ADS5281IPFPR datasheet(32/41 Pages) TI

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Driving Circuit

4.7

0.1 F

1:1

0.1 F

200

0.1 F

1:2

2:1

ADS5281

ADS5282

SBAS397E – DECEMBER 2006 – REVISED JANUARY 2008

At high input frequencies, the mismatch in the

transformer

parasitic

capacitance

(between

the

For optimum performance, the analog inputs must be

windings) results in degraded even-order harmonic

driven differentially. This approach improves the

performance.

Connecting

two

identical

common-mode

noise

immunity

and

even-order

transformers back-to-back helps to minimize this

harmonic rejection. Input configurations using RF

mismatch, and good performance is obtained for

transformers

suitable

for

low

and

high

input

high-frequency

input

signals.

additional

frequencies are shown in Figure 36 and Figure 37,

termination resistor pair is required between the two

respectively. The single-ended signal is fed to the

transformers, as shown in Figure 37. The center point

primary

winding

the

transformer.

The

of this termination is connected to ground to improve

transformer is terminated by 50

Ω resistor on the

the balance between the positive and negative sides.

secondary side. Placing the termination on the

The

values

the

terminations

between

the

secondary side helps to shield the kicks caused by

transformers and on the secondary side must be

the

input

sampling

capacitors

from

the

chosen to achieve an overall 50

Ω (in the case of 50Ω

transformer leakage inductances. The termination is

source impedance).

accomplished by two 25

Ω resistors, connected in

series, with the center point connected to the 1.5V

common-mode. The 4.7

Ω resistor in series with each

input pin is required to damp the ringing caused by

the device package parasitics.

Figure 36. Drive Circuit at Low Input Frequencies

Figure 37. Drive Circuit at High Input Frequencies

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