mode must be in this range to guarantee the
functionality of the AD7933/AD7934.
+1.25V
–1.25V
*ADDITIONAL PINS OMITTED FOR CLARITY
0V
COMMON-MODE
Figure 26. Single-Ended Mode Connection Diagram
VOLTAGE
V
IN
igure 27
Figure 27. Differential Input Definition
REF
3R
R
increases, the common-mode range
*ADDITIONAL PINS OMITTED FOR CLARITY
REF
Preliminary Technical Data
REF
IN+
defines the fully differential analog
to +V
for both a 5 V and a 3 V power supply.
− V
V
p-p
V
p-p
R
REF
REF
IN−
REF
). V
peak-to-peak (i.e., 2 × V
IN+
+2.5V
IN+
0V
and V
and V
V
V
IN+
IN–
AD7934*
AD7933/
V
V
IN−
IN+
IN−
IN0
IN3
REF
should be
+ V
pins in each
AD7934*
AD7933/
/2. This
V
IN−
REFOUT
)/2, and is
0.47µF
REF
REF
).
REF
.