AD8223 Analog Devices, AD8223 Datasheet - Page 17

• Manufacturer Part Number: AD8223
• Description: Single Supply, Rail-to-Rail, Low Cost Instrumentation Amplifier
• Manufacturer: Analog Devices

Preliminary Technical Data
Driving a Cable
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 AD8223’s output response. To
reduce the peaking, use a resistor between the AD8223 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 AD8232 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.
A SINGLE-SUPPLY DATA ACQUISITION SYSTEM
Interfacing bipolar signals to single-supply analog-to-digital
converters (ADCs) presents a challenge. The bipolar signal
must be mapped into the input range of the ADC. Figure 39
shows how this translation can be achieved.
5V
Figure 39. A Single Supply Data Acquisition System
(SINGLE OUT)
AD8223
AD8223
(DIFF OUT)
±10mV
Figure 38. Driving a Cable
1.02kΩ
R
G
+
–
AD8223
5V
0.1µF
REF
A
REF
REF
AD7776
IN
5V
OUT
IN
0.1µF
Rev. PrA | Page 17 of 20
The bridge circuit is excited by a +5 V supply. The full-scale
output voltage from the bridge (±10 mV) therefore has a
common-mode level of 2.5 V. The AD8223 removes the
common-mode component and amplifies the input signal by a
factor of 100 (R
of ±1 V. To prevent this signal from running into the AD8223
ground rail, the voltage on the REF pin has to be raised to at
least 1 V. In this example, the 2 V reference voltage from the
AD7776 ADC is used to bias the AD8223 output voltage to 2 V
± 1 V. This corresponds to the input range of the ADC.
AMPLIFYING SIGNALS WITH LOW COMMON-
MODE VOLTAGE
Because the common-mode input range of the AD8223 extends
0.1 V below ground, it is possible to measure small differential
signals that have low, or no, common-mode components. Figure 40
shows a thermocouple application where one side of the J-type
thermocouple is grounded.
Over a temperature range from −200°C to +200°C, the J-type
thermocouple delivers a voltage ranging from −7.890 mV
to 10.777 mV. A programmed gain on the AD8223 of 100
(R
in the AD8223 output voltage ranging from 1.110 V to 3.077 V
relative to ground.
Figure 40. Amplifying Bipolar Signals with Low Common-Mode Voltage
G
THERMOCOUPLE
= 845) and a voltage on the AD8223 REF pin of 2 V results
J-TYPE
GAIN
= 1.02 kΩ). This results in an output signal
1.02kΩ
R
G
+
–
AD8223
5V
0.1µF
REF
AD8223
V
2V
OUT