AD8222_16 Datasheet, PDF (21/25 Pages) Analog Devices – Precision, Dual-Channel Instrumentation Amplifier

AD8222
Data Sheet
10µF ⁺ 0.1µF
100pF
+IN
1kΩ
NPO
5%
1000pF
1kΩ
–IN
100pF
NPO
5%
10µF ₊ 0.1µF
+12V
+5V
0.1µF
AD8222
(DIFF OUT)
+OUT 1kΩ
1kΩ
–OUT
REF2
+IN2
+5V REF
–12V
2200pF
2200pF
VDD
IN+
AD7688
IN–
GND REF
0.1µF
+12V
10µF
X5R
V_IN
V_OUT
ADR435
GND
+5V REF
0.1µF
Figure 52. Driving a Differential ADC
DRIVING A DIFFERENTIAL INPUT ADC
The AD8222 can be configured in differential output mode
to drive a differential analog-to-digital converter. Figure 52
illustrates several of the concepts.
RFI and Antialiasing Filter
The 1 kΩ resistors, 1000 pF capacitor, and 100 pF capacitors in
front of the in-amp form filter circuitry that performs many
functions. The 1 kΩ and 100 pF capacitors form common-mode
filters that protect the amplifier from incoming radio frequency
signals. Without the filtering, these RFI signals can be rectified
in the in-amp. The 1 kΩ resistors provide some overvoltage
protection. The 1 kΩ resistors and 1000 pF capacitor form a
76 kHz antialiasing filter for the ADC.
Note that the 100 pF capacitors are 5% COG/NPO types. These
capacitors match well over time and temperature, which keeps
the system CMRR high over frequency.
Second Antialiasing Filter
A 1 kΩ resistor and 2200 pF capacitor are placed between each
AD8222 output and ADC input. They create a 72 kHz low-pass
filter for another stage of antialiasing protection.
These four elements also improve distortion performance. The
2200 pF capacitor provides charge to the switched capacitor
front end of the ADC, and the 1 kΩ resistor shields the AD8222
from driving any sharp current changes. If the application
requires a lower frequency antialiasing filter and is distortion
sensitive, increase the value of the capacitor rather than the
resistor.
The 1 kΩ resistors can also protect an ADC from overvoltages.
Because the AD8222 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 AD8222. However, other conver-
ters have less robust inputs and may need the added protection.
Reference
The ADR435 supplies a reference voltage to both the ADC and
the AD8222. Because REF2 on the AD8222 is grounded, the
common-mode output voltage is precisely half the reference
voltage, exactly where it needs to be for the ADC.
PRECISION STRAIN GAGE
The low offset and high CMRR over frequency of the AD8222
make it an excellent candidate for both ac and dc bridge measure-
ments. As shown in Figure 53, the bridge can be connected to
the inputs of the amplifier directly.
5V
10µF 0.1µF
350Ω
350Ω
350Ω
350Ω
+IN ₊
R_G AD8222
–IN ^–
2.5V
Figure 53. Precision Strain Gauge
Rev. B | Page 20 of 24