Datasheet

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f − Frequency − MHz

0 100 200 300 400 500 600

C − Capacitance − pF

G084

Using RF-Transformer Based Drive Circuits

INP

INM

VCM

1 :1

5 W

0.1

25 W

TF_ADC

25 W

5 W

ADS6125, ADS6124

ADS6123, ADS6122

SLAS560A – OCTOBER 2007 – REVISED MARCH 2008

Figure 92. ADC Input Capacitance, Cin

Figure 93 shows a configuration using a single 1:1 turns ratio transformer (for example, Coilcraft WBC1-1) that

can be used for low input frequencies (about 100 MHz).

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 93. Single Transformer Drive Circuit

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. Figure 94 shows an

example using two transformers (Coilcraft WBC1-1). An additional termination resistor pair (enclosed within the

shaded box in Figure 94 ) 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.

Product Folder Link(s): ADS6125, ADS6124 ADS6123, ADS6122