AD694

REV. B

–11–

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-

termined by the reaction of the voltage reference. The supply

reduced significantly from 15 V the differential nonlinearity of

the DAC will increase and the linearity will be degraded.

In some applications it is desirable to have some underrange and

overrange in the 4–20 mA output. For example, assume an over

and under range capability of

±5% of span is needed, then the

output current range corresponding to the full scale of the DAC

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

AD694 would be increased 10% to 17.6 mA by adding a nonin-

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

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

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.

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

some resistors, an instrumentation amplifier front end can be

added which easily handles these types of low level signals.

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

Figure 15. The AD694’s 2 V reference is used to provide a

“ground” of 2 V that ensures proper operation of the in amp

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

subtractor circuit is tied to the 2 V reference (point C). A 2 k

Ω

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

to sink any subtractor current. The 2 V FS (Pin 4) is attached to

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

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

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

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

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

is determined according to the equation: