Rainbow-electronics MAX1418 Manual do Utilizador descarregar pdf (Página 15)

MAX1418

15-Bit, 65Msps ADC with -78.2dBFS

Noise Floor for IF Applications

______________________________________________________________________________________ 15

turns ratio, the larger the effect of the differential input

resistance of the MAX1418 on the primary referred

input resistance. At a turns ratio of 1:4.47, the 1kΩ dif-

ferential input resistance of the MAX1418 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 MAX1418

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 MAX1418 may be

observed. The core must be demagnetized to return to

balanced operation.

MAX1418

56Ω

0.1µF

0.1µF 0.1µF

T2-1T-KK81

D0–D14

DRV

GND

CLKP CLKN

INP

INN

SINGLE-ENDED

INPUT TERMINAL

Figure 6. Transformer-Coupled Analog Input Configuration

MAX1418

56Ω

0.1µF

0.1µF 0.1µF

T2-1T-KK81

D0–D14

DRV

GND

CLKP CLKN

INP

INN

POSITIVE

TERMINAL

Figure 7. Transformer-Coupled Analog Input Configuration with Primary-Side Transformer

1 2 ... 10 11 12 13 14 15 16 17 18 19 20

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