AD8227ARZ-R71 datasheet(24/25 Pages) AD

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AD8227

Rev. 0 | Page 23 of 2

DRIVING AN ADC

Figure 66 shows several different methods for 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 AD8227

from the capacitance. To keep the AD8227 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 AD8227 needs to

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

In normal operation, the AD8227 output stays within the ADC

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

condition, the output of the AD8227 may go outside the supply

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

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

limits the current into the AD8616 to a safe level.

AD8227

REF

100nF

100Ω

10Ω

ADC0

ADC1

3.3V

OPTION 1: DRIVING LOW FREQUENCY SIGNALS

OPTION 2: DRIVING HIGH FREQUENCY SIGNALS

3.3V

10kΩ

10nF

ADC2

AGND

OPTION 3: PROTECTING ADC FROM LARGE VOLTAGES

3.3V

AD8227

AD8616

ADuC7026

REF

3.3V

10Ω

10nF

AD8227

AD8616

REF

+15V

–15V

AVDD

Figure 66. Driving an ADC

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