AD624
ERROR BUDGET ANALYSIS
+V
S
To illustrate how instrumentation amplifier specifications are
applied, we will now examine a typical case where an AD624 is
required to amplify the output of an unbalanced transducer.
Figure 47 shows a differential transducer, unbalanced by ≈5 Ω,
supplying a 0 to 20 mV signal to an AD624C. 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.)
+10V
10k⍀
RG
1
350⍀
350⍀
350⍀
350⍀
14-BIT
ADC
0 TO 2V
F.S.
G = 100
AD624C
RG
2
–V
S
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 (20 ppm =
0.002%) are significant. Furthermore, if a system has an intelli-
gent processor monitoring the A to D output, the addition of an
autogain/autozero cycle will remove all reducible errors and may
eliminate the requirement for initial calibration. This will also
reduce errors to 0.002%.
Figure 47. Typical Bridge Application
Table II. Error Budget Analysis of AD624CD in Bridge Application
Effect on
Absolute
Accuracy
Effect on
Absolute
Accuracy
Effect
on
AD624C
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
±0.1%
10 ppm
±0.001%
±25 µV, RTI
±0.25 µV/°C
±0.1% = 1000 ppm
(10 ppm/°C) (60°C) = 600 ppm
±0.001% = 10 ppm
±25 µV/20 mV = ±1250 ppm
(±0.25 µV/°C) (60°C)= 15 µV
15 µV/20 mV = 750 ppm
±2.0 mV/20 mV = 1000 ppm
(±10 µV/°C) (60°C) = 600 µV
600 µV/20 mV = 300 ppm
(±15 nA)(5 Ω ) = 0.075 µV
0.075 µV/20mV = 3.75 ppm
(±10 nA)(5 Ω) = 0.050 µV
0.050 µV/20 mV = 2.5 ppm
(10 nA) (175 Ω) = 1.75 µV
1.75 µV/20 mV = 87.5 ppm
(100 pA/°C) (175 Ω) (60°C) = 1 µV
1 µV/20 mV = 50 ppm
1000 ppm
_
–
1000 ppm
600 ppm
–
–
–
10 ppm
–
1250 ppm
1250 ppm
–
750 ppm
1000 ppm
–
–
Output Offset Voltage1
±2.0 mV
1000 ppm
Output Offset Voltage Drift1 ±10 µV/°C
–
300 ppm
3.75 ppm
2.5 ppm
87.5 ppm
50 ppm
–
–
–
–
–
–
Bias Current–Source
Imbalance Error
Offset Current–Source
Imbalance Error
Offset Current–Source
Resistance Error
Offset Current–Source
Resistance–Drift
Common-Mode Rejection
5 V dc
±15 nA
3.75 ppm
2.5 ppm
87.5 ppm
–
±10 nA
±10 nA
±100 pA/°C
115 dB
115 dB = 1.8 ppm × 5 V = 9 µV
9 µV/20 mV = 444 ppm
450 ppm
450 ppm
Noise, RTI
(0.1 Hz–10 Hz)
0.22 µV p-p
0.22 µV p-p/20 mV = 10 ppm
_
–
10 ppm
20 ppm
Total Error
3793.75 ppm
5493.75 ppm
NOTE
1Output offset voltage and output offset voltage drift are given as RTI figures.
For a comprehensive study of instrumentation amplifier design
and applications, refer to the Instrumentation Amplifier Application
Guide, available free from Analog Devices.
REV. C
–14–