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)

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

5V

+

–

REFOUT

REFIN

0.1µF

0.1µF

1.02kΩ

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

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

5V

AD8223

REF

0.1µF

1.02kΩ

+

–

J-TYPE

THERMOCOUPLE

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

in the AD8223 output voltage ranging from 1.110 V to 3.077 V

relative to ground.