Datasheet
10W
10W
0.1 Fm
100W
100W
INP
INM
VCM
ADS41xx
Differential
InputSignal
Bandpassor
Low-Pass
Filter
ADS4126
,
ADS4129
ADS4146
,
ADS4149
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SBAS483G – NOVEMBER 2009–REVISED JANUARY 2011
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 may be
required between the two transformers, as shown in Figure 116 and Figure 117. The center point of this
termination is connected to ground to improve the balance between the P (positive) and M (negative) sides. The
values of the terminations between the transformers and on the secondary side must be chosen to obtain an
effective 50Ω (for a 50Ω source impedance).
Figure 116 and Figure 117 use 1:1 transformers with a 50Ω source. As explained in the Drive Circuit
Requirements section, this architecture helps to present a low source impedance to absorb sampling glitches.
With a 1:4 transformer, the source impedance is 200Ω. The higher source impedance is unable to absorb the
sampling glitches effectively and can lead to degradation in performance (compared to using 1:1 transformers).
In almost all cases, either a bandpass or low-pass filter is needed to get the desired dynamic performance, as
shown in Figure 118. Such a filter presents low source impedance at the high frequencies corresponding to the
sampling glitch and helps avoid the performance loss with the high source impedance.
Figure 118. Drive Circuit with 1:4 Transformer
Input Common-Mode
To ensure a low-noise, common-mode reference, the VCM pin is filtered with a 0.1µF low-inductance capacitor
connected to ground. The VCM pin is designed to directly drive the ADC inputs. Each ADC input pin sinks a
common-mode current of approximately 0.6µA per MSPS of clock frequency.
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