AD8224 Analog Devices, AD8224 Datasheet - Page 24

• Manufacturer Part Number: AD8224
• Description: Precision, Dual-Channel, JFET Input Rail-to-Rail Instrumentation Amplifier
• Manufacturer: Analog Devices

AD8224
DRIVING A DIFFERENTIAL INPUT ADC
The AD8224 can be configured in differential output mode
to drive a differential analog-to-digital converter. Figure 61
illustrates several of the concepts.
First Antialiasing Filter
The 1 kΩ 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 system’s CMRR high over frequency.
Second Antialiasing Filter
An 806 Ω 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
AD8224. The 806 Ω resistor shields the AD8224 from the
ADC’s switched capacitor input, 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 Ω 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 the AD7688.
Its input can handle a 130 mA overdrive, which is much higher
than the short-circuit limit of the AD8224.
10µF
10µF
+IN
–IN
+
+
0.1µF
0.1µF
1kΩ
1kΩ
1000pF
100pF
100pF
NPO
NPO
5%
5%
+12V
–12V
(DIFF OUT)
AD8224
+5V REF
+IN2
Figure 61. Driving a Differential ADC
REF2
+OUT
–OUT
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806Ω
806Ω
2.7nF
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
the AD8224. Because REF2 on the AD8224 is grounded, the
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 the 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 Ω.
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.
0.1µF
2.7nF
(SINGLE OUT)
+12V
AD8224
AD8224
V
(DIFF OUT)
ADR435
IN+
IN–
IN
GND
V
GND
OUT
VDD
+5V
AD7688
Figure 62. Driving a Cable
10µF
X5R
0.1µF
REF
0.1µF
+5V REF