Datasheet

XTR108
20
SBOS187C
www.ti.com
6) Choose PGA gain from the available list and calculate
the initial excitation current using:
I
II GRRR
ARR
REF
OUT OUT LIN MAX Z VI
PGA MAX MIN
MAX MIN
12
1
50
,
–––
–
=
()
•
()
()
•
••
()
Important: the PGA gain value should be chosen such
that the I
REF
value is within ±35% of 5V
REF
/R
SET
to
allow room for calibration adjustments without having
to go to another span step.
7) The required DAC zero offset current value can be
calculated by:
II
AI R R
R
ZERO OUT
PGA REF MIN Z
VI
MIN
=
•
()
–
–50
Example:
Measurement Range: T
MIN
= –20°C, T
MAX
= 50°C; 100Ω
RTD.
1) R
MIN
= 92.16Ω, R
MAX
= 119.40Ω, R
MID
= 105.85Ω;
2) Sensor relative nonlinearity: B
V
= 0.0026;
3) Choosing R
Z
= 90.9Ω (closest to R
MIN
2% value);
4) Linearization coefficient: G
LIN
= 0.3804mA/V;
5) 4-20mA output span;
6) PGA voltage gain A
PGA
= 200, sensor excitation current
I
REF1,2
= 368.39mA;
7) Zero offset DAC: I
ZERO
= 3.268mA
CALIBRATION PROCEDURE FOR RTD SENSORS
Step 1 Initial parameters calculation.
• Using the procedure above, compute I
REF
, A
PGA
, I
ZERO
,
and G
LIN
based on T
MIN
, T
MAX
, and nominal values of
R
Z
, R
SET
, and R
VI
. Use the equation in Table XV to
calculate the DAC register values.
• Configure the input MUX, write PGA gain, reference,
and offset DAC registers of the XTR108 with calcu-
lated settings. Note: write G
LIN
= 0 (no linearization) to
XTR108 at this step;
Step 2 Measurement.
• Set RTD resistor value (or oven temperature) to mini-
mum scale, measure output signal I
MEAS1
;
• Set RTD resistor value (or oven temperature) to maxi-
mum scale, measure output signal I
MEAS2
;
Step 3.
• Calculate corrections using the following equations:
I
IIR
AR R
RR
IIR
AI
G
B
BR BR BR
REF
MEAS MEAS VI
PGA MAX MIN
ZMIN
ZERO MEAS VI
PGA REFA
LINA
V
V MAX V MIN V ZA
A
A
=
()
()
=+
()
=
+
()()
21
1
50
50
2
05 05 2
–
–
–
.–.––
I
II GRRR
ARR
III II
Adjusted I fine DAC Code N N round
IR
V
II I
AI
REF
B
OUT
MAX
OUT
MIN
LIN A MAX Z
A
VI
PGA
MAX MIN
REF REF REF
A
REF REF
B
REF
A
REF
SET
REF
ZERO OUT
MIN
ZERO
PGA
REF
=
(
)
•
( )
( )
•
••
( )
=
( )
+
( )
=+
•






=
•
–– –
–
––
:
––
_
1
50
1024
50
10 10
∆
∆
∆
BB
MIN Z
A
VI
ZERO
A
ZERO
VI
REF
RR
R
Adjusted I fine DAC Code N N round
IR
V
–
:
( )
=+
•
•






12 12
512
5
∆
This takes into account resistor value deviations, all
offsets and gain errors of the coarse DACs and PGA. If
the adjusted abs(N
12A
) > 128 or abs(N
10A
) > 128, adjust
the coarse DAC first, then recalculate the fine DAC
value;
• Update all the DAC register value, including lineariza-
tion DAC.
Step 4 (optional).
Measure output signal I
MEAS3
with maximum RTD value
still connected to the input from step 2;
Step 5 (optional).
Compute G
LIN
correction and update LinDAC register;
Step 6 (optional).
Make verification measurements at min- and max-input
signal; If linearity check is needed: make a measurement at
mid-scale; write EEPROM data.
Step 7.
Set the desired over-scale, under-scale signal limits and
sensor burnout indication configuration. Verify and adjust
the over-scale and under-scale levels by applying the posi-
tive and negative overdriving differential signals to the PGA
inputs.