Datasheet

AD7760
Rev. A | Page 26 of 36
DRIVING THE AD7760
The AD7760 has an on-chip differential amplifier that operates
with a supply voltage (AV
DD
3) within the 3.15 V to 5.25 V range.
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 50. 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. Using the values in the table as an example yields a
10 dB attenuation at the first alias point of 19 MHz.
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
+
0
4975-040
Figure 50. Differential Amplifier Configuration
Table 8. 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 52 shows the signal conditioning that occurs using the
circuit shown in
Figure 50 with a ±2.5 V input signal biased
around ground and the component values and conditions listed
in
Table 8. 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.
With a 4.096 V reference, a 5 V supply must be provided to the
reference buffer (AV
DD
4). With a 2.5 V reference, a 3.3 V supply
must be provided to AV
DD
4.
Figure 51 shows the transfer function in terms of the 24-bit
digital output codes (twos complement coding) of the AD7760
vs. the voltage signals V
A
and V
B
applied to the on-board
differential amplifier A1, as shown in
Figure 52.
011…111
011…110
000…010
000…001
000…000
111…111
111…110
100…000
100…001
24 BITS
AD7760
24-BIT
O
UTPUT
B=+2.5V
A = –2.5V
A=0V
B=0V
A=+2.5V
B = –2.5V
04975-056
Figure 51. Transfer Function for the AD7760 Filtered Output Where V
A
and V
B
are Inputs to the On-Board Differential Amplifier A1
+2.5V
0V
–2.5V
A
+2.5V
0V
–2.5V
B
INPUTS TO THE AD7760 DIFFERENTIAL AMPLIFIER
OUTPUTS OF THE AD7760 DIFFERENTIAL AMPLIFIER
+3.685V
+2.048V
+0.410V
+3.685V
+2.048V
+0.410V
V
IN
+
V
IN
–
0
4975-055
Figure 52. Differential Amplifier Signal Conditioning