REV. C

AD624

–14–

AD624C

G = 100

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

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 = 9 µV

5V dc

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

Total Error

3793.75 ppm

5493.75 ppm

20 ppm

NOTE

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%.