AD8226ARMZ-R7 datasheet(25/29 Pages) AD

25 / 29 page

AD8226

Data Sheet

Rev. C | Page 24 of 28

PRECISION STRAIN GAGE

The low offset and high CMRR over frequency of the AD8226

make it an excellent candidate for performing bridge measure-

ments. The bridge can be connected directly to the inputs of the

amplifier (see Figure 65).

2.5V

10µF

0.1µF

AD8226

+IN

–IN

350Ω

–

Figure 65. Precision Strain Gage

DRIVING AN ADC

Figure 66 shows several methods for driving an ADC. The

ADuC7026 microcontroller was chosen for this example because it

contains ADCs with 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

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 while the resistor shields the AD8226

from the capacitance. To keep the AD8226 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 speed signals. It uses a

precision op amp (AD8616) with relatively high bandwidth 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 AD8226 needs to

run off a large voltage supply but drive a single-supply ADC.

In normal operation, the AD8226 output stays within the ADC

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

condition, the output of the AD8226 may go outside the supply

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

the circuit, however, because the 10 kΩ resistor between the two

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

AD8226

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

AD8226

AD8616

ADuC7026

REF

3.3V

10Ω

10nF

AD8226

AD8616

REF

+15V

–15V

AVDD

Figure 66. Driving an ADC

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