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AD693AD-AD693AQ-AD693AQ.
Loop-Powered 4.20 mA Sensor Transmitter
FUNCTIONAL BLOCK DIAGRAMREV.A
Loop-Powered 4–20 mA
Sensor Transmitter
FEATURES
Instrumentation Amplifier Front End
Loop-Powered Operation
Precalibrated 30mV or 60mV Input Spans
Independently Adjustable Output Span and Zero
Precalibrated Output Spans:4–20 mA Unipolar
0–20 mA Unipolar
12 6 8mA Bipolar
Precalibrated 100V RTD Interface
6.2 V Reference with Up to 3.5 mA of Current Available
Uncommitted Auxiliary Amp for Extra Flexibility
Optional External Pass Transistor to Reduce
Self-Heating Errors
PRODUCT DESCRIPTIONThe AD693 is a monolithic signal conditioning circuit which
accepts low-level inputs from a variety of transducers to control a
standard 4–20mA, two-wire current loop. An on-chip voltage
reference and auxiliary amplifier are provided for transducer
excitation; up to 3.5mA of excitation current is available when the
device is operated in the loop-powered mode. Alternatively, the
device may be locally powered for three-wire applications when
0–20mA operation is desired.
Precalibrated 30 mV and 60 mV input spans may be set by
simple pin strapping. Other spans from 1 mV to 100 mV may
be realized with the addition of external resistors. The auxiliary
amplifier may be used in combination with on-chip voltages to
provide six precalibrated ranges for 100Ω RTDs. Output span
and zero are also determined by pin strapping to obtain the
standard ranges: 4–20mA, 12 ± 8 mA and 0–20 mA.
Active laser trimming of the AD693’s thin-film resistors result
in high levels of accuracy without the need for additional
adjustments and calibration. Total unadjusted error is tested on
every device to be less than 0.5% of full scale at +25°C, and less
than 0.75% over the industrial temperature range. Residual
nonlinearity is under 0.05%. The AD693 also allows for the use
of an external pass transistor to further reduce errors caused by
self-heating.
For transmission of low-level signals from RTDs, bridges and
pressure transducers, the AD693 offers a cost-effective signal
conditioning solution. It is recommended as a replacement for
discrete designs in a variety of applications in process control,
factory automation and system monitoring.
The AD693 is packaged in a 20-pin ceramic side-brazed DIP,
20-pin Cerdip, and 20-pin LCCC and is specified over the
–40°C to +85°C industrial temperature range.
PRODUCT HIGHLIGHTSThe AD693 is a complete monolithic low-level voltage-to-
current loop signal conditioner.Precalibrated output zero and span options include
4–20 mA, 0–20 mA, and 12 ± 8 mA in two- and three-wire
configurations.Simple resistor programming adds a continuum of ranges
to the basic 30 mV and 60 mV input spans.The common-mode range of the signal amplifier input
extends from ground to near the device’s operating voltage.Provision for transducer excitation includes a 6.2 V
reference output and an auxiliary amplifier which may be
configured for voltage or current output and signal
amplification.The circuit configuration permits simple linearization of
bridge, RTD, and other transducer signals.A monitored output is provided to drive an external pass
transistor. This feature off-loads power dissipation to
extend the temperature range of operation, enhance
reliability, and minimize self-heating errors.Laser-wafer trimming results in low unadjusted errors and
affords precalibrated input and output spans.Zero and span are independently adjustable and noninteractive
to accommodate transducers or user defined ranges.
10.Six precalibrated temperature ranges are available with a
100Ω RTD via pin strapping.
AD693–SPECIFICATIONS
(@ +258C and VS = +24 V. Input Span = 30 mV or 60 mV. Output Span = 4–20mA,
RL = 250 V, VCM = 3.1V, with external pass transistor unless otherwise noted.)
