AD8494/AD8495/AD8496/AD8497

Rev. 0 | Page 15 of 16

SETPOINT CONTROLLER

The AD849x can be used as a temperature setpoint controller,

with a thermocouple input from a remote location or with the

AD849x itself being used as a temperature sensor. When the

measured temperature is below the setpoint temperature, the

For best accuracy and CMRR performance, the setpoint voltage

should be created with a low impedance source. If the setpoint

voltage is generated with a voltage divider, a buffer is

recommended.

IN-AMP

AD849x

OUT

SENSE

SETPOINT

VOLTAGE

REF

5V

COLD JUNCTION

COMPENSATION

2

3

6

7

5

+IN

THERMO-

COUPLE

–IN

1MΩ

8

1

Figure 35. Setpoint Controller

Hysteresis can be added to the setpoint controller by using a

resistor divider from the output to the reference pin, as shown

in Figure 36. The hysteresis in °C is

C

mV/

5

)

/(









R2

R1

R1

V

T

S

HYST

IN-AMP

AD849x

OUT

SENSE

SETPOINT

VOLTAGE

R2

100kΩ

R1

1kΩ

R1

1kΩ

REF

5V

COLD JUNCTION

COMPENSATION

2

3

7

5

6

+IN

THERMO-

COUPLE

–IN

1MΩ

8

1

Figure 36. Adding 10 Degrees of Hysteresis

A resistor equivalent to the output resistance of the divider should

be connected to the sense pin to ensure good CMRR.

MEASURING NEGATIVE TEMPERATURES

The AD849x can measure negative temperatures on dual

supplies and on a single supply. When operating on dual

supplies with the reference pin grounded, a negative output

voltage indicates a negative temperature at the thermocouple

measurement junction.

When operating the AD849x on a single supply, level-shift

a negative temperature at the thermocouple measurement

junction.

REFERENCE PIN ALLOWS OFFSET ADJUSTMENT

The reference pin can be used to level-shift the AD849x output

voltage. This is useful for measuring negative temperatures on a

single supply and to match the AD849x output voltage range to

the input voltage range of the subsequent electronics in the

signal chain.

The reference pin can also be used to offset any initial calibra-

tion errors. Apply a small reference voltage proportional to the

error to nullify the effect of the calibration error on the output.