AD7763 Analog Devices, AD7763 Datasheet - Page 20

• Manufacturer Part Number: AD7763
• Description: 24-Bit, 625 kSPS, 109 dB Sigma-Delta ADC with On-Chip Buffers, Serial Interface
• Manufacturer: Analog Devices

Specifications of AD7763

• Resolution (bits): 24bit
• # Chan: 1
• Sample Rate: 40MSPS
• Interface: I2S,Ser
• Analog Input Type: Diff-Bip
• Ain Range: 4 V p-p,6.5 V p-p
• Adc Architecture: Sigma-Delta
• Pkg Type: QFP

AD7763
DRIVING THE AD7763
The AD7763 has an on-chip differential amplifier that operates
with a supply voltage (AV
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.
Table 10. Normal Mode Component Values
V
4.096 V
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
The signal is also scaled to give the maximum allowable voltage
swing with this reference value. This is calculated as 80% of
V
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
must be provided to AV
REF
REF
; that is, 0.8 × 4.096 V ≈ 3.275 V p-p on each input.
R
1 kΩ
IN
Figure 31. Differential Amplifier Configuration
A
B
DD4
R
655 Ω
). With a 2.5 V reference, a 3.3 V supply
FB
DD4
R
R
DD3
IN
C
IN
S
.
) from 3.15 V to 5.25 V. For a 4.096 V
R
C
C
R
R
18 Ω
A1
FB
FB
FB
FB
M
REF
/2, in this case, 2.048 V.
R
R
M
M
C
5.6 pF
S
V
V
IN
IN
–
+
C
33 pF
FB
Rev. A | Page 20 of 32
+2.5V
–2.5V
+2.5V
–2.5V
The AD7763 employs a double sampling front end, as shown in
Figure 34. For simplicity, only the equivalent input circuit for
V
V
IN
IN
0V
0V
+ is shown. The equivalent input circuitry for V
V
IN
+
CPA
Figure 32. Differential Amplifier Signal Conditioning
A
B
Figure 33. Single-Ended-to-Differential Conversion
2R
Figure 34. Equivalent Input Circuit
R
SS1
SH1
SS2
SH2
CPB1
CPB2
AD8021
2R
CS1
CS2
SS3
SS4
+3.685V
+2.048V
+0.410V
+3.685V
+2.048V
+0.410V
R
R
IN
IN
C
SH3
SH4
S
R
C
C
R
A1
FB
FB
MODULATOR
FB
FB
ANALOG
IN
− is the same.
R
R
V
V
M
M
IN
IN
+
–
V
V
IN
IN
–
+