Preliminary Technical Data

AD8223

Rev.

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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.

AD8223

(DIFF OUT)

AD8223

(SINGLE OUT)

Figure 38. 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 39

shows how this translation can be achieved.

±10mV

5V

AD8223

REF

5V

AD7776

5V

+

–

0.1µF

0.1µF

1.02k

Ω

Figure 39. 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

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.

2V

5V

AD8223

REF

0.1µF

1.02k

Ω

+

–

J-TYPE

THERMOCOUPLE

Figure 40. Amplifying Bipolar Signals with Low Common-Mode Voltage

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

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

relative to ground.