REV. C
AD624
–14–
AD624C
–V
S
+V
S
G = 100
RG
1
RG
2
10k⍀
350⍀
+10V
14-BIT
ADC
0 TO 2V
F.S.
350⍀
350⍀
350⍀
Figure 47. Typical Bridge Application
Table II. Error Budget Analysis of AD624CD in Bridge Application
Effect on Effect on
Absolute Absolute Effect
AD624C Accuracy Accuracy on
Error Source Specifications Calculation at T
A
= +25ⴗC at T
A
= +85ⴗC Resolution
Gain Error ±0.1% ± 0.1% = 1000 ppm 1000 ppm 1000 ppm –
Gain Instability 10 ppm (10 ppm/°C) (60°C) = 600 ppm _ 600 ppm –
Gain Nonlinearity ±0.001% ±0.001% = 10 ppm ––10 ppm
Input Offset Voltage ±25 µV, RTI ±25 µV/20 mV = ±1250 ppm 1250 ppm 1250 ppm –
Input Offset Voltage Drift ±0.25 µV/°C(±0.25 µV/°C) (60°C)= 15 µV
15 µV/20 mV = 750 ppm – 750 ppm –
Output Offset Voltage
1
±2.0 mV ±2.0 mV/20 mV = 1000 ppm 1000 ppm 1000 ppm –
Output Offset Voltage Drift
1
±10 µV/°C(±10 µV/°C) (60°C) = 600 µV
600 µV/20 mV = 300 ppm – 300 ppm –
Bias Current–Source ±15 nA (±15 nA)(5 Ω ) = 0.075 µV
Imbalance Error 0.075 µV/20mV = 3.75 ppm 3.75 ppm 3.75 ppm –
Offset Current–Source ±10 nA (±10 nA)(5 Ω) = 0.050 µV
Imbalance Error 0.050 µV/20 mV = 2.5 ppm 2.5 ppm 2.5 ppm –
Offset Current–Source ±10 nA (10 nA) (175 Ω) = 1.75 µV
Resistance Error 1.75 µV/20 mV = 87.5 ppm 87.5 ppm 87.5 ppm –
Offset Current–Source ±100 pA/°C (100 pA/°C) (175 Ω) (60°C) = 1 µV
Resistance–Drift 1 µV/20 mV = 50 ppm – 50 ppm –
Common-Mode Rejection 115 dB 115 dB = 1.8 ppm × 5V = 9µV
5V dc 9µV/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
Total Error 3793.75 ppm 5493.75 ppm 20 ppm
NOTE
1
Output 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.
ERROR BUDGET ANALYSIS
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.)
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%.
