AD8275

Rev. A | Page 14 of 16

APPLICATIONS INFORMATION

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.

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.

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.

VREF

(ADR444,

ADR445)

AD8275

7

4

5

6

8

2

50k

Ω

0.1µF

50k

Ω

20k

Ω

20k

Ω

33

Ω

10k

Ω

3

+IN

–IN

VIN

REF2

REF1

+5V

OUT

SENSE

0.1µF

2.7nF

10µF

1

AD7685

VDD

GND

REF

IN+

IN–

Figure 38. Driving a Single-Ended ADC

10

0

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

–130

–140

–150

–160

–170

0

1

4

7

10

FREQUENCY (kHz)

2

5

8

3

6

9

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.

DIFFERENTIAL OUTPUTS

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.

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.

set using a resistor divider from the ADC reference to make the

output ratiometric with the ADC.

AD8275

7

4

5

6

8

2

50k

Ω

0.1µF

0.1µF

8.2µF

50k

Ω

20k

Ω

20k

Ω

2k

Ω

2k

Ω

10k

Ω

3

+IN

–IN

REF2

REF1

+5V

+10V

–10V

+5V

OUT

SENSE

1

AD8655

+3.5V

+1.5V

+2.5V

+3.5V

+1.5V

+2.5V

Figure 40. AD8275 Configured for Differential Output (for Driving a Differential ADC)