AD8132ARZ-R7 datasheet(27/33 Pages) AD

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AD8132

Rev. I | Page 26 of 32

APPLICATIONS INFORMATION

ANALOG-TO-DIGITAL DRIVER

Many of the newer high speed ADCs are single supply and have

differential inputs. Thus, the driver for these devices is able to

convert from a single-ended signal to a differential signal and

provide output common-mode level shifting in addition to

having low distortion and noise. The AD8132 conveniently

performs these functions when driving the AD9203, a 10-bit,

40 MSPS ADC.

In Figure 73, a 1 V p-p signal drives the input of an AD8132

configured for unity gain. Both the AD8132 and the AD9203 are

powered from a single 3 V supply. A voltage divider biases VOCM

at midsupply and in turn drives VOUT, cm to half of the supply

voltage. This is within the common-mode range of the AD9203.

Between the ADC and the driver is a 1-pole, differential filter that

helps to filter some of the noise and assists the switched-capacitor

inputs of the ADC. Each of the ADC inputs is driven by a 0.5 V p-p

signal that ranges from 1.25 V dc to 1.75 V dc. Figure 72 is an

FFT plot of the performance of the circuit when running at a

clock rate of 40 MSPS and an input frequency of 2.5 MHz.

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

2.5

5.0

7.5

10.0

12.5

15.0

17.5

20.0

2ND

3RD

4TH

5TH

7TH

8TH

9TH

6TH

fS = 40MHz

fIN = 2.5MHz

INPUT FREQUENCY (MHz)

FUND

Figure 72. FTT Response for AD8132 Driving AD9203

BALANCED CABLE DRIVER

When driving a twisted pair cable, it is desirable to drive only

a pure differential signal onto the line. If the signal is purely

differential (that is, fully balanced), and the transmission line is

twisted and balanced, there is minimum radiation of any signal.

The complementary electrical fields are confined mostly to

the space between the two twisted conductors and does not

significantly radiate out from the cable. The current in the cable

creates magnetic fields that radiate to some degree. However, the

amount of radiation is mitigated by the twists, because for

each twist, the two adjacent twists have an opposite polarity

magnetic field. If the twist pitch is tight enough, these small

magnetic field loops contain most of the magnetic flux, and

the magnetic farfield strength is negligible.

0.1µF

10µF

348Ω

0.1µF

348Ω

49.9Ω

348Ω

24.9Ω

10kΩ

1V p-p

348Ω

60.4Ω

20pF

AINN

AINP

AVDD

DRVDD

AVSS

DRVSS

AD9203

DIGITAL

OUTPUTS

0.1µF

AD8132

Figure 73. AD8132 Driving AD9203, a 10-Bit, 40 MSPS ADC

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