ad693.pdf

Loop-Powered 4 –20 mA

Sensor Transmitter

AD693

FEATURES

Instrumentation Amplifier Front End

Loop-Powered Operation

Precalibrated 30 mV or 60 mV Input Spans

Independently Adjustable Output Span and Zero

Precalibrated Output Spans: 4–20 mA Unipolar

0–20 mA Unipolar

FUNCTIONAL BLOCK DIAGRAM

8 mA Bipolar

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 DESCRIPTION

The AD693 is a monolithic signal conditioning circuit which

accepts low-level inputs from a variety of transducers to control a

standard 4–20 mA, two-wire current loop. An on-chip voltage

reference and auxiliary amplifier are provided for transducer

excitation; up to 3.5 mA 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

0–20 mA 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 100 W 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 HIGHLIGHTS

1. The AD693 is a complete monolithic low-level voltage-to-

current loop signal conditioner.

2. Precalibrated output zero and span options include

4–20 mA, 0–20 mA, and 12 ± 8 mA in two- and three-wire

configurations.

3. Simple resistor programming adds a continuum of ranges

to the basic 30 mV and 60 mV input spans.

4. The common-mode range of the signal amplifier input

extends from ground to near the device’s operating voltage.

5. 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.

6. The circuit configuration permits simple linearization of

bridge, RTD, and other transducer signals.

7. 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.

8. Laser-wafer trimming results in low unadjusted errors and

affords precalibrated input and output spans.

9. Zero and span are independently adjustable and noninteractive

to accommodate transducers or user defined ranges.

10. Six precalibrated temperature ranges are available with a

100 W RTD via pin strapping.

REV. A

Information furnished by Analog Devices is believed to be accurate and

reliable. However, no responsibility is assumed by Analog Devices for its

use, nor for any infringements of patents or other rights of third parties

which may result from its use. No license is granted by implication or

otherwise under any patent or patent rights of Analog Devices.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.

Tel: 617/329-4700

Precalibrated 100

Fax: 617/326-8703

AD693–SPECIFICATIONS

(@ +25 8 C and V S = +24 V. Input Span = 30 mV or 60 mV. Output Span = 4–20 mA,

R L = 250 V , V CM = 3.1 V, with external pass transistor unless otherwise noted.)

Model

AD693AD/AQ/AE

Conditions

Min

Typ

Max

Units

LOOP-POWERED OPERATION

TOTAL UNADJUSTED ERROR 1, 2

± 0.25

6 0.5

% Full Scale

T MIN to T MAX

± 0.4

6 0.75

% Full Scale

100 W RTD CALIBRATION ERROR 3

(See Figure 17)

± 0.5

± 2.0

°C

LOOP POWERED OPERATION 2

Zero Current Error 4

Zero = 4 mA

Zero = 12 mA

120

Zero = 0 mA 5

+35

+100

vs. Temp.

Zero = 4 mA

0.5

1.5

Power Supply Rejection (RTI)

12 V

V OP

36V 6

3.0

5.6

V/V

0 V

V CM

6.2 V

Common-Mode Input Range

(See Figure 3)

+V OP – 4 V 6

Common-Mode Rejection (RTI)

0 V

V CM

6.2 V

V/V

Input Bias Current 7

+20

T MIN to T MAX

+25

Input Offset Current 7

V SIG = 0

0.5

3.0

Transconductance

Nominal

30 mV Input Span

0.5333

A/V

60 mV Input Span

0.2666

A/V

Unadjusted Error

0.05

0.2

vs. Common-Mode

6.2 V

30 mV Input Span

V CM

0.03

0.04

%/V

60 mV Input Span

0.05

0.06

%/V

Error vs. Temp.

ppm/

Nonlinearity 8

30 mV Input Span

0.01

0.05

% of Span

60 mV Input Span

0.02

0.07

% of Span

OPERATIONAL VOLTAGE RANGE

Operational Voltage, V OP 6

+12

+36

Quiescent Current

Into Pin 9

+500 +700

OUTPUT CURRENT LIMIT

+21

+25

+32

COMPONENTS OF ERROR

SIGNAL AMPLIFIER 9

Input Voltage Offset

± 40

6 200

vs. Temp

± 1.0

± 2.5

mV/°C

Power Supply Rejection

12 V £ V OP £ 36 V 6

± 3.0

6 5.6

mV/V

0 V £ V CM £ 6.2 V

V/I CONVERTER 9, 10

Zero Current Error

Output Span = 4–20 mA

± 30

± 80

Power Supply Rejection

12 V £ V OP £ 36 V 6

± 1.0

± 3.0

mA/V

Transconductance

Nominal

0.2666

A/V

Unadjusted Error

± 0.05

± 0.2

6.200 V REFERENCE 9, 12

Output Voltage Tolerance

± 3

6 12

vs. Temp.

