Datasheet

INP
INM
VCM
1:1 1:1
5 W
50 W
50 W
50 W
50 W
5 W
0.1 Fm
0.1 Fm
S0164-04
ADS6xxx
INPUT COMMON MODE
155 mAxFs
125 MSPS
(1)
REFERENCE
ADS6425
www.ti.com ......................................................................................................................................................... SLWS197B – MARCH 2007 – REVISED JUNE 2009
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 32 shows an
example using two transformers (Coilcraft WBC1-1). An additional termination resistor pair (enclosed within the
shaded box in Figure 32 ) 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.
Figure 32. Two Transformer Drive Circuit
To ensure a low-noise common-mode reference, the VCM pin is filtered with a 0.1- µ F 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 155 µ A at 125 MSPS (per input pin). 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.
The ADS6425 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 on-chip integration of the
requisite reference capacitors 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 programming the register bit < REF> (Table 11 ).
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