ADS6445 Datasheet, PDF (48/75 Pages) STMicroelectronics – QUAD CHANNEL, 14-BIT, 125/105/80/65 MSPS ADC WITH SERIAL LVDS OUTPUTS

ADS6445, ADS6444
ADS6443, ADS6442
SLAS531 – MAY 2007
APPLICATION INFORMATION (continued)
www.ti.com
F1
Freq = 50 MHz
S(1, 1) = 0.967 / –13.241
Impedance = 62.211 – j421.739
1000
900
800
700
600
500
400
300
200
100
0
0
F1
Frequency = 50 MHz
Mag(Zin1) = 426.302
F2
Frequency = 400 MHz
F1
Mag(Zin1) = 65.193
F2
50 100 150 200 250 300 350 400 450 500
f_I -- Input Frequency -- MHz
F2
F1
Frequency (100 kHz to 500 MHz)
F2
Freq = 400 MHz
S(1, 1) = 0.273 / –59.329
Impedance = 58.132 – j29.510
M0087-01
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.
48
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