lt1993-2 Linear Technology Corporation, lt1993-2 Datasheet - Page 16

• Manufacturer Part Number: lt1993-2
• Description: 800mhz Low Distortion, Low Noise Differential Ampli? Er/ Adc Driver Av = 2v/v
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LT1993-2
that look like a series 25Ω resistor with a 36pF capacitor
shunting each fi ltered output to AC ground, giving a –3dB
bandwidth of 175MHz.
The fi lter cutoff frequency is easily modifi ed 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 –OUTFIL-
TERED (Figure 7). These resistors are in parallel with the
internal 25Ω resistor, lowering the overall resistance and
increasing fi lter bandwidth. To double the fi lter bandwidth,
for example, add two external 25Ω resistors to lower
the series resistance to 12.5Ω. The 36pF of capacitance
remains unchanged, so fi lter bandwidth doubles.
To decrease fi lter 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
16
Figure 6. LT1993-2 Internal Filter Topology –3dB BW ≈175MHz
Figure 7. LT1993-2 Internal Filter Topology Modifi ed for
2x Filter Bandwidth (2 External Resistors)
LT1993-2
LT1993-2
25Ω
25Ω
25Ω
25Ω
12pF
12pF
12pF
12pF
U
V
V
V
V
EE
EE
EE
EE
12pF
12pF
U
8
7
6
5
8
7
6
5
–OUT
–OUTFILTERED
+OUTFILTERED
+OUT
–OUT
+OUT
–OUTFILTERED
+OUTFILTERED
25Ω
25Ω
W
19932 F06
19932 F07
FILTERED OUTPUT
(175MHz)
FILTERED OUTPUT
(350MHz)
U
it will appear at each fi ltered output as a single-ended
capacitance of twice the value. To halve the fi lter band-
width, for example, two 36pF capacitors could be added
(one from each fi ltered output to ground). Alternatively
one 18pF capacitor could be added between the fi ltered
outputs, again halving the fi lter bandwidth. Combinations
of capacitors could be used as well; a three capacitor
solution of 12pF from each fi ltered output to ground plus
a 12pF capacitor between the fi ltered outputs would also
halve the fi lter bandwidth (Figure 8).
Bandpass fi ltering 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 fi lter with a 71MHz
center frequency, –3dB points of 55MHz and 87MHz, and
1.6dB of insertion loss (Figure 9).
Figure 9. LT1993-2 Output Filter Topology Modifi ed for Bandpass
Filtering (1 External Inductor, 1 External Capacitor)
LT1993-2
LT1993-2
Figure 8. LT1993-2 Internal Filter Topology Modifi ed for
1/2x Filter Bandwidth (3 External Capacitors)
25Ω
25Ω
25Ω
25Ω
12pF
12pF
12pF
12pF
V
V
EE
EE
V
V
12pF
EE
EE
12pF
8
7
6
5
8
7
6
5
–OUT
+OUT
–OUTFILTERED
+OUTFILTERED
–OUT
–OUTFILTERED
+OUTFILTERED
+OUT
39nH
19932 F09
19932 F08
120pF
12pF
12pF
12pF
FILTERED OUTPUT
(71MHz BANDPASS,
–3dB @ 55MHz/87MHz)
FILTERED OUTPUT
(87.5MHz)
19932fa