LTC2272 datasheet(23/44 Pages) LINER

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LTC2273/LTC2272

22732f

APPLICATIONS INFORMATION

INPUT DRIVE CIRCUITS

Input Filtering

A ﬁrst order RC lowpass ﬁlter at the input of the ADC

can serve two functions: limit the noise from input cir-

cuitry and provide isolation from ADC S/H switching. The

LTC2273/LTC2272 have very broadband S/H circuits, DC

to 700MHz; it can be used in a wide range of applications;

therefore, it is not possible to provide a single recom-

mended RC ﬁlter.

Figures 3, 4a and 4b show three examples of input RC

ﬁltering at three ranges of input frequencies. In general,

it is desirable to make the capacitors as large as can be

tolerated—this will help suppress random noise as well

as noise coupled from the digital circuitry. The LTC2273/

LTC2272 do not require any input ﬁlter to achieve data sheet

speciﬁcations; however, no ﬁltering will put more stringent

noise requirements on the input drive circuitry.

Transformer Coupled Circuits

Figure 3 shows the LTC2273/LTC2272 being driven by

an RF transformer with a center-tapped secondary. The

secondary center tap is DC biased with VCM, setting the

ADC input signal at its optimum DC level. Figure 3 shows

a 1:1 turns ratio transformer. Other turns ratios can be

used; however, as the turns ratio increases so does the

impedance seen by the ADC. Source impedance greater

than 50

Ω can reduce the input bandwidth and increase

high frequency distortion. A disadvantage of using a

transformer is the loss of low frequency response. Most

small RF transformers have poor performance at frequen-

cies below 1MHz.

Center-tapped transformers provide a convenient means

of DC biasing the secondary; however, they often show

poor balance at high input frequencies, resulting in large

2nd order harmonics.

Figure 4a shows transformer coupling using a transmis-

sion line balun transformer. This type of transformer has

much better high frequency response and balance than

ﬂux coupled center tap transformers. Coupling capacitors

are added at the ground and input primary terminals to

allow the secondary terminals to be biased at 1.25V. Figure

4b shows the same circuit with components suitable for

higher input frequencies.

0.1μF

AIN

–

4.7pF

2.2μF

4.7pF

VCM

LTC2273/

LTC2272

ANALOG

INPUT

0.1μF

1:1

T1 = MA/COM ETC1-1-13

RESISTORS, CAPACITORS

ARE 0402 PACKAGE SIZE

EXCEPT 2.2μF

22732 F04a

5Ω

10Ω

25Ω

10Ω

5Ω

Figure 4a. Using a Transmission Line Balun Transformer.

Recommended for Input Frequencies from 100MHz to 250MHz

0.1μF

AIN

–

2.2μF

2.2pF

VCM

LTC2273/

LTC2272

ANALOG

INPUT

0.1μF

1:1

T1 = MA/COM ETC1-1-13

RESISTORS, CAPACITORS

ARE 0402 PACKAGE SIZE

EXCEPT 2.2μF

22732 F04b

5Ω

25Ω

5Ω

Figure 4b. Using a Transmission Line Balun Transformer.

Recommended for Input Frequencies from 250MHz to 500MHz

Figure 3. Single-Ended to Differential Conversion

Using a Transformer. Recommended for Input

Frequencies from 5MHz to 150MHz

35Ω

50Ω

35Ω

10Ω

5Ω

0.1μF

AIN

–

8.2pF

2.2μF

8.2pF

VCM

LTC2273/

LTC2272

T1 = MA/COM ETC1-1T

RESISTORS, CAPACITORS

ARE 0402 PACKAGE SIZE

EXCEPT 2.2μF

22732 F03

LTC2272 Datasheet(PDF) 23 Page - Linear Technology

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