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Using RF Transformer-Based Drive Circuits

Input Common-Mode

190MSPS

(310 A

m ) xFs

(1)

Reference

ADS5546

SLWS183C – NOVEMBER 2005 – REVISED OCTOBER 2006

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 45 and Figure 46.

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 45) 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).

To ensure a low-noise common-mode reference, the VCM pin is filtered with a 0.1-

µF low-inductance capacitor

connected to ground. The VCM pin is designed to directly drive the ADC inputs. The input stage of the ADC

sinks a common-mode current in the order of 310

µA (at 190 MSPS). Equation 1 describes the dependency of

the common-mode current and the sampling frequency.

This equation helps to design the output capability and impedance of the CM driving circuit accordingly.

ADS5546 has built-in internal references REFP and REFM, requiring no external components. Design schemes

are used to linearize the converter load seen by the references; this and the integration of the requisite

reference capacitors on-chip eliminates the need for external decoupling. The full-scale input range of the

converter can be controlled in the external reference mode as explained below. The internal or external

reference modes can be selected by controlling the MODE pin 23 (see Table 7 for details) or by programming

the serial interface register bit <REF>.

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