AD693

REV. A

–5–

converter’s inverting input (Pin 12). Arranging the zero offset in

this way makes the zero signal output current independent of

input span. When the input to the signal amp is zero, the

noninverting input of the V/I is at 6.2 V.

Since the standard offsets are laser trimmed at the factory,

adjustment is seldom necessary except to accommodate the zero

offset of the actual source. (See “Adjusting Zero.”)

SIGNAL AMPLIFIER

The Signal Amplifier is an instrumentation amplifier used to

buffer and scale the input to match the desired span. Inputs

applied to the Signal Amplifier (at Pins 17 and 18) are amplified

and referred to the 6.2 V reference output in much the same way as

the level translation occurs in the V/I converter. Signals from the

two preamplifiers are subtracted, the difference is amplified, and

the result is fed back to the upper preamp to minimize the

difference. Since the two preamps are identical, this minimum will

occur when the voltage at the upper preamp just matches the

differential input applied to the Signal Amplifier at the left.

Since the signal which is applied to the V/I is attenuated across

the two 800

Ω resistors before driving the upper preamp, it will

necessarily be an amplified version of the signal applied between

Pins 17 and 18. By changing this attenuation, you can control

the span referred to the Signal Amplifier. To illustrate: a 75 mV

signal applied to the V/I results in a 20 mA loop current.

Nominally, 15 mV is applied to offset the zero to 4 mA leaving a

60 mV range to correspond to the span. And, since the nominal

attenuation of the resistors connected to Pins 16, 15 and 14 is

2.00, a 30 mV input signal will be doubled to result in 20 mA of

loop current. Shorting Pins 15 and 16 results in unity gain and

permits a 60 mV input span. Other choices of span may be

implemented with user supplied resistors to modify the

attenuation. (See section “Adjusting Input Span.”)

The Signal Amplifier is specially designed to accommodate a

large common-mode range. Common-mode signals anywhere up

to and beyond the 6.2 V reference are easily handled as long as

extended range will be useful when measuring sensors driven,

for example, by the auxiliary amplifier which may go above the

6.2 V potential. In addition, the PNP input stage will continue

to operate normally with common-mode voltages of several

hundred mV, negative, with respect to common. This feature

accommodates self-generating sensors, such as thermocouples,

which may produce small negative normal-mode signals as well

as common-mode noise on “grounded” signal sources.

AUXILIARY AMPLIFIER

The Auxiliary Amplifier is included in the AD693 as a signal

conditioning aid. It can be used as an op amp in noninverting

applications and has special provisions to provide a controlled

current output. Designed with a differential input stage and an

unbiased Class A output stage, the amplifier can be resistively

loaded to common with the self-contained 100

Ω resistor or

with a user supplied resistor.

As a functional element, the Auxiliary Amplifier can be used in

dynamic bridges and arrangements such as the RTD signal

conditioner shown in Figure 17. It can be used to buffer, amplify

and combine other signals with the main Signal Amplifier. The

Auxiliary Amplifier can also provide other voltages for excitation

FUNCTIONAL DESCRIPTION

The operation of the AD693 can be understood by dividing the

circuit into three functional parts (see Figure 9). First, an

instrumentation amplifier front-end buffers and scales the low-

level input signal. This amplifier drives the second section, a V/I

converter, which provides the 4-to-20mA loop current. The

third section, a voltage reference and resistance divider, provides

application voltages for setting the various “live zero” currents.

In addition to these three main sections, there is an on-chip

auxiliary amplifier which can be used for transducer excitation.

VOLTAGE-TO-CURRENT (V/I) CONVERTER

The output NPN transistor for the V/I section sinks loop current

when driven on by a high gain amplifier at its base. The input for

this amplifier is derived from the difference in the outputs of the

matched preamplifiers having gains, G2. This difference is caused

to be small by the large gain, +A, and the negative feedback

through the NPN transistor and the loop current sampling resistor

to the input of the left preamp and servos the loop current until

both signals are equal. Accurate voltage-to-current transformation

is thereby assured. The preamplifiers employ a special design

which allows the active feedback amplifier to operate from the most

The V/I stage is designed to have a nominal transconductance of

0.2666 A/V. Thus, a 75 mV signal applied to the inputs of the

V/I (Pin 16, noninverting; Pin 12, inverting) results in a

full-scale output current of 20 mA.

The current limiter operates as follows: the output of the feed-

back preamp is an accurate indication of the loop current. This

output is compared to an internal setpoint which backs off the

drive to the NPN transistor when the loop current approaches

25 mA. As a result, the loop and the AD693 are protected from the

consequences of voltage overdrive at the V/I input.

VOLTAGE REFERENCE AND DIVIDER

A stabilized bandgap voltage reference and laser-trimmed

resistor divider provide for both transducer excitation as well as

precalibrated offsets for the V/I converter. When not used for

external excitation, the reference should be loaded by approxi-

mately 1 mA (6.2 k

Ω to common).

The 4 mA and 12 mA taps on the resistor divider correspond to

–15 mV and –45 mV, respectively, and result in a live zero of

4 mA or 12 mA of loop current when connected to the V/I

Figure 9. Functional Flock Diagram