ADS5546 datasheet(30/50 Pages) TI

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-16

-10

-8

-6

-2

f − Frequency − MHz

500

700

1000

900

100

-4

-12

-14

300

200

400

600

800

150

250

300

400

450

f − Frequency − MHz

500

700

1000

900

100

500

350

100

300

200

400

600

800

200

Drive Circuit Requirements

Example Drive Circuits

INP

INM

VCM

ADT1-1T

1:1

S11,Z

ADS5546

25 W

5 W

0.1 F

5 W

ADS5546

SLWS183C – NOVEMBER 2005 – REVISED OCTOBER 2006

APPLICATION INFORMATION (continued)

Transfer Function - ADC Only

ADC Input Impedance, ZI

Figure 41. Analog Input Bandwidth (Data From Actual

Figure 42. Impedance Looking Into INP, INM (Data From

Silicon)

Simulation)

A 5-

Ω resistor in series with each input pin is recommended to damp out ringing caused by the package

parasitics. It is also necessary to present a low impedance (< 50

Ω) for the common-mode switching currents.

For example, this is achieved by using two resistors from each input terminated to the common-mode voltage

(VCM).

In addition to the above ADC requirements, the drive circuit may have to be designed to provide a low insertion

loss over the desired frequency range and matched impedance to the source. For this, the ADC input

impedance has to be considered, see Figure 42.

A configuration suitable for low input frequency ranges (< 100 MHz) is shown in Figure 43. Note the 5-

Ω series

resistors and the low common-mode impedance (using 25-

Ω resistors terminated to VCM). In addition, the circuit

has low insertion loss, and good impedance match at low input frequencies, see Figure 44.

Figure 43. Configuration for Low Input Frequencies

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