Datasheet

f − Frequency − MHz
0
1
2
3
4
5
6
7
8
9
0 100 200 300 400 500 600
C − Capacitance − pF
G082
S0163-03
INP
INM
VCM
1:1
ADS62P4x
25 W
25 W
0.1 Fm
0.1 Fm
ADS62P45, ADS62P44
ADS62P43, ADS62P42
www.ti.com SLAS561C –JULY 2007– REVISED FEBRUARY 2012
Figure 85. ADC Analog Input Capacitance (Cin) Across Frequency
Using RF-Transformer Based Drive Circuits
Figure 86 shows a configuration using a single 1:1 turns ratio transformer (for example, Coilcraft WBC1-1) that
can be used for low input frequencies (about 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). 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 currents.
Figure 86. Drive Circuit at Low Input Frequencies
At high input frequencies, the mismatch in the transformer parasitic capacitance (between the windings) results
in degraded even-order harmonic performance. Connecting two identical RF transformers back-to-back helps
minimize this mismatch, and good performance is obtained for high frequency input signals. Figure 87 shows an
example using two transformers (Coilcraft WBC1-1). An additional termination resistor pair (enclosed within the
shaded box) may be required between the two transformers to improve the balance between the P and M sides.
The center point of this termination must be connected to ground.
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