INA330

6

SBOS260

www.ti.com

APPLICATIONS INFORMATION

OVERVIEW

Precision temperature controllers are generally adjusted to

their set-point temperature to achieve the desired system

performance and to compensate for tolerance of the ther-

mistor and reference circuitry. After this adjustment, the

crucial issue is the stability of this set-point temperature.

When used in a temperature control loop (Figure 1), the

INA330 provides excellent control-point stability over time

and ambient temperature changes. Low 1/f noise assures

excellent short-term stability. Internal auto-zero circuitry as-

sures excellent stability throughout product life.

SOURCES OF ERRORS

The largest source of error in a control system will occur due

The INA330 errors are extremely low. The primary errors in

the INA330 occur in the current conveyer circuitry, as shown

small output current error of 200nA (maximum), and some

variation with temperature of 40nA (maximum). The offset is

calibrated out. Only the variation affects set-point stability.

The variation can be referred to the input as a set-point temp

variation: 10k

Ω thermistor with a 4.5% temperature coeffi-

cient, (

α = –0.045) changes resistance by 450Ω/°C. This

at the thermistor. Therefore, the 40nA maximum current

offset error variation with ambient temperature results in a

0.009

°C variation in set-point temperature over –40°C to

+85

°C ambient (40nA/4500nA/°C = 0.009°C set-point/°C

FIGURE 2. Current Conveyor Portion of the INA330.

8

7

I

Current Conveyor: measures

the current difference between

pins 10 and 1.

2

3

10

1

R

THERM

10k

Ω at 25°C

9.55k

Ω at 26°C

∆I = 4500nA

R

SET

I

2

I

1

1V

D/A

Converter

2

3

10

1

95

4

V+

V

2

V

EXCITE

1V

V

1

6

Enable

High = On

Low = Off

8

+0.9V/

°C

for increasing

temperature.

7

I

1

I

2

R

SET

10k

Ω

(Selection of R

SET

significantly affects

control system—see

“Selecting Components”

section.)

R

G

200k

Ω

Thermistor

R

C

FILTER

500pF

I

V

O

V

REF

2.5V

PID CONTROLLER

V

TEC

Power

Amp

0.1

µF

Place 0.1

µF capacitor close

to and across the power-

supply pins.

FIGURE 1. The INA330 In Simplified Temperature Control Loop.

ambient). This is the variation in set-point temperature due to

variation in ambient temperature of the INA330.

Insignificant Errors

Input offset voltage of the voltage excitation buffers are auto-

zeroed to approximately 60

µV and match to 30µV. Drift with

temperature is very low. They contribute negligible error.

Voltage excitation buffers have an input bias current of

0.2nA. With a source impedance of less than 10k

Ω, errors

produced by the input bias current will be negligible.

Output buffer errors are auto-zeroed. When referred to the

input, their errors are negligible.

Gain error does not produce any significant temperature set-

point error when used in a temperature set-point control loop.