Datasheet

LTC6400-20
12
640020f
swing. In general, the single-ended input impedance and
termination resistor R
T
are determined by the combination
of R
S
, R
G
and R
F
. For example, when R
S
is 50Ω, it is found
that the single-ended input impedance is 202Ω and R
T
is
66.5Ω in order to match to a 50Ω source impedance.
The LTC6400-20 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 LTC6400-20 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 voltage gain at the inputs. A trade-off
between gain and noise is obvious when constant noise
fi gure circle and constant gain circle are plotted within
the same input Smith Chart, based on which users can
choose the optimal source impedance for a given gain
and noise requirement.
Figure 4. Calculate Differential Gain
Output Match and Filter
The LTC6400-20 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 fi lter outputs at +OUTF/–OUTF
have a –3dB bandwidth of 590MHz. External capacitors
can reduce the low pass fi lter bandwidth as shown in
Figure 5. A bandpass fi lter is easily implemented with
only a few components as shown in Figure 6. Three
39pF capacitors and a 16nH inductor create a bandpass
fi lter with 165MHz center frequency, –3dB frequencies at
138MHz and 200MHz.
Output Common Mode Adjustment
The output common mode voltage is set by the V
OCM
pin,
which is a high impedance input. The output common mode
voltage is capable of tracking V
OCM
in a range from 1V to
APPLICATIONS INFORMATION
Figure 3. Input Termination for Single-Ended 50Ω Input
Impedance
Figure 5. LTC6400-20 Internal Filter Topology Modifi ed for Low
Filter Bandwidth (Three External Capacitors)
640020 F03
+OUT
+OUTF
–OUTF
–OUT
+IN
IN+ OUT–
IN– OUT+
+IN
–IN
–IN
100Ω
R
T
66.5Ω
0.1μF
0.1μF
12.5Ω
1000Ω
LTC6400-20
100Ω
R
S
50Ω
R
S
50Ω
V
IN
1000Ω
12.5Ω
50Ω
50Ω
1.7pF
13
14
15
16
7
5
6
8
+
–
0.1μF
R
T
66.5Ω
640020 F04
+OUT
+OUTF
–OUTF
–OUT
+IN
IN+ OUT–
IN– OUT+
+IN
–IN
–IN
100Ω 12.5Ω
1000Ω
LTC6400-20
100Ω
1/2 R
S
1/2 R
S
V
IN
V
OUT
1000Ω
12.5Ω
50Ω
50Ω
1.7pF
13
14
15
16
7
5
6
8
+
–
1/2 R
L
1/2 R
L
640020 F05
IN+ OUT–
IN– OUT+
100Ω 12.5Ω
1000Ω
LTC6400-20
100Ω
1000Ω
12.5Ω
50Ω
50Ω
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. LTC6400-20 Internal Filter Topology Modifi ed
for Bandpass Filtering (Three External Capacitors, One
External Inductor)
640020 F06
IN+ OUT–
IN– OUT+
100Ω 12.5Ω
1000Ω
LTC6400-20
LTC2208
100Ω
1000Ω
12.5Ω
10Ω
10Ω
4.99Ω
4.99Ω
50Ω
50Ω
13
14
15
16
7
5
6
8
39pF
16nH
39pF
+OUT
+OUTF
–OUTF
–OUT
+IN
+IN
–IN
–IN
1.7pF
39pF