LT5514 datasheet(16/22 Pages) AD

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LT1993-2

Rev B

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that look like a series 25 resistor with a 36pF capacitor

shunting each filtered output to AC ground, giving a –3dB

bandwidth of 175MHz.

it will appear at each filtered output as a single-ended

capacitance of twice the value. To halve the filter band-

width, for example, two 36pF capacitors could be added

(one from each filtered output to ground). Alternatively

one 18pF capacitor could be added between the filtered

outputs, again halving the filter bandwidth. Combinations

of capacitors could be used as well; a three capacitor

solution of 12pF from each filtered output to ground plus

a 12pF capacitor between the filtered outputs would also

halve the filter bandwidth (Figure8).

The filter cutoff frequency is easily modified with just a few

external components. To increase the cutoff frequency,

simply add 2 equal value resistors, one between +OUT

and +OUTFILTERED and the other between –OUT and –

OUTFILTERED (Figure 7). These resistors are in parallel

with the internal 25 resistor, lowering the overall resis-

tance and increasing filter bandwidth. To double the filter

bandwidth, for example, add two external 25 resistors to

lower the series resistance to 12.5 . The 36pF of capac-

itance remains unchanged, so filter bandwidth doubles.

Figure6. LT1993-2 Internal Filter Topology –3dB BW ≈175MHz

Figure7. LT1993-2 Internal Filter Topology Modified for

2x Filter Bandwidth (2 External Resistors)

Bandpass filtering is also easily implemented with just a

few external components. An additional 120pF and 39nH,

each added differentially between +OUTFILTERED and –

OUTFILTERED creates a bandpass filter with a 71MHz

center frequency, –3dB points of 55MHz and 87MHz, and

1.6dB of insertion loss (Figure9).

Figure8. LT1993-2 Internal Filter Topology Modified for

1/2x Filter Bandwidth (3 External Capacitors)

19932 F06

Ω

VEE

12pF

LT1993-2

–OUT

+OUT

Ω

12pF

–OUTFILTERED

+OUTFILTERED

FILTERED OUTPUT

(175MHz)

VEE

19932 F07

Ω

12pF

LT1993-2

–OUT

+OUT

Ω

12pF

–OUTFILTERED

FILTERED OUTPUT

(350MHz)

+OUTFILTERED

To decrease filter bandwidth, add two external capacitors,

one from +OUTFILTERED to ground, and the other from

–OUTFILTERED to ground. A single differential capacitor

connected between +OUTFILTERED and –OUTFILTERED

can also be used, but since it is being driven differentially

VEE

19932 F08

Ω

12pF

LT1993-2

–OUT

+OUT

Ω

12pF

–OUTFILTERED

+OUTFILTERED

FILTERED OUTPUT

(87.5MHz)

Figure9. LT1993-2 Output Filter Topology Modified for Bandpass

Filtering (1 External Inductor, 1 External Capacitor)

VEE

19932 F09

Ω

12pF

LT1993-2

–OUT

+OUT

Ω

12pF

120pF

39nH

–OUTFILTERED

+OUTFILTERED

FILTERED OUTPUT

(71MHz BANDPASS,

–3dB @ 55MHz/87MHz)

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