ltc6400cud-26-trpbf Linear Technology Corporation, ltc6400cud-26-trpbf Datasheet - Page 12

• Manufacturer Part Number: ltc6400cud-26-trpbf
• Description: 1.8ghz Low Noise, Low Distortion Differential Adc Driver For 300mhz If
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LTC6400-20
swing. In general, the single-ended input impedance and
termination resistor R
of R
that the single-ended input impedance is 202Ω and R
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:
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.
12
A
S
Figure 3. Input Termination for Single-Ended 50Ω Input
Impedance
V
, R
+
–
+
–
0.1μF
66.5Ω
1/2 R
1/2 R
V
66.5Ω
=
V
50Ω
50Ω
R
IN
R
G
IN
S
S
R
R
S
S
T
T
V
and R
V
OUT
0.1μF
0.1μF
IN
13
14
15
16
Figure 4. Calculate Differential Gain
+IN
+IN
–IN
–IN
13
14
15
16
F
=
. For example, when R
+IN
+IN
–IN
–IN
R
100Ω
100Ω
S
2000
100Ω
100Ω
+
T
IN+
IN–
are determined by the combination
200 25
IN+
IN–
1000Ω
1000Ω
•
OUT–
OUT+
1000Ω
1000Ω
R
+
OUT–
OUT+
L
R
L
12.5Ω
12.5Ω
50Ω
50Ω
S
is 50Ω, it is found
12.5Ω
12.5Ω
LTC6400-20
50Ω
50Ω
1.7pF
+OUTF
–OUTF
LTC6400-20
+OUT
–OUT
640020 F04
1.7pF
+OUTF
–OUTF
640020 F03
+OUT
–OUT
8
7
6
5
1/2 R
1/2 R
V
OUT
8
7
6
5
L
L
T
is
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
which is a high impedance input. The output common mode
voltage is capable of tracking V
Figure 5. LTC6400-20 Internal Filter Topology Modifi ed for Low
Filter Bandwidth (Three External Capacitors)
13
14
15
16
Figure 6. LTC6400-20 Internal Filter Topology Modifi ed
for Bandpass Filtering (Three External Capacitors, One
External Inductor)
13
14
15
16
+IN
+IN
–IN
–IN
+IN
+IN
–IN
–IN
100Ω
100Ω
100Ω
100Ω
IN+
IN–
IN+
IN–
1000Ω
1000Ω
1000Ω
1000Ω
OUT–
OUT+
OUT–
OUT+
12.5Ω
12.5Ω
50Ω
50Ω
LTC6400-20
12.5Ω
12.5Ω
50Ω
50Ω
1.7pF
+OUTF
–OUTF
640020 F06
+OUT
–OUT
OCM
LTC6400-20
1.7pF
8
7
6
5
+OUTF
–OUTF
640020 F05
+OUT
–OUT
10Ω
10Ω
16nH
in a range from 1V to
39pF
39pF
8
7
6
5
39pF
4.99Ω
4.99Ω
12pF
8.2pF
8.2pF
FILTERED OUTPUT
(87.5MHz)
OCM
LTC2208
640020f
pin,