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Using RF transformers

Using Differential Amplifier Drive Circuits

FIL

0.1 F

10 F

||R

INP

INM

ADS5547

THS4509

VCM

500 W

200 W

5 W

500 W

0.1 F

–V

ADS5527

SLWS196A – DECEMBER 2006 – REVISED MAY 2007

APPLICATION INFORMATION (continued)

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 40) 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).

Figure 43 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 43). RFIL helps to isolate the amplifier

outputs from the switching input of the ADC. Together with CFIL it also forms a low-pass filter that band-limits the

noise (& 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.

Figure 43. Drive Circuit Using the THS4509

See the EVM User Guide (SLWU028) for more information.

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