AD694 データシートの表示（PDF） - Analog Devices

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AD694

4–20 mA Transmitter

Analog Devices 

AD694

Figure 13. Digital to 4–20 mA Interface Using a Current Steering DAC

Figure 14. Single-Supply Digital Input to 4–20 mA Output

code dependent, and the response time of the circuit will be de- some resistors, an instrumentation amplifier front end can be

termined by the reaction of the voltage reference. The supply

added which easily handles these types of low level signals.

voltage to the AD7541A should be kept close to 15 V. If VS is

reduced significantly from 15 V the differential nonlinearity of

the DAC will increase and the linearity will be degraded.

The traditional 3 op amp instrumentation amplifier is built us-

ing an AD708 dual op amp for the front end, and the AD694’s

buffer amplifier is used for the subtractor circuit, as shown in

In some applications it is desirable to have some underrange and Figure 15. The AD694’s 2 V reference is used to provide a

overrange in the 4–20 mA output. For example, assume an over “ground” of 2 V that ensures proper operation of the in amp

and under range capability of ± 5% of span is needed, then the

over a wide common mode range. The reference pin of the

output current range corresponding to the full scale of the DAC subtractor circuit is tied to the 2 V reference (point C). A 2 kΩ

is 3.2 mA to 20.8 mA. To accomplish this, the span of the

pull-down resistor ensures that the voltage reference will be able

AD694 would be increased 10% to 17.6 mA by adding a nonin- to sink any subtractor current. The 2 V FS (Pin 4) is attached to

verting gain of 1.1 to the buffer amplifier. The 4 mA offset

the 2 V reference; this offsets the input range of the V/I con-

would then be reduced by 0.8 mA, by utilizing the adjustment

verter 2 volts positive, to match the “ground” of the in amp.

scheme explained in Adjusting 4 mA Zero section. Then a digi-

tal input from all zero code to full scale would result in an out-

put current of 3.2 mA to 20.8 mA.

The AD694 will now output a 4–20 mA output current for a

0 V to 2 V differential swing across VA. The gain of the in amp

front end is adjusted so that the desired full-scale input signal at

LOW COST SENSOR TRANSMITTER

Sensor bridges typically output differential signals in the 10 mV

to 100 mV full-scale range. With an AD694, a dual op amp, and

VIN results in a VA of 2 V. For example a sensor that has a 100

mV full scale will require a gain of 20 in the front end. The gain

is determined according to the equation:

G = [2RS/Rg] + 1

REV. B

–11–

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