Datasheet

AD8227
Rev. 0 | Page 23 of 24
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 k resistor between the two amplifiers
limits the current into the AD8616 to a safe level.
AD8227
REF
100nF
100Ω
10Ω
ADC0
ADC1
3.3V
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
AV
DD
07759-066
Figure 66. Driving an ADC