AD8553 datasheet(27/28 Pages) AD

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AD8426

Rev. 0 | Page 27 of 28

DRIVING AN ADC

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.

Figure 72 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 3 is useful for applications where the AD8426 must

operate from a large voltage supply but drives a single-supply

ADC. In normal operation, the AD8426 output signal stays

within the ADC range, and the AD8616 simply buffers the signal.

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 a problem in this circuit, because the 10 kΩ resistor

between the two amplifiers limits the current into the AD8616

to a safe level.

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.

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 72. Driving an ADC

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