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

FUNCTIONAL DESCRIPTION

The operation of the AD694 can best be understood by dividing

the circuit into three functional parts (see Figure 1). First, a

single supply input amplifier buffers the high level, single-ended

input signal. The buffer amplifier drives the second section, a

voltage to current (V/I) converter, that makes a 0 to 16 mA sig-

nal dependent current.

Typical Minimum Supply Voltage vs. Temperature for 2 V

and 10 V Full Scale

Maximum RL vs. Supply Voltage

Voltage Reference Power Supply Rejection

IOUT: Voltage Compliance vs. Temperature

Figure 1. Functional Block Diagram

The third section, a voltage reference and offset generator, is re-

sponsible for providing the 4 mA offset current signal.

BUFFER AMPLIFIER

The buffer amplifier is a single supply amplifier that may be

used as a unity gain buffer, an output amplifier for a current

output DAC, or as a gain block to amplify low level signals. The

amplifier’s PNP input stage has a common-mode range that ex-

tends from a few hundred mV below ground to within 2.5 V of

VS. The Class A output of the amplifier appears at Pin 1 (FB).

The output range extends from about 1 mV above common to

within 2.5 V of VS when the amplifier is operated as a follower.

The amplifier can source a maximum load of 5 kΩ, but can sink

only as much as its internal 10 kΩ pulldown resistor allows.

V/I CONVERTER

The ground referenced, input signal from the buffer amplifier is

converted to a 0 to 0.8 mA current by A2 and level shifted to

the positive supply. A current mirror then multiplies this signal

by a factor of 20 to make the signal current of 0 to 16 mA. This

technique allows the output stage to drive a load to within 2 V

of the positive supply (VS). Amplifier A2 forces the voltage at

Pin 1 across resistors R1 and R2 by driving the Darlington tran-

sistor, Q2. The high gain Darlington transmits the resistor cur-

rent to its collector and to R3 (900 Ω). A3 forces the level

shifted signal across the 45 Ω resistor to get a current gain of 20.

The transfer function of the V/I stage is therefore:

( ) IOUT = 20 ×VPIN1 / R1 + R2

resulting in a 0-16 mA output swing for a 0–10 V input. Tying

Pin 4 (2 V FS) to ground shorts out R2 and results in a 2 V

full-scale input for a 16 mA output span.

The output stage of the V/I converter is of a unique design that

allows the IOUT pin to drive a load below the common (sub-

strate) potential of the device. The output transistor can always

drive a load to a point 36 V below the positive supply (VS). An

–4–

REV. B

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