Datasheet

AD8295
Rev. A | Page 25 of 28
If the application requires a lower frequency antialiasing filter
than the one shown, increasing the capacitor values produces
much better distortion results than increasing the resistor values.
DRIVING DIFFERENTIAL ADCs
Figure 71 shows how to configure the AD8295 to drive a differ-
ential ADC. The circuit shown uses very little board space and
consumes little power. With the AD7690, this configuration
gives excellent dc performance and a THD of 83 dB (10 kHz
input). For applications that need better distortion performance,
a dedicated ADC driver, such as the ADA4941-1 or ADA4922-1
is recommended.
The 500  resistors also protect the ADC against overvoltage.
Because the AD8295 runs on wider supply voltages than a typ-
ical ADC, there is a possibility of overdriving some converters.
This is not an issue with a PulSAR® ADC, such as the AD7690,
because its input can handle a 130 mA overdrive, which is much
higher than the short-circuit limit of the AD8295. However,
other converters have less robust inputs and may benefit from
the resistive protection.
The 500  resistors and the 2.2 nF capacitors form a low-pass,
antialiasing filter at 144 kHz. The four elements of the filter also
prevent the switching transients produced by a typical SAR ADC
from destabilizing the AD8295. The capacitors provide charge to
the switched-capacitor front end of the ADC, and the resistors
shield the AD8295 from driving any sharp current changes.
+7V
A1
OUT
A1
R2
A2
+IN
A2
–IN
–IN
–INPUT
+INPUT
0.1µF
–V
S
–7V
REF
+IN
IN–
GND
REF
R
G
+V
S
+7V
OUT
R
G
VDD
IN+
1
2
3
4
8765
13141516
A1
IA
A2
R1
20kΩ
R2
20kΩ
A2
OUT
AD8295
ADR435
A1
+IN
+2.5V
+OUT
500Ω
10kΩ
10kΩ
2.2nF
A1
R1
+5V
+
AD7690
10µF
500Ω
2.2nF
–OUT
A1
–IN
12
11
10
9
0.1µF
+5V
0.1µF
0.1µF
+5V
0.1µF
07343-014
Figure 71. Driving a Differential ADC