ppm/

Line Regulation

12 V £ V OP £ 36 V 6

± 200

6 300

mV/V

Load Regulation 11

0 mA £ I REF £ 3 mA

± 0.3

6 0.75

mV/mA

Output Current 13

Loop Powered, (Figure 10)

+3.0

+3.5

3-Wire Mode, (Figure 15)

+5.0

–2–

REV. A

0 V

AD693

Model

AD693AD

Conditions

Min

Typ

Max

Units

AUXILIARY AMPLIFIER

Common-Mode Range

+V OP – 4 V 6

Input Offset Voltage

± 50

± 200

Input Bias Current

+20

Input Offset Current

+0.5

± 3.0

Common-Mode Rejection

Power Supply Rejection

105

Output Current Range

Pin I X OUT

+0.01

Output Current Error

Pin V X – Pin I X

± 0.005

TEMPERATURE RANGE

Case Operating 14

T MIN to T MAX

–40

+85

°C

Storage

–65

+150

°C

NOTES

1 Total error can be significantly reduced (typically less than 0.1%) by trimming the zero current. The remaining unadjusted error sources are transconductance and

nonlinearity.

2 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.

3 Error from ideal output assuming a perfect 100 W RTD at 0 and +100°C.

4 Refer to the Error Analysis to calculate zero current error for input spans less than 30 mV.

5 By forcing the differential signal amplifier input sufficiently negative the 7 mA zero current can always be achieved.

6 The operational voltage (V OP ) 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, V OP = V S –

(I LOOP ´ R L ) in two-wire mode (refer to Figure 10).

7 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.

8 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.

9 Specifications for the individual functional blocks are components of error that contribute to, and that are included in, the Loop Powered Operation specifications.

10 Includes error contributions of V/I converter and Application Voltages.

11 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.

12 If not used for external excitation, the reference should be loaded by approximately 1 mA (6.2 k

T C .

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 RATINGS

Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +36 V

Reverse Loop Current . . . . . . . . . . . . . . . . . . . . . . . . . 200 mA

Signal Amp Input Range . . . . . . . . . . . . . . . . . . –0.3 V to V OP

Reference Short Circuit to Common . . . . . . . . . . . . Indefinite

Auxiliary Amp Input Voltage Range . . . . . . . . . . 0.3 V to V OP

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

AD693 PIN CONFIGURATION

(AD, AQ, AE Packages)

ORDERING GUIDE

Package

Model

Description

Option

AD693AD

Ceramic Side-Brazed DIP

D-20

AD693AQ

Cerdip

Q-20

AD693AE

Leadless Ceramic Chip

E-20A

Functional Diagram

Carrier (LCCC)

REV. A

–3–

to common).

13 In the loop powered mode up to 5 mA can be drawn from the reference, however, the lower limit of the output span will be increased accordingly. 3.5 mA is the

maximum current the reference can source while still maintaining a 4 mA zero.

14 The AD693 is tested with a pass transistor so T A

AD693–Typical Characteristics

Figure 1. Maximum Load Resistance

vs. Power Supply

Figure 4. Bandwidth vs. Series Load

Resistance

Figure 7. Input Current Noise vs.

Frequency

Figure 2. Differential Input Current vs.

Input Signal Voltage Normalized to +IN

Figure 5. Signal Amplifier PSRR vs.

Frequency

Figure 8. Input Voltage Noise vs.

Frequency

Figure 3. Maximum Common-Mode

Voltage vs. Supply

Figure 6. CMRR (RTI) vs. Frequency

–4–

REV. A

AD693

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.

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 W 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

V IN is sufficiently positive. The Signal Amplifier is biased with

respect to V IN 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.

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 I IN 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, 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.

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 kW 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

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 W 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

REV. A

–5–

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: