ADS6445 datasheet(48/75 Pages) STMICROELECTRONICS

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M0087-01

Frequency(100kHzto500MHz)

Freq=50MHz

S(1,1)=0.967/ –13.241

Impedance=62.211 – j421.739

Freq=400MHz

S(1,1)=0.273/ –59.329

Impedance=58.132 – j29.510

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f -- Input Frequency -- MHz

Frequency = 50 MHz

Mag(Zin1) = 426.302

Frequency = 400 MHz

Mag(Zin1) = 65.193

Using RF-Transformers Based Drive Circuits

ADS6445, ADS6444

ADS6443, ADS6442

SLAS531 – MAY 2007

APPLICATION INFORMATION (continued)

Figure 84. ADC Input Impedance, Zin

Figure 85 shows a configuration using a single 1:1 turns ratio transformer (for example, Coilcraft WBC1-1) that

can be used for low input frequencies up to 100 MHz.

The single-ended signal is fed to the primary winding of the RF transformer. The transformer is terminated on

the secondary side. Putting the termination on the secondary side helps to shield the kickbacks caused by the

sampling circuit from the RF transformer’s leakage inductances. The termination is accomplished by two

resistors connected in series, with the center point connected to the 1.5 V common mode (VCM pin). The value

of the termination resistors (connected to common mode) has to be low (< 100

Ω) to provide a low-impedance

path for the ADC common-mode switching current.

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