Data Sheet
AD8224
Rev. C | Page 25 of 28
AD8224
(DIFF OUT)
100pF
NPO
5%
100pF
NPO
5%
1000pF
–IN
+IN
0.1F
10F
0.1F
10F
–12V
+12V
1k
+
IN+
VDD
GND
REF
10F
X5R
AD7688
IN–
0.1F
+5V
ADR435
GND
VIN
VOUT
0.1F
+12V
0.1F
+5V REF
+IN2
REF2
+5V REF
+OUT
–OUT
806
2.7nF
806
06286-
066
Figure 65. Driving a Differential ADC
DRIVING A DIFFERENTIAL INPUT ADC
Th
e AD8224 can be configured in differential output mode
to drive a differential ADC.
Figure 65 illustrates several of the
concepts.
First Antialiasing Filter
The 1 k resistor, 1000 pF capacitor, and 100 pF capacitors in
front of the in-amp form a 76 kHz filter. This is the first of two
antialiasing filters in the circuit and helps to reduce the noise of
the system. The 100 pF capacitors protect against common-
mode RFI signals. Note that they are 5% COG/NPO types.
These capacitors match well over time and temperature,
which keeps the CMRR of the system high over frequency.
Second Antialiasing Filter
An 806 resistor and a 2.7 nF capacitor are located between
each
AD8224 output and ADC input. These components create
a 73 kHz low-pass filter for another stage of antialiasing
protection.
These four elements also isolate the ADC from loading the
switched capacitor input of the ADC, which looks like a time-
varying load. The 2.7 nF capacitor provides a charge to the
switched capacitor front end of the ADC. If the application
requires a lower frequency antialiasing filter, increase the value
of the capacitor rather than the resistor.
The 806 resistors can also protect an ADC from overvoltages.
Because the
AD8224 runs on wider supply voltages than a
typical ADC, there is a possibility of overdriving the ADC. This
is not an issue with a PulSAR converter, such as t
he AD7688.Its input can handle a 130 mA overdrive, which is much higher
than the short-circuit limit of the
AD8224.However, other converters have less robust inputs and may need
the added protection.
Reference
The
ADR435 supplies a reference voltage to both the ADC and
common-mode output voltage is precisely half the reference
voltage, exactly where it needs to be for the ADC.
DRIVING CABLING
All cables have a certain capacitance per unit length, which
varies widely with cable type. The capacitive load from the cable
may cause peaking in th
e AD8224 output response. To reduce
peaking, use a resistor between the
AD8224 and the cable.
Because cable capacitance and desired output response vary
widely, this resistor is best determined empirically. A good
starting point is 50 .
The
AD8224 operates at a low enough frequency that
transmission line effects are rarely an issue; therefore, the
resistor need not match the characteristic impedance of
the cable.
AD8224
(DIFF OUT)
AD8224
(SINGLE OUT)
06286-
067
Figure 66. Driving a Cable