AD8553 datasheet(16/20 Pages) AD

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AD8426

Preliminary Technical Data

Rev. PrD | Page 16 of 20

PRECISION STRAIN GAGE

The low offset and high CMRR over frequency of the AD8426

make it an excellent candidate for bridge measurements. The

bridge can be connected directly to the inputs of the amplifier

(see Figure 15).

2.5V

10µF

0.1µF

AD8426

+IN

–IN

350Ω

–

Figure 15. Precision Strain Gage

DRIVING AN ADC

Figure 16 shows several different methods of driving an ADC.

The ADC in the ADuC7026 microcontroller was chosen for this

example because it has an unbuffered, charge sampling

architecture that is typical of most modern ADCs. This type of

architecture typically requires an RC buffer stage between the

ADC and the amplifier to work correctly.

Option 1 shows the minimum configuration required to drive

a charge sampling ADC. The capacitor provides charge to the

ADC sampling capacitor, and the resistor shields the AD8426

from the capacitance. To keep the AD8426 stable, the RC time

constant of the resistor and capacitor needs to stay above 5 μs.

This circuit is mainly useful for lower frequency signals.

Option 2 shows a circuit for driving higher frequency signals.

It uses a precision op amp (AD8616) with relatively high band-

width and output drive. This amplifier can drive a resistor and

capacitor with a much higher time constant and is, therefore,

suited for higher frequency applications.

Option 3 is useful for applications where the AD8426 needs to

run off a large voltage supply, but drives a single supply ADC.

In normal operation, the AD8426 output stays within the ADC

range, and the AD8616 simply buffers it. However, in a fault

condition, the output of the AD8426 may go outside the supply

range of both the AD8616 and the ADC. This is not an issue in

this circuit, because the 10 kΩ resistor between the two

amplifiers limits the current into the AD8616 to a safe level.

AD8426

REF

100nF

100Ω

10kΩ

10Ω

10nF

ADC0

ADC1

ADC2

AGND

3.3V

OPTION 1: DRIVING LOW FREQUENCY SIGNALS

OPTION 2: DRIVING HIGH FREQUENCY SIGNALS

OPTION 3: PROTECTING ADC FROM LARGE VOLTAGES

3.3V

AD8426

AD8616

ADuC7026

REF

3.3V

10Ω

10nF

AD8426

AD8616

REF

+15V

–15V

AVDD

Figure 16. Driving an ADC

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