
Preliminary Technical Data
AD7760
DRIVING THE AD7760
The AD7760 has an on-chip differential amplifier. This
amplifier will operate with a supply voltage (AV
DD3
) from 3V to
5.5V. For a 4.096V reference, the supply voltage must be 5V.
Rev. PrN | Page 15 of 22
To achieve the specified performance in full power mode, the
differential amplifier should be configured as a first order anti-
alias filter as shown in Figure 16. 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 7. Using the first row as an example would yield a 10dB
attenuation at the first alias point of 19MHz.
Figure 16. Differential Amplifier Configuration
Table 7. Full Power Component Values
ODR
V
REF
R
IN
R
FB
C
S
C
FB
2.5MHz
4.096v
1k
655
5.6pF
33pF
2.5MHz
2.5v
TBD
TBD
TBD
TBD pF
48kHz
4.096v
TBD
TBD
TBD
TBD pF
48kHz
2.5v
TBD
TBD
TBD
TBD pF
Figure 17 shows the signal conditioning that occurs using the
circuit in Figure 16 with a ±2.5v input signal biased around
ground using the component values and conditions in the first
row of Table 7. The differential amplifier will always bias the
output signal to sit on the optimum common mode of V
REF
/2, in
this case 2.048V. The signal is also scaled to give the maximum
allowable voltage swing with this reference value. This is
calculated as 80% of V
REF
, i.e. 0.8 × 4.096V
≈
3.275V peak to
peak on each input.
(
) % (
* % (
(
) % (
* % (
) %(
%(
) %(
(
)
(
*
) %(
%(
) %(
Figure 17. Differential Amplifier Signal Conditioning
To obtain maximum performance from the AD7760, it is
advisable to drive the ADC with differential signals. However, it
is possible to drive the AD7760 with a single ended signal once
the common mode of the signal is within the range of +0.7V to
+2.1V with V
DD3
= 5V or +0.7 to +1.25V with V
DD3
= 3.3V. In
this case the on-chip differential amplifier can be used to
convert the signal from single-ended to differential before being
fed into the modulator inputs. Figure 18 shows how a bipolar
single-ended signal biased around ground can be used to drive
the AD7760 with the use of an external op-amp such as the
AD8021.
Figure 18. Single Ended to Differential Conversion