ADS5481 Datasheet, PDF (33/54 Pages) Texas Instruments – 16-Bit, 80/105/135-MSPS Analog-to-Digital Converters

ADS5517
www.ti.com
SLWS203 – DECEMBER 2007
Using RF transformers
For optimum performance, the analog inputs have to be driven differentially. This improves the common-mode
noise immunity and even order harmonic rejection. 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 13).
At higher 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. An additional
termination resistor pair (Figure 34) 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. (Note that the
drive circuit has to be tuned to account for this additional termination, to get the desired S11 and impedance
match).
Using Differential Amplifier Drive Circuits
Figure 37 shows a drive circuit using a differential amplifier (TI's THS4509) to convert a single-ended input to
differential output that can be interface to the ADC analog input pins. In addition to the single-ended to differential
conversion, the amplifier also provides gain (10 dB in Figure 37). R_FIL helps to isolate the amplifier outputs from
the switching input of the ADC. Together with C_FIL, it forms a low-pass filter that band-limits the noise (and signal)
at the ADC input. As the amplifier output is ac-coupled, the common-mode voltage of the ADC input pins is set
using two 200 Ω resistors connected to VCM.
The amplifier output can also be dc-coupled. Using the output common-mode control of the THS4509, the ADC
input pins can be biased to 1.5 V. In this case, use +4 V and -1 V supplies for the THS4509 so that its output
common-mode voltage (1.5 V) is at mid-supply.
R_F
+V_S
R_S
R_T
0.1 mF _RG
500 W
0.1 mF 10 mF
0.1 mF
R_G
CM THS4509
R_S || R_T
0.1 mF
500 W
–V_S
0.1 mF 10 mF
R_FIL
0.1 mF
5W
INP
C_FIL
200 W
R_FIL
C_FIL
0.1 mF
200 W
5W
INM
VCM ADS5517
0.1 mF
R_F
Figure 37. Drive Circuit Using the THS4509
See the EVM User Guide (SLWU028) for more information.
Copyright © 2007, Texas Instruments Incorporated
Product Folder Link(s): ADS5517
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