NOTESTotal error can be significantly reduced (typically less than 0.1%) by trimming the zero current. The remaining unadjusted error sources are transconductance and
nonlinearity.The AD693 is tested as a loop powered device with the signal amp, V/I converter, voltage reference, and application voltages operating together. Specifications are
valid for preset spans and spans between 30 mV and 60 mV.Error from ideal output assuming a perfect 100Ω RTD at 0 and +100°C.Refer to the Error Analysis to calculate zero current error for input spans less than 30 mV.By forcing the differential signal amplifier input sufficiently negative the 7μA zero current can always be achieved.The operational voltage (VOP) is the voltage directly across the AD693 (Pin 10 to 6 in two-wire mode, Pin 9 to 6 in local power mode). For example, VOP = VS –
(ILOOP × RL) in two-wire mode (refer to Figure 10).Bias currents are not symmetrical with input signal level and flow out of the input pins. The input bias current of the inverting input increases with input signal volt-
age, see Figure 2.Nonlinearity is defined as the deviation of the output from a straight line connecting the endpoints as the input is swept over a 30 mV and 60 mV input span.Specifications for the individual functional blocks are components of error that contribute to, and that are included in, the Loop Powered Operation specifications.Includes error contributions of V/I converter and Application Voltages.Changes in the reference output voltage due to load will affect the Zero Current. A 1% change in the voltage reference output will result in an error of 1% in the
value of the Zero Current.If not used for external excitation, the reference should be loaded by approximately 1mA (6.2kΩ to common).In the loop powered mode up to 5mA can be drawn from the reference, however, the lower limit of the output span will be increased accordingly. 3.5mA is the
maximum current the reference can source while still maintaining a 4mA zero.The AD693 is tested with a pass transistor so TA ≅ TC.
Specifications subject to change without notice.
Specifications shown in boldface are tested on all production units at final electrical test. Results from those tests are used to calculate outgoing quality levels. All min
and max specifications are guaranteed, although only those shown in boldface are tested on all production units.
ABSOLUTE MAXIMUM RATINGSSupply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +36 V
Reverse Loop Current . . . . . . . . . . . . . . . . . . . . . . . . . 200 mA
Signal Amp Input Range . . . . . . . . . . . . . . . . . . –0.3 V to VOP
Reference Short Circuit to Common . . . . . . . . . . . . Indefinite
Auxiliary Amp Input Voltage Range . . . . . . . . . . 0.3 V to VOP
Auxiliary Amp Current Output . . . . . . . . . . . . . . . . . . . 10 mA
Storage Temperature . . . . . . . . . . . . . . . . . . –65°C to +150°C
Lead Temperature, 10 sec Soldering . . . . . . . . . . . . . +300°C
Max Junction Temperature . . . . . . . . . . . . . . . . . . . . . +150°C
ORDERING GUIDE
AD693 PIN CONFIGURATION
(AD, AQ, AE Packages)Functional Diagram
AD693
Figure 1.Maximum Load Resistance
vs. Power Supply
Figure 2.Differential Input Current vs.
Input Signal Voltage Normalized to +IN
Figure 3.Maximum Common-Mode
Voltage vs. Supply
AD693–Typical CharacteristicsFigure 4.Bandwidth vs. Series Load
Resistance
Figure 5.Signal Amplifier PSRR vs.
Frequency
Figure 6.CMRR (RTI) vs. Frequency
Figure 7.Input Current Noise vs.
Frequency
Figure 8.Input Voltage Noise vs.
Frequency
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 6.2V.
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 AMPLIFIERThe 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 6.2V 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 800Ω 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 75mV
signal applied to the V/I results in a 20mA loop current.
Nominally, 15mV is applied to offset the zero to 4mA leaving amV 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 30mV input signal will be doubled to result in 20mA of
loop current. Shorting Pins 15 and 16 results in unity gain and
permits a 60mV 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 6.2V reference are easily handled as long as
VIN 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
6.2V 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 AMPLIFIERThe 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 100Ω 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
FUNCTIONAL DESCRIPTIONThe 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) CONVERTERThe 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
positive point in the circuit, IIN.
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 approachesmA. As a result, the loop and the AD693 are protected from the
consequences of voltage overdrive at the V/I input.
