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AD693AE

Loop-Powered 4–20 mA Sensor Transmitter

Analog Devices 

AD693

Figure 19. Thermocouple Inputs with Cold Junction Compensation

Table II. Thermocouple Application—Cold Junction Compensation

POLARITY MATERIAL

AMBIENT

TYPE TEMP RCOMP RZ

30 mV 60 mV

TEMP TEMP

RANGE RANGE

+

IRON

–

CONSTANTAN

J

25°

75°

51.7 Ω

53.6 Ω

301K

294K

546°C

1035°C

+

NICKEL-CHROME

25°

40.2 Ω 392K

721°C

—

_

NICKEL-ALUMINUM

K

75°

42.2 Ω 374K

+

NICKEL-CHROME

25°

60.4 Ω 261K

E

413°C 787°C

–

COPPER-NICKEL

75°

64.9 Ω 243K

+

COPPER

–

COPPER-NICKEL

25°

T

75°

40.2 Ω

45.3 Ω

392K

USE WITH GAIN >2

340K

via a set of thermocouple tables referenced to °C. For example,

the output of a properly referenced type J thermocouple is

60 mV when the hot junction is at 1035°C. Table II lists the

maximum measurement temperature for several thermocouple

types using the preadjusted 30 mV and 60 mV input ranges.

More convenient temperature ranges can be selected by deter-

mining the full-scale input voltages via standard thermocouple

tables and adjusting the AD693 span. For example, suppose

only a 300°C span is to be measured with a type K thermo-

couple. From a standard table, the thermocouple output is

12.207 mV; since 60 mV at the signal amplifier corresponds to a

16 mA span at the output a gain of 5, or more precisely 60 mV/

12.207 mV = 4.915 will be needed. Using a 12.207 mV span in

the gain resistor formula given in “Adjusting Input Span” yields

a value of about 270 Ω as the minimum from P1 to 6.2 V. Adding

a 50 Ω potentiometer will allow ample adjustment range.

With the connection illustrated, the AD693 will give a full-scale

indication with an open thermocouple.

ERROR BUDGET ANALYSIS

Loop-Powered Operation specifications refer to parameters

tested with the AD693 operating as a loop-powered transmitter.

The specifications are valid for the preset spans of 30 mV,

60 mV and those spans in between. The section, “Components

of Error,” refers to parameters tested on the individual functional

blocks, (Signal Amplifier, V/I Converter, Voltage Reference, and

Auxiliary Amplifier). These can be used to get an indication of

device performance when the AD693 is used in local power

mode or when it is adjusted to spans of less than 30 mV.

Table III lists the expressions required to calculate the total

error. The AD693 is tested with a 250 Ω load, a 24 V loop supply

Table III. RTI Contributions to Span and Offset Error

RTI Contributions to Offset Error

Error Source

Expression for RTI Error at Zero

IZE Zero Current Error

IZE/XS

PSRR Power Supply Rejection Ratio (|VLOOP – 24 V| + [|RL – 250 Ω| × IZ]) × PSRR

CMRR Common-Mode Rejection Ratio |VCM – 3.1 V| × CMRR

IOS Input Offset Current

RS × IOS

RTI Contributions to Span Error

Error Source

XSE Transconductance Error

XPSRR Transconductance PSRR1

XCMRR Transconductance CMRR

XNL Nonlinearity

IDIFF Differential Input Current2

Expression for RTI Error at Full Scale

VSPAN × XSE

|RL – 250 Ω| × IS × PSRR

|VCM – 3.1 V| × VSPAN × XCMRR

VSPAN × XNL

RS × IDIFF

Abbreviations

IZ

IS

RS

RL

VLOOP

VCM

VSPAN

XS

Zero Current (usually 4 mA)

Output span (usually 16 mA)

Input source impedance

Load resistance

Loop supply voltage

Input common-mode voltage

Input span

Nominal transconductance in A/V

1The 4–20 mA signal, flowing through the metering resistor, modulates the power supplyvoltage seen

by the AD693. The change in voltage causes a power supply rejection error that varies with the

output current, thus it appears as a span error.

2The input bias current of the inverting input increases with input signal voltage. The differential

input current, IDIFF, equals the inverting input current minus the noninverting input current; see

Figure 2. IDIFF, flowing into an input source impedance, will cause an input voltage error that var-

ies with signal. If the change in differential input current with input signal is approximated as a

linear function, then any error due to source impedance may be approximated as a span error. To

calculate IDIFF, refer to Figure 2 and find the value for IDIFF/ + In corresponding to the full-scale

input voltage for your application. Multiply by + In max to get IDlFF. Multiply IDIFF by the source

impedance to get the input voltage error at full scale.

REV. A

–11–

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