Datasheet

XTR108
21
SBOS187C
www.ti.com
SAMPLE ERROR ANALYSIS
Table XVII shows a detailed computation of the error
accumulation. The sample error budget is based on a typical
RTD circuit (Pt100, 200°C measurement span). Note that
these calculations are based on typical characteristics where
no maximum or minimum characteristic is available. The
assumption is made that all errors are positive and additive.
As the various error sources are independent, a closer
approximation to nominal performance might be to accumu-
late the errors with a root-sum-square calculation.
CALIBRATED ERROR
ERROR SOURCE ERROR EQUATION SAMPLE ERROR CALCULATION (ppm of Full Scale)
INPUT
Input Offset Voltage Note (1) 0
vs Common Mode CMRR • ∆CM/(V
IN MAX
) • 10
6
5µV/V • 0.1V/0.04V • 10
6
12.5
Input Bias Current Note (1) 0
Input Offset Current Note (1) 0
Total Input Error: 12.5
EXCITATION
Current Reference Accuracy Note (1) 0
vs Common Mode ∆CM/R
OUT
• R
RTD MIN
/(V
IN MAX
) • 10
6
0.1V/100MΩ • 100Ω/40mV 2.5
Current Reference Matching Note (1)
DAC Resolution and Linearity 1LSB
FINE
• R
RTD MIN
/(V
IN MAX
) • 10
6
96nA • 100Ω/40mV • 10
6
240
Total Excitation Error: 242.5
GAIN
Span Note (1) 0
Nonlinearity Nonlinearity (%)/100% • 10
6
0.01%/100% • 10
6
100
Total Gain Error: 100
OUTPUT
Zero Output Note (1) 0
vs Supply (I
ZERO
vs V
+
) • ∆V
+
/16mA • 10
6
Note (2) 6
DAC Resolution and Linearity 2LSB
FINE
/16mA • 10
6
2 • 1.8µA/16mA • 10
6
225
Total Output Error: 231
DRIFT (∆T
A
= 20°C)
Input Offset Voltage Drift • ∆T
A
/(V
IN MAX
) • 10
6
0.02µV/°C • 20°C/40mV • 106 10
Current Reference Accuracy Drift • ∆T
A
35ppm • 20°C 700
Current Reference Matching Drift • ∆T
A
• I
REF
• R
RTD MIN
/(V
IN MAX
) 15ppm • 20°C • 518.9µA • 100/40mV 390
Span Drift • ∆T
A
30ppm • 20°C 600
Zero Output Drift • ∆T
A
Note (1) 250
Total Drift Error: 1950
NOISE (0.1Hz to 10Hz, Typ)
Input Offset Voltage V
N
/(V
IN MAX
) • 10
6
6µV/40mV • 10
6
150
Current Reference I
REF
Noise • R
RTD MIN
/(V
IN MAX
) • 10
6
0.015µA • 100Ω/40mV • 10
6
37.5
Zero Output I
ZERO
Noise/16mA • 10
6
1.1µA/16mA • 10
6
68.5
Total Noise Error: 256
TOTAL ERROR: 2792 (1997)
(3)
0.28% (0.20%)
(3)
SAMPLE ERROR CALCULATION
RTD value at 4mA Output (R
RTD MIN
) 100Ω: RTD Measurement Range 200°C; Ambient Temperature Range (∆T
A
) 20°C; Supply Voltage Change (∆V
+
) 5V; Common-
Mode Voltage Change (∆CM) 0.1V.
Chosen XTR108 parameters: PGIA gain = 50; I
REF
= 518.9µA; Full-scale V
IN
= 40mW. Register 06 = 0
H
03; Register 11 = 0
H
11; Register 13 = 0
H
FC; Register 14 = 0
H
70.
NOTES: (1) Does not contribute to the output error due to calibration. (2) All errors are referred to input unless otherwise stated. (3) Calculated as root-
sum–square.
TABLE XVII. Sample Error Budget Calculation.