AD8275
APPLICATIONS INFORMATION
10
0
–10
DRIVING A SINGLE-ENDED ADC
The AD8275 provides the common-mode rejection that SAR
ADCs often lack. In addition, it enables designers to use cost-
effective, precision, 16-bit ADCs such as the AD7685, yet still
condition 10 V signals.
–20
–30
–40
–50
–60
–70
–80
–90
One important factor in selecting an ADC driver is its ability to
settle within the acquisition window of the ADC. The AD8275
is able to drive medium speed SAR ADCs.
–100
–110
–120
–130
–140
–150
–160
–170
In Figure 38, the 2.7 nF capacitor serves to store and deliver
necessary charge to the switched capacitor input of the ADC.
The 33 Ω series resistor reduces the burden of the 2.7 nF load
from the amplifier and isolates it from the kickback current
injected from the switched capacitor input of the AD7685. The
output impedance of the amplifier can affect the THD of the
ADC. In this case, the combined impedance of the 33 Ω resistor
and the output impedance of the AD8275 provides extremely
low THD of −112 dB. Figure 39 shows the ac response of the
AD8275 driving the AD7685.
0
1
4
7
10
2
5
8
3
6
9
FREQUENCY (kHz)
Figure 39. FFT of AD8275 Directly Driving the AD7685 Using the 5 V
Reference of the Evaluation Board (Input = 20 V p-p, 1 kHz, THD = −112 dB)
The AD8275 can condition signals for higher resolution ADCs
such as 18-bit SAR converters, provided that a narrower
bandwidth is sampled to limit noise.
+5V
DIFFERENTIAL OUTPUTS
0.1µF
0.1µF
In certain applications, it is necessary to create a differential signal.
For example, high resolution ADCs often require a differential
input. In other cases, transmission over a long distance can require
differential signals for better immunity to interference.
7
+V
S
50kΩ
–IN
10kΩ
2
5
SENSE
33Ω
VDD
AD7685
IN+
Figure 40 shows how to configure the AD8275 to output a
differential signal. The AD8655 op amp is used in an inverting
topology to create a differential voltage. VREF sets the output
midpoint. Errors from the op amp are common to both outputs
and are thus common mode. Likewise, errors from using
mismatched resistors cause a common-mode dc offset error.
Such errors are rejected in differential signal processing by
differential input ADCs or by instrumentation amplifiers.
OUT
6
2.7nF
IN–
REF
GND
50kΩ
+IN
20kΩ
20kΩ
3
8
1
VIN
REF2
REF1
VREF
(ADR444,
ADR445)
10µF
AD8275 –V
S
4
Figure 38. Driving a Single-Ended ADC
When using this circuit to drive a differential ADC, VREF can be
set using a resistor divider from the ADC reference to make the
output ratiometric with the ADC.
+5V
0.1µF
7
+V
S
50kΩ
–IN
10kΩ
2
5
6
SENSE
OUT
+3.5V
+V
OUT
+2.5V
+1.5V
AD8655
2kΩ
2kΩ
V
= 2.5V
REF
+10V
–10V
50kΩ
+IN
20kΩ
20kΩ
3
8
1
REF2
REF1
8.2µF
+5V
0.1µF
AD8275
+3.5V
+2.5V
+1.5V
–V
4
S
–V
OUT
Figure 40. AD8275 Configured for Differential Output (for Driving a Differential ADC)
Rev. A | Page 14 of 16
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