AD7760 Analog Devices, AD7760 Datasheet - Page 26

• Manufacturer Part Number: AD7760
• Description: 2.5 MSPS, 24-Bit, 100 dB Sigma-Delta ADC with On-Chip Buffer
• Manufacturer: Analog Devices

Specifications of AD7760

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

AD7760
DRIVING THE AD7760
The AD7760 has an on-chip differential amplifier that operates
with a supply voltage (AV
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.
Table 8. Normal Mode Component Values
V
4.096 V
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
REF
R
1 kΩ
IN
Figure 50. Differential Amplifier Configuration
A
B
R
655 Ω
R
R
FB
IN
C
IN
DD
S
INPUTS TO THE AD7760 DIFFERENTIAL AMPLIFIER
3) within the 3.15 V to 5.25 V range.
+2.5V
–2.5V
+2.5V
–2.5V
R
C
0V
0V
C
R
A1
FB
FB
FB
FB
R
18 Ω
M
R
R
M
M
A
B
C
5.6 pF
S
Figure 52. Differential Amplifier Signal Conditioning
V
V
IN
IN
–
+
C
33 pF
FB
Rev. A | Page 26 of 36
OUTPUTS OF THE AD7760 DIFFERENTIAL AMPLIFIER
+3.685V
+2.048V
+0.410V
+3.685V
+2.048V
+0.410V
OUTPUT
signal to sit on the optimum common mode of V
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
on each input.
With a 4.096 V reference, a 5 V supply must be provided to the
reference buffer (AV
must be provided to AV
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
differential amplifier A1, as shown in Figure 52.
AD7760
Figure 51. Transfer Function for the AD7760 Filtered Output Where V
24-BIT
011…111
011…110
000…010
000…001
000…000
111…111
111…110
100…001
100…000
24 BITS
are Inputs to the On-Board Differential Amplifier A1
B = +2.5V
A = –2.5V
DD
REF
4). With a 2.5 V reference, a 3.3 V supply
V
V
A
DD
IN
IN
, that is, 0.8 × 4.096 V ≈ 3.275 V p-p
and V
+
–
4.
B
applied to the on-board
A = 0V
B = 0V
REF
/2, in this
A = +2.5V
B = –2.5V
A
and V
B