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Using RF Transformer-Based Drive Circuits

Using Differential Amplifier Drive Circuits

FIL

0.1 F

10 F

||R

INP

INM

ADS5545

THS4509

VCM

500 W

200 W

5 W

500 W

0.1 F

–V

ADS5545

SLWS180C – NOVEMBER 2005 – REVISED MAY 2007

APPLICATION INFORMATION (continued)

For optimum performance, the analog inputs must be driven differentially. This improves the common-mode

noise immunity and even order harmonic rejection. Some examples of input configurations using RF

transformers suitable for low and high input frequencies are shown in Figure 46 and Figure 47.

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). 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.

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. An additional

termination resistor pair (enclosed within the shaded box in Figure 46) 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 48 shows a drive circuit using a differential amplifier (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 48). 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 (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 the output

common-mode voltage (1.5V) is at the supply mid-point.

Figure 48. Drive Circuit Using the THS4509

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

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