ADS5547 datasheet(32/54 Pages) TI

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Example Drive Circuits

WBC1-1TLB

1:1

0.1 F

INP

INM

VCM

25 W

100 W

25 W

3.3pF

0.1 F

(Note A)

WBC1-1TLB

100 W

33 W

Z andTFADC

-6

-5

-1

f − Frequency − MHz

1000

-4

100

200

500

700

-2

-3

400

300

600

800

900

250

350

f − Frequency − MHz

1000

500

100

200

500

700

200

150

300

400

300

600

800

900

400

450

ADS5517

SLWS203 – DECEMBER 2007

Using 10-

Ω series resistance and 25 Ω-3.3 pF-25 Ω as the R-C-R filter, high effective bandwidth (700 MHz) can

be achieved, (see Figure 35, transfer function from the analog input pins to the voltage across the sampling

capacitors).

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 taken into account (Figure 36).

A suitable configuration using RF transformers and including the R-C-R filter is shown in Figure 34. Note the

15-

Ω series resistors and the low common-mode impedance (using 33-Ω resistors terminated to VCM).

Use lower series resistance (

≈ 5 Ω to 10 Ω) at high input frequencies (> 100 MHz)

Figure 34. Example Drive Circuit With RF Transformers

Figure 35. Analog Input Bandwidth, TFADC (Actual

Figure 36. Input Impedance, ZI

Silicon Data)

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