AD8223
Rev. 0 | Page 18 of 20
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 output response of the AD8223.
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 75 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.
AD8223
(DIFF OUT)
AD8223
(SINGLE OUT)
06925-047
Figure 40. Driving a 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 41
shows how this translation can be achieved.
±10mV
5V
AD8223
REF
5V
AD7776
5
+
–
A
IN
REFOUT
REFIN
0.1µF
0.1µF
R
G
1.02kΩ
6925-048
Figure 41. A Single-Supply Data Acquisition System
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
G
= 1.02 k). This results in an output signal of ±1 V. To
prevent this signal from running into the AD8223 ground rail, the
voltage on the REF pin must 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, which
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
0.15 V below ground, it is possible to measure small differential
signals that have low, or no, common-mode components. Figure 42
shows a thermocouple application in which one side of the J-type
thermocouple is grounded.
2V
5
AD8223
REF
0.1µF
V
OUT
R
G
1.02kΩ
+
–
J-TYPE
THERMOCOUPLE
06925-049
Figure 42. Amplifying Bipolar Signals with Low Common-Mode Voltage
Over a temperature range of −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
G
= 845) and a voltage on the AD8223 REF pin of 2 V results
in the AD8223 output voltage ranging from 1.110 V to 3.077 V
relative to ground.
