AD524
ERROR BUDGET ANALYSIS
In many applications, differential linearity and resolution are of
prime importance. This would be so in cases where the absolute
value of a variable is less important than changes in value. In
these applications, only the irreducible errors (45 ppm = 0.004%)
are significant. Furthermore, if a system has an intelligent pro-
cessor monitoring the A-to-D output, the addition of a auto-
gain/autozero cycle will remove all reducible errors and may
eliminate the requirement for initial calibration. This will also
reduce errors to 0.004%.
To illustrate how instrumentation amplifier specifications are
applied, we will now examine a typical case where an AD524 is
required to amplify the output of an unbalanced transducer.
Figure 46 shows a differential transducer, unbalanced by 100 Ω,
supplying a 0 to 20 mV signal to an AD524C. The output of the
IA feeds a 14-bit A-to-D converter with a 0 to 2 volt input volt-
age range. The operating temperature range is –25°C to +85°C.
Therefore, the largest change in temperature ∆T within the
operating range is from ambient to +85°C (85°C – 25°C = 60°C).
+V
S
+10V
10k⍀
350⍀
350⍀
350⍀
350⍀
RG
1
14-BIT
ADC
0V TO 2V
F.S.
G = 100
AD524C
RG
2
–V
S
Figure 46. Typical Bridge Application
Table II. Error Budget Analysis of AD524CD in Bridge Application
Effect on
Absolute
Accuracy
Effect on
Absolute
Accuracy
Effect
on
AD524C
Error Source
Specifications Calculation
at TA = +25؇C at TA = +85؇C Resolution
Gain Error
Gain Instability
Gain Nonlinearity
Input Offset Voltage
Input Offset Voltage Drift
±0.25%
25 ppm
±0.003%
±50 µV, RTI
±0.5 µV/°C
–
±0.25% = 2500 ppm
(25 ppm/°C)(60°C) = 1500 ppm
±0.003% = 30 ppm
2500 ppm
–
–
2500 ppm
1500 ppm
–
–
–
30 ppm
–
±50 µV/20 mV = ±2500 ppm
(±0.5 µV/°C)(60°C) = 30 µV
30 µV/20 mV = 1500 ppm
±2.0 mV/20 mV = 1000 ppm
(±25 µV/°C)(60°C)= 1500 µV
1500 µV/20 mV = 750 ppm
(±15 nA)(100 Ω) = 1.5 µV
1.5 µV/20 mV = 75 ppm
(±100 pA/°C)(100 Ω)(60°C) = 0.6 µV
0.6 µV/20 mV= 30 ppm
(±10 nA)(100 Ω) = 1 µV
1 µV/20 mV = 50 ppm
(100 pA/°C)(100 Ω)(60°C) = 0.6 µV
0.6 µV/20 mV = 30 ppm
2500 ppm
2500 ppm
–
1500 ppm
1000 ppm
–
–
Output Offset Voltage*
Output Offset Voltage Drift* ±25 µV/°C
±2.0 mV
1000 ppm
–
750 ppm
75 ppm
30 ppm
50 ppm
30 ppm
87.5 ppm
50 ppm
444 ppm
–
–
–
–
–
–
–
–
Bias Current-Source
Imbalance Error
Bias Current-Source
Imbalance Drift
Offset Current-Source
Imbalance Error
Offset Current-Source
Imbalance Drift
Offset Current-Source
Resistance-Error
Offset Current-Source
Resistance-Drift
Common Mode Rejection
5 V dc
±15 nA
75 ppm
±100 pA/°C
±10 nA
–
50 ppm
±100 pA/°C
±10 nA
–
(10 nA)(175 Ω) = 3.5 µV
3.5 µV/20 mV = 87.5 ppm
(100 pA/°C)(175 Ω)(60°C) = 1 µV
1 µV/20 mV = 50 ppm
115 dB = 1.8 ppm × 5 V = 8.8 µV
8.8 µV/20 mV = 444 ppm
87.5 ppm
–
±100 pA/°C
115 dB
444 ppm
Noise, RTI
(0.1 Hz–10 Hz)
0.3 µV p-p
0.3 µV p-p/20 mV = 15 ppm
–
–
15 ppm
45 ppm
Total Error
6656.5 ppm
10516.5 ppm
*Output offset voltage and output offset voltage drift are given as RTI figures.
–14–
REV. E