Datasheet

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M0087-01

S(1,1)

Frequency(100kHzto500MHz)

Freq=50MHz

S(1,1)=0.967/ –13.241

Impedance=62.211 – j421.739

Freq=400MHz

S(1,1)=0.273/ –59.329

Impedance=58.132 – j29.510

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0 50 100 150 200 250 300 350 400 450 500

Magnitude of Zin -- W

f -- Input Frequency -- MHz

Frequency = 50 MHz

Mag(Zin1) = 426.302

Frequency = 400 MHz

Mag(Zin1) = 65.193

Using RF-Transformer Based Drive Circuits

ADS6424

ADS6423

ADS6422

SLAS532A – MAY 2007 – REVISED JUNE 2007

APPLICATION INFORMATION (continued)

Figure 66. ADC Input Impedance, Zin

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

noise immunity and even order harmonic rejection. An example of input drive using RF transformers is shown in

Figure 67 and Figure 68 .

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.

Figure 67 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 100 MHz.

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