EVAL-AD7762EB View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
'EVAL-AD7762EB' PDF : 28 Pages View PDF
AD7762
DRIVING THE AD7762
The AD7762 has an on-chip differential amplifier that operates
with a supply voltage (AVDD3) 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 25. 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 listed in
Table 8. The values in Table 8 yield a 10 dB attenuation at the
first alias point of 19 MHz.
CFB
RFB
RIN
A
CS
B
RIN
A1
RFB
CFB
RM
VIN–
RM
VIN+
Figure 25. Differential Amplifier Configuration
Table 8. Normal Mode Component Values
VREF
RIN
RFB
RM
CS
4.096 V 1 kΩ
655 Ω 18 Ω
5.6 pF
CFB
33 pF
Figure 26 shows the signal conditioning that occurs using the
circuit in Figure 25 with a ±2.5 V input signal biased around
ground and having the component values and conditions in
Table 8. The differential amplifier always biases the output
signal to sit on the optimum common mode of VREF/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 VREF, that is, 0.8 × 4.096 V ≈ 3.275 V p-p
on each input.
To obtain maximum performance from the AD7762, it is
advisable to drive the ADC with differential signals. Figure 27
shows how a bipolar, single-ended signal biased around ground
can drive the AD7762 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 (AVDD4). With a 2.5 V reference, a 3.3 V supply
must be provided to AVDD4.
+2.5V
0V
A
–2.5V
+3.685V
+2.048V
+0.410V
VIN+
+2.5V
0V
–2.5V
VIN
+3.685V
B
+2.048V
VIN–
+0.410V
Figure 26. Differential Amplifier Signal Conditioning
CFB
2R
R
2R
AD8021
RIN
CS
RFB
A1
RIN
RFB
CFB
RM
VIN–
RM
VIN+
Figure 27. Single-Ended-to-Differential Conversion
VIN+
CPA
SS1
CS1
SH3
SH1
SS3
CPB1
ANALOG
MODULATOR
SS2
CS2
SH4
SH2
SS4
CPB2
Figure 28. Equivalent Input Circuit
The AD7762 employs a double sampling front end, as shown in
Figure 28. For simplicity, only the equivalent input circuit for VIN+
is shown. The equivalent input circuitry for VIN− is the same.
Rev. 0 | Page 16 of 28
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