VOLTAGE REFERENCE AND DIVIDERA 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 1mA (6.2 kΩ 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 ofmA or 12mA of loop current when connected to the V/I
AD693if the 6.2 V of the reference is unsuitable. Configured as a simple
follower, it can be driven from a user supplied voltage divider
or the precalibrated outputs of the AD693 divider (Pins 3 and
4) to provide a stiff voltage output at less than the 6.2 level, or
by incorporating a voltage divider as feedback around the amplifier,
one can gain-up the reference to levels higher than 6.2V. If
large positive outputs are desired, IX, the Auxiliary Amplifier
output current supply, should be strapped to either VIN or
Boost. Like the Signal Amplifier, the Auxiliary requires about
3.5 V of headroom with respect to VIN at its input and about 2V
of difference between IX and the voltage to which VX is required
to swing.
The output stage of the Auxiliary Amplifier is actually a high
gain Darlington transistor where IX is the collector and VX is the
emitter. Thus, the Auxiliary Amplifier can be used as a V/I
converter when configured as a follower and resistively loaded.
IX functions as a high-impedance current source whose current
is equal to the voltage at VX divided by the load resistance. For
example, using the onboard 100Ω resistor and the 75mV or
150mV application voltages, either a 750μA or 1.5mA current
source can be set up for transducer excitation.
The IX terminal has voltage compliance within 2V of VX. If the
Auxiliary Amplifier is not to be used, then Pin 2, the noninverting
input, should be grounded.
REVERSE VOLTAGE PROTECTION FEATUREIn the event of a reverse voltage being applied to the AD693
through a current-limited loop (limited to 200mA), an internal
shunt diode protects the device from damage. This protection
mode avoids the compliance voltage penalty which results from
a series diode that must be added if reversal protection is
required in high-current loops.
Applying the AD693
CONNECTIONS FOR BASIC OPERATIONFigure 10 shows the minimal connections for basic operation:
0–30mV input span, 4–20mA output span in the two-wire,
loop-powered mode. If not used for external excitation, the
6.2V reference should be loaded by approximately 1mA
(6.2kΩ to common).
USING AN EXTERNAL PASS TRANSISTORThe emitter of the NPN output section, IOUT, of the AD693 is
usually connected to common and the negative loop connection
(Pins 7 to 6). Provision has been made to reconnect IOUT to the
base of a user supplied NPN transistor as shown in Figure 11.
This permits the majority of the power dissipation to be moved
off chip to enhance performance, improve reliability, and extend
the operating temperature range. An internal hold-down resistor
of about 3k is connected across the base emitter of the external
transistor.
The external pass transistor selected should have a BVCEO greater
than the intended supply voltage with a sufficient power rating for
continuous operation with 25mA current at the supply voltage.
Ft should be in the 10MHz to 100MHz range and β should be
greater than 10 at a 20mA emitter current. Some transistors
that meet this criteria are the 2N1711 and 2N2219A. Heat
sinking the external pass transistor is suggested.
The pass transistor option may also be employed for other
applications as well. For example, IOUT can be used to drive an
LED connected to Common, thus providing a local monitor of
loop fault conditions without reducing the minimum compliance
voltage.
ADJUSTING ZEROIn general, the desired zero offset value is obtained by
connecting an appropriate tap of the precision reference/voltage
divider network to the inverting terminal of the V/I converter.
As shown in Figure 9, precalibrated taps at Pins 14, 13 and 11
result in zero offsets of 0mA, 4mA and 12mA, respectively,
when connected to Pin 12. The voltages which set the 4mA andmA zero operating points are 15mV and 45mV negative
with respect to 6.2 V, and they each have a nominal source
resistance of 450Ω. While these voltages are laser trimmed to
high accuracy, they may require some adjustment to
accommodate variability between sensors or to provide
additional ranges. You can adjust zero by pulling up or down on
the selected zero tap, or by making a separate voltage divider to
drive the zero pin.
The arrangement of Figure 12 will give an approximately linear
adjustment of the precalibrated options with fixed limits. To
find the proper resistor values, first select IA, the desired range
