Datasheet

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
to R
SET
, see “Selecting Components” section.
The INA330 errors are extremely low. The primary errors in
the INA330 occur in the current conveyer circuitry, as shown
in Figure 2. Equal currents in R
SET
and R
THERM
produce a
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: 10kΩ thermistor with a 4.5% temperature coeffi-
cient, (α = –0.045) changes resistance by 450Ω/°C. This
results in 4500nA change in I
1
for a 1°C temperature change
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
O
= I
1
– I
2
+ I
ERROR
+ ∆I
ERROR
/∆T
Current Conveyor: measures
the current difference between
pins 10 and 1.
2
3
10
1
R
THERM
10kΩ at 25°C
9.55kΩ 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
10kΩ
(Selection of R
SET
significantly affects
control system—see
“Selecting Components”
section.)
R
G
200kΩ
Thermistor
R
THERM
= 10kΩ
C
FILTER
500pF
I
O
= I
1
– I
2
V
O
V
REF
2.5V
PID CONTROLLER
V
ADJUST
= +2.5V
TEC
Power
Amp
0.1µF
Place 0.1µF 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 60µV 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 10kΩ, 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.