AD693
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.
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.
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.
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
between IIN and Boost. The signal across this resistor is compared
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
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
positive point in the circuit, I
IN.
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.
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
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 kΩ to common).
V
IN is sufficiently positive. The Signal Amplifier is biased with
respect to VIN and requires about 3.5 volts of headroom. The
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.
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
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
Figure 9. Functional Flock Diagram
REV. A
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