Datasheet

0.1 Fm
50W
50W
200W
200W
IN
P
V
CM
IN
N
0.1 Fm
1:2 2:1
4.7W
4.7W
0.1 Fm
0.1 Fm
25W
25W
1:1
IN
P
V
CM
IN
N
ADS5281
ADS5282
SBAS397I –DECEMBER 2006–REVISED JUNE 2012 www.ti.com
Driving Circuit
At high input frequencies, the mismatch in the
transformer parasitic capacitance (between the
For optimum performance, the analog inputs must be
windings) results in degraded even-order harmonic
driven differentially. This approach improves the
performance. Connecting two identical RF
common-mode noise immunity and even-order
transformers back-to-back helps to minimize this
harmonic rejection. Input configurations using RF
mismatch, and good performance is obtained for
transformers suitable for low and high input
high-frequency input signals. An additional
frequencies are shown in Figure 36 and Figure 37,
termination resistor pair is required between the two
respectively. The single-ended signal is fed to the
transformers, as shown in Figure 37. The center point
primary winding of the RF transformer. The
of this termination is connected to ground to improve
transformer is terminated by 50Ω resistor on the
the balance between the positive and negative sides.
secondary side. Placing the termination on the
The values of the terminations between the
secondary side helps to shield the kicks caused by
transformers and on the secondary side must be
the input sampling capacitors from the RF
chosen to achieve an overall 50Ω (in the case of 50Ω
transformer leakage inductances. The termination is
source impedance).
accomplished by two 25Ω resistors, connected in
series, with the center point connected to the 1.5V
common-mode. The 4.7Ω resistor in series with each
input pin is required to damp the ringing caused by
the device package parasitics.
Figure 36. Drive Circuit at Low Input Frequencies
Figure 37. Drive Circuit at High Input Frequencies
34 Copyright © 2006–2012, Texas Instruments Incorporated