Datasheet

M0087-01
S(1,1)
Frequency(100kHzto500MHz)
F2
F1
F1
Freq=50MHz
S(1,1)=0.967/ –13.241
Impedance=62.211 – j421.739
F2
Freq=400MHz
S(1,1)=0.273/ –59.329
Impedance=58.132 – j29.510
0
100
200
300
400
500
600
700
800
900
1000
0 50 100 150 200 250 300 350 400 450 500
Magnitude of Zin -- W
f -- Input Frequency -- MHz
I
F1
F1
Frequency = 50 MHz
Mag(Zin1) = 426.302
F2
F2
Frequency = 400 MHz
Mag(Zin1) = 65.193
ADS6445, ADS6444
ADS6443, ADS6442
SLAS531B –MAY 2007–REVISED DECEMBER 2009 www.ti.com
Figure 84. ADC Input Impedance, Zin
Using RF-Transformers Based Drive Circuits
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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