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AD596

Thermocouple Conditioner and Setpoint Controller

Analog Devices 

AD596/AD597

TEMPERATURE PROPORTIONAL OUTPUT MODE

The AD596/AD597 can be used to generate a temperature

proportional output of 10 mV/°C when operated with J and K

type thermocouples as shown in Figure 1. Thermocouples pro-

duce low level output voltages which are a function of both the

temperature being measured and the reference or cold junction

temperature. The AD596/AD597 compensates for the cold

junction temperature and amplifies the thermocouple signal to

produce a high level 10 mV/°C voltage output which is a func-

tion only of the temperature being measured. The temperature

stability of the part indicates the sensitivity of the output voltage

to changes in ambient or device temperatures. This is typically

0.02°C/°C over the +25°C to +100°C recommended ambient

temperature range. The parts will operate over the extended

ambient temperature ranges from –55°C to +125°C, but ther-

mocouple nonlinearity at the reference junction will degrade the

temperature stability over this extended range. Table I is a list of

ideal AD596/AD597 output voltages as a function of Celsius

temperature for type J and K ANSI standard thermocouples

with package and reference junction at 60°C. As is normally the

case, these outputs are subject to calibration and temperature

sensitivity errors. These tables are derived using the ideal trans-

fer functions:

AD596 output = (Type J voltage + 301.5 µV) × 180.57

AD597 output = (Type K voltage) × 245.46

CONSTANTAN

(ALUMEL)

+5V TO +30V

IRON

(CHROMEL) +15V

100k⍀

OPTIONAL

OFFSET 10k⍀

ADJUST

100k⍀

1M⍀

–15V

AD596/

AD597*

0.01␮F

VOUT

SPAN OF

+5V TO +30V

0.01␮F

0 TO –25V

*H PACKAGE PINOUT SHOWN

Figure 1. Temperature Proportional Output Connection

The offsets and gains of these devices have been laser trimmed

to closely approximate thermocouple characteristics over mea-

surement temperature ranges centered around 175°C with the

AD596/AD597 at an ambient temperature between 25°C and

100°C. This eliminates the need for additional gain or offset

adjustments to make the output voltage read:

VOUT = 10 mV/°C × (thermocouple temperature in °C) (within

specified tolerances).

Excluding calibration errors, the above transfer function is accu-

rate to within 1°C from +80°C to +550°C for the AD596 and

–20°C to +350°C for the AD597. The different temperature

ranges are due to the differences in J and K type thermocouple

curves.

European DIN FE-CuNi thermocouple vary slightly from ANSI

type J thermocouples. Table I does not apply when these types

of thermocouples are used. The transfer functions given previ-

ously and a thermocouple table should be used instead.

Figure 1 also shows an optional trimming network which can be

used to change the device’s offset voltage. Injecting or sinking

200 nA from Pin 3 will offset the output approximately 10 mV

(1°C).

The AD596/AD597 can operate from a single supply from 5 V

to 36 V or from split supplies totalling 36 V or less as shown.

Since the output can only swing to within 2 V of the positive

supply, the usable measurement temperature range will be re-

stricted when positive supplies less than 15 V for the AD597

and 10 V for the AD596 are used. If the AD596/AD597 is to be

used to indicate negative Celsius temperatures, then a negative

supply is required.

Common-mode voltages on the thermocouple inputs must

remain within the common-mode voltage range of the AD596/

AD597, with a return path provided for the bias currents. If the

thermocouple is not remotely grounded, then the dotted line

connection shown in Figure 1 must be made to one of the ther-

mocouple inputs. If there is no return path for the bias currents,

the input stage will saturate, causing erroneous output voltages.

In this configuration, the AD596/AD597 H package option has

circuitry which detects the presence of an open thermocouple. If

the thermocouple loop becomes open, one or both of the inputs

to the device will be deprived of bias current causing the output

to saturate. It is this saturation which is detected internally and

used to activate the alarm circuitry. The output of this feature

has a flexible format which can be used to source or sink up to

20 mA of current. The collector (+ALM) should not be allowed

to become more positive than (–VS + 36 V), however, it may be

permitted to be more positive than +VS. The emitter voltage

(–ALM) should be constrained such that it does not become

more positive than 4 V below +VS. If the alarm feature is not

used, this pin should be connected to Pins 4 or 5 as shown in

Figure 1. The alarm function is unavailable on the AR package

option.

–4–

REV. B

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