Datasheet

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 k 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
100
10k
10
10nF
ADC0
ADC1
ADC2
AGND
3.3V
3.3V
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
10
10nF
AD8426
AD8616
REF
+15V
–15V
AV
DD
09490-065
Figure 72. Driving an ADC