Datasheet AD8223 (Analog Devices) - 18

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AD8223 Driving a Cable
The bridge circuit is excited by a +5 V supply. The full-scale output All cables have a certain capacitance per unit length, which voltage from the bridge (±10 mV), therefore, has a common- varies widely with cable type. The capacitive load from the mode level of 2.5 V. The AD8223 removes the common-mode cable may cause peaking in the output response of the AD8223. component and amplifies the input signal by a factor of 100 To reduce the peaking, use a resistor between the AD8223 and (RG = 1.02 kΩ). This results in an output signal of ±1 V. To the cable. Because cable capacitance and desired output response prevent this signal from running into the AD8223 ground rail, the vary widely, this resistor is best determined empirically. A good voltage on the REF pin must be raised to at least 1 V. In this starting point is 75 Ω. example, the 2 V reference voltage from the AD7776 ADC is used to bias the AD8223 output voltage to 2 V ± 1 V, which The AD8232 operates at a low enough frequency that transmission corresponds to the input range of the ADC. line effects are rarely an issue; therefore, the resistor need not match the characteristic impedance of the cable.
AMPLIFYING SIGNALS WITH LOW COMMON- MODE VOLTAGE
Because the common-mode input range of the AD8223 extends
AD8223
0.15 V below ground, it is possible to measure small differential
(DIFF OUT)
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.
5V 0.1µF AD8223 (SINGLE OUT) +
7
J-TYPE RG
04
AD8223 V THERMOCOUPLE 1.02kΩ OUT
5- 92 06
– REF 2V
Figure 40. Driving a Cable 9 04 5- 92 06 Figure 42. Amplifying Bipolar Signals with Low Common-Mode Voltage
A SINGLE-SUPPLY DATA ACQUISITION SYSTEM
Over a temperature range of −200°C to +200°C, the J-type Interfacing bipolar signals to single-supply analog-to-digital thermocouple delivers a voltage ranging from −7.890 mV converters (ADCs) presents a challenge. The bipolar signal to +10.777 mV. A programmed gain on the AD8223 of 100 must be mapped into the input range of the ADC. Figure 41 (RG = 845) and a voltage on the AD8223 REF pin of 2 V results shows how this translation can be achieved. in the AD8223 output voltage ranging from 1.110 V to 3.077 V
5V
relative to ground.
5V 5V 0.1µF 0.1µF AD7776 + R ±10mV G AD8223 AIN 1.02kΩ – REF REFOUT REFIN
8 -04 25 69 0 Figure 41. A Single-Supply Data Acquisition System Rev. 0 | Page 18 of 20 Document Outline FEATURES APPLICATIONS CONNECTION DIAGRAM GENERAL DESCRIPTION REVISION HISTORY SPECIFICATIONS SINGLE SUPPLY DUAL SUPPLY ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION AMPLIFIER ARCHITECTURE GAIN SELECTION INPUT VOLTAGE RANGE REFERENCE TERMINAL INPUT PROTECTION RF INTERFERENCE (RFI) GROUND RETURNS FOR INPUT BIAS CURRENTS APPLICATIONS INFORMATION BASIC CONNECTION DIFFERENTIAL OUTPUT OUTPUT BUFFERING CABLES Receiving from a Cable Driving a Cable A SINGLE-SUPPLY DATA ACQUISITION SYSTEM AMPLIFYING SIGNALS WITH LOW COMMON-MODE VOLTAGE OUTLINE DIMENSIONS ORDERING GUIDE