AD8270 datasheet(18/20 Pages) AD

18 / 20 page

AD8270

Rev. 0 | Page 18 of 20

DRIVING AN ADC

The AD270 high slew rate and drive capability, combined with

its dc accuracy, make it a good ADC driver. The AD8270 can

drive both single-ended and differential input ADCs. Many

converters require the output to be buffered with a small value

resistor combined with a high quality ceramic capacitor. See the

converter data sheet for more details. Figure 51 shows the AD8270

in differential configuration, driving the AD7688 ADC. The

AD8270 divides down the 5 V reference voltage from the ADR435,

so that the common-mode output voltage is 2.5 V, which is

precisely where the AD7688 needs it.

DRIVING CABLING

All cables have a certain capacitance per unit length, which varies

widely with cable type. The capacitive load from the cable may

cause peaking or instability in output response, especially when the

AD8270 is operating in a gain of 0.5.

To reduce the peaking, use a resistor between the AD8270 and the

cable. Because cable capacitance and desired output response vary

widely, this resistor is best determined empirically. A good starting

point is 20 Ω.

AD8270

(DIFF OUT)

AD8270

(SINGLE OUT)

Figure 50. Driving Cabling

10kΩ

20kΩ

10kΩ

0.1µF

5V_REF

AD8270

–IN

+IN

–IN

+IN

+12V –12V

33Ω

2.7nF

COG

33Ω

2.7nF

COG

4 –IN

+IN

REF

AD7688

0.1µF

ADR435

GND

VIN

VOUT

+12V

10µF

5V_REF

NOTE:

POWER SUPPLY DECOUPLING

NOT SHOWN.

Figure 51. Driving an ADC

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