ADS6425_14 Datasheet, PDF (32/60 Pages) Texas Instruments – QUAD CHANNEL, 12-BIT, 125-MSPS ADC WITH SERIAL LVDS INTERFACE

ADS6425
SLWS197B – MARCH 2007 – REVISED JUNE 2009 ......................................................................................................................................................... www.ti.com
1
0
−1
−2
−3
−4
−5
−6
0
100 200 300 400 500 600 700
f_IN − Input Frequency − MHz
G022
Figure 30. Analog Input Bandwidth (represented by magnitude of TF_ADC, see Figure 31)
Drive Circuit Requirements
For optimum performance, the analog inputs must be driven differentially. This improves the common-mode
noise immunity and even order harmonic rejection.
A 5-Ω resistor in series with each input pin is recommended to damp out ringing caused by the package
parasitics. It is also necessary to present low impedance (< 50 Ω) for the common mode switching currents. For
example, this is achieved by using two resistors from each input terminated to the common mode voltage (VCM).
Using RF-Transformer Based Drive Circuits
Figure 31 shows a configuration using a single 1:1 turns ratio transformer (for example, WBC1-1) that can be
used for low input frequencies up to 100MHz.
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.
0.1 mF
TF_ADC
5W
0.1 mF
25 W
25 W
1:1
5W
ADS6xxx
INP
INM
VCM
S0256-01
Figure 31. Single Transformer Drive Circuit
At high input frequencies, the mismatch in the transformer parasitic capacitance (between the windings) results
32
Submit Documentation Feedback
Product Folder Link(s): ADS6425
Copyright © 2007–2009, Texas Instruments Incorporated