Datasheet

AD7763 Data Sheet
Rev. B | Page 20 of 32
DRIVING THE AD7763
The AD7763 has an on-chip differential amplifier that operates
with a supply voltage (AV
DD3
) from 3.15 V to 5.25 V. For a 4.096 V
reference, the supply voltage must be 5 V.
To achieve the specified performance in normal mode, the
differential amplifier should be configured as a first-order
antialias filter, as shown in Figure 31. Any additional filtering
should be carried out in previous stages using low noise, high
performance op amps, such as the AD8021.
Suitable component values for the first-order filter are shown in
Table 10. The values in Table 10 yield a 10 dB attenuation at the
first alias point of 19 MHz.
05476-016
A1
R
IN
R
FB
C
FB
R
IN
R
M
R
M
C
S
R
FB
C
FB
V
IN
A
B
V
IN
+
Figure 31. Differential Amplifier Configuration
Table 10. Normal Mode Component Values
V
REF
R
IN
R
FB
R
M
C
S
C
FB
4.096 V
1 kΩ
655 Ω
18 Ω
5.6 pF
33 pF
Figure 32 shows the signal conditioning that occurs using the
circuit in Figure 18 with a ±2.5 V input signal biased around
ground and having the component values and conditions in
Table 10.
The differential amplifier always biases the output signal to sit
on the optimum common mode of V
REF
/2, in this case, 2.048 V.
The signal is also scaled to give the maximum allowable voltage
swing with this reference value. This is calculated as 80% of
V
REF
; that is, 0.8 × 4.096 V ≈ 3.275 V p-p on each input.
To obtain maximum performance from the AD7763, it is
advisable to drive the ADC with differential signals. Figure 33
shows how a bipolar, single-ended signal biased around ground
can drive the AD7763 with the use of an external op amp, such as
the AD8021.
With a 4.096 V reference, a 5 V supply must be provided to the
reference buffer (AV
DD4
). With a 2.5 V reference, a 3.3 V supply
must be provided to AV
DD4
.
05476-017
+2.5V
0V
–2.5V
+2.5V
0V
–2.5V
+3.685V
+2.048V
+0.410V
+3.685V
+2.048V
+0.410V
A
V
IN
+
V
IN
B
Figure 32. Differential Amplifier Signal Conditioning
05476-018
A1
R
IN
R
FB
C
FB
R
IN
R
M
R
M
C
S
R
FB
C
FB
V
IN
V
IN
V
IN
+
AD8021
2R
2R
R
Figure 33. Single-Ended-to-Differential Conversion
05476-019
CS2
CPB2
SS4
SH4
CPA
SS2
SH2
CS1
CPB1
SS3
SH3
SS1
SH1
ANALOG
MODULATOR
V
IN
+
Figure 34. Equivalent Input Circuit
The AD7763 employs a double sampling front end, as shown in
Figure 34. For simplicity, only the equivalent input circuit for
V
IN
+ is shown. The equivalent input circuitry for V
IN
− is the same.