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LT1813 Datasheet(PDF) 11 Page - Linear Technology

Part # LT1813
Description  1.3GHz Low Noise, Low Distortion Differential ADC Driver for 140MHz IF
PDF  16 Pages
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LT1813 Datasheet(HTML) 11 Page - Linear Technology

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LTC6401-20
11
640120f
two outputs have the same gain and thus symmetrical
swing. In general, the single-ended input impedance and
termination resistor RT are determined by the combination
of RS, RG and RF. For example, when RS is 50Ω, it is found
that the single-ended input impedance is 200Ω and RT is
66.5Ω in order to match to a 50Ω source impedance.
The LTC6401-20 is unconditionally stable. However, the
overall differential gain is affected by both source imped-
ance and load impedance as shown in Figure 4:
A
V
VR
R
R
V
OUT
IN
S
L
L
==
++
2000
200 25
•
The noise performance of the LTC6401-20 also depends
upon the source impedance and termination. For example,
an input 1:4 balun transformer in Figure 2 improves SNR
by adding 6dB of gain at the inputs. A trade-off between
gain and noise is obvious when constant noise figure
circle and constant gain circle are plotted within the same
Figure 4. Calculate Differential Gain
input Smith Chart, based on which users can choose the
optimal source impedance for a given gain and noise
requirement.
Output Match and Filter
The LTC6401-20 can drive an ADC directly without
external output impedance matching. Alternatively, the
differential output impedance of 25Ω can be matched to
higher value impedance, e.g. 50Ω, by series resistors or
an LC network.
The internal low pass filter outputs at +OUTF/–OUTF
have a –3dB bandwidth of 590MHz. External capacitor
can reduce the low pass filter bandwidth as shown in
Figure 5. A bandpass filter is easily implemented with
only a few components as shown in Figure 6. Three
39pF capacitors and a 16nH inductor create a bandpass
filter with 165MHz center frequency, –3dB frequencies at
138MHz and 200MHz.
APPLICATIONS INFORMATION
Figure 3. Input Termination for Single-Ended 50Ω Input
Impedance
Figure 5. LTC6401-20 Internal Filter Topology Modified for Low
Filter Bandwidth (Three External Capacitors)
640120 F03
IN+
OUT–
IN–
OUT+
100
Ω
RT
66.5
Ω
0.1
μF
0.1
μF
12.5
Ω
1000
Ω
LTC6401-20
100
Ω
RS
50
Ω
RS
50
Ω
VIN
1000
Ω
12.5
Ω
50
Ω
50
Ω
13
14
15
16
7
5
6
8
+
–
0.1
μF
RT
66.5
Ω
+OUT
+OUTF
–OUTF
–OUT
+IN
+IN
–IN
–IN
1.7pF
640120 F04
IN+
OUT–
IN–
OUT+
100
Ω
12.5
Ω
1000
Ω
LTC6401-20
100
Ω
1/2 RS
1/2 RS
VIN
VOUT
1000
Ω
12.5
Ω
50
Ω
50
Ω
13
14
15
16
7
5
6
8
+
–
1/2 RL
1/2 RL
+OUT
+OUTF
–OUTF
–OUT
+IN
+IN
–IN
–IN
1.7pF
640120 F05
IN+
OUT–
IN–
OUT+
100
Ω
12.5
Ω
1000
Ω
LTC6401-20
100
Ω
1000
Ω
12.5
Ω
50
Ω
50
Ω
13
14
15
16
7
5
6
8
8.2pF
8.2pF
12pF
FILTERED OUTPUT
(87.5MHz)
+OUT
+OUTF
–OUTF
–OUT
+IN
+IN
–IN
–IN
1.7pF
Figure 6. LTC6401-20 Application Circuit for Bandpass
Filtering (Three External Capacitors, One External Inductor)
LTC2208
10
Ω
10
Ω
4.99
Ω
4.99
Ω
39pF
16nH
39pF
39pF
640120 F06
IN+
OUT–
IN–
OUT+
100
Ω
12.5
Ω
1000
Ω
LTC6401-20
100
Ω
1000
Ω
12.5
Ω
50
Ω
50
Ω
13
14
15
16
7
5
6
8
+OUT
+OUTF
–OUTF
–OUT
+IN
+IN
–IN
–IN
1.7pF



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