MAX1428 Datasheet, PDF (13/18 Pages) Maxim Integrated Products – 15-Bit, 80Msps ADC with -78.4dBFS Noise Floor for IF Applications

15-Bit, 80Msps ADC with -78.4dBFS
Noise Floor for IF Applications
SINGLE-ENDED
INPUT TERMINAL ^0.1µF _T2-1T-KK81
56Ω
56Ω
0.1µF
Figure 6. Transformer-Coupled Analog Input Configuration
AV_CC DV_CC DRV_CC
INP
D0–D14
MAX1428
INN
15
0.1µF
GND
CLKP CLKN
POSITIVE
TERMINAL ^0.1µF
T2-1T-KK81
T2-1T-KK81
56Ω
56Ω
AV_CC DV_CC DRV_CC
INP
D0–D14
MAX1428
INN
15
0.1µF
0.1µF
GND
CLKP CLKN
Figure 7. Transformer-Coupled Analog Input Configuration with Primary-Side Transformer
turns ratio, the larger the effect of the differential input
resistance of the MAX1428 on the primary referred
input resistance. At a turns ratio of 1:4.47, the 1kΩ dif-
ferential input resistance of the MAX1428 by itself
results in a primary referred input resistance of 50Ω.
Although the center tap of the transformer in Figure 6 is
shown floating, it may be AC-coupled to ground.
However, experience has shown that better balance is
achieved if the center tap is left floating.
As stated previously, the signal inputs to the MAX1428
must be accurately balanced to achieve the best even-
order distortion performance. Figure 7 provides
improved balance over the circuit of Figure 6 by adding
a balun on the primary side of the transformer, and can
yield substantial improvement in even-order distortion
terms over the circuit of Figure 6.
One note of caution in relation to transformers is impor-
tant. Any DC current passed through the primary or
secondary windings of a transformer may magnetically
bias the transformer core. When this happens, the
transformer is no longer accurately balanced and a
degradation in the distortion of the MAX1428 may be
observed. The core must be demagnetized to return to
balanced operation.
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