AD7450BRM-REEL Datasheet, PDF (11/22 Pages) Analog Devices – Differential Input, 1 MSPS ADC in
SOIC-8 and SO-8

AD7450
V_REF p-p
V_REF p-p
0.1␮F
3V/5V
SUPPLY
10␮F
SERIAL
INTERFACE
V_DD
CM* V_IN+
SCLK
AD7450
SDATA
CM* V_IN–
V_REF
CS
GND
␮C/␮P
1.25V/2.5V
V_REF
0.1␮F
*CM = COMMON-MODE VOLTAGE
Figure 6. Typical Connection Diagram
THE ANALOG INPUT
The analog input of the AD7450 is fully differential. Differential
signals have a number of benefits over single-ended signals,
including noise immunity based on the device’s common-mode
rejection, improvements in distortion performance, doubling of
the device’s available dynamic range, and flexibility in input ranges
and bias points.
Figure 7 defines the fully differential analog input of the AD7450.
Figures 8 and 9 show how the common-mode range typically
varies with V_REF for both a 5 V and a 3 V power supply. The
common mode must be in this range to guarantee the
functionality of the AD7450.
For ease of use, the common mode can be set up to be equal to
V_REF, resulting in the differential signal being ± V_REF centered
on V_REF. When a conversion takes place, the common mode is
rejected resulting in a virtually noise free signal of amplitude
–V_REF to +V_REF corresponding to the digital codes of 0 to 4095.
5.0
4.5
4.0
3.5
3.25V
3.0
2.5
COMMON-MODE RANGE
2.0
1.75V
1.5
1.0
0.5
0
0.25 0.75
1.25
1.75
2.25
V_REF
2.75
3.25 3.50
Figure 8. Input Common-Mode Range vs. V_REF
(V_DD = 5 V and V_REF (Max) = 3.5 V)
3.0
COMMON-MODE
VOLTAGE
V_REF p-p
V_REF p-p
V_IN+
AD7450
V_IN–
2.5
2.0
2V
1.5
COMMON-MODE RANGE
Figure 7. Differential Input Definition
The amplitude of the differential signal is the difference between
the signals applied to the V_IN+ and V_IN– pins (i.e., V_IN+ – V_IN–).
V_IN+ and V_IN– are simultaneously driven by two signals each of
amplitude V_REF that are 180° out of phase. The amplitude of
the differential signal is therefore –V_REF to +V_REF p-p
(i.e., 2 Ï« V_REF). This is regardless of the common mode (CM).
The common mode is the average of the two signals, i.e.,
(V_IN+ + V_IN–)/2, and is therefore the voltage that the two inputs
are centered on. This results in the span of each input being
CM ± V_REF/2. This voltage has to be set up externally and its
range varies with V_REF. As the value of V_REF increases, the com-
mon-mode range decreases. When driving the inputs with an
amplifier, the actual common-mode range will be determined
by the amplifier’s output voltage swing.
1.0
1V
0.5
0
0.25 0.50 0.75
1.00
1.25
V_REF
1.50
1.75
2.00 2.20
Figure 9. Input Common-Mode Range vs. V_REF (V_DD = 3 V
and V_REF (Max) = 2.2 V)
Figure 10 shows examples of the inputs to V_IN+ and V_IN– for
different values of V_REF for V_DD = 5 V. It also gives the maxi-
mum and minimum common-mode voltages for each reference
value according to Figure 8.
Rev. A
–11–