LTC6401-8
10
64018f
Referring to Figure 3, LTC6401-8 can be easily confi gured
for single-ended input and differential output without a
balun. The signal is fed to one of the inputs through a
matching network while the other input is connected to
the same matching network and a source resistor. Because
the return ratios of the two feedback paths are equal, the
two outputs have the same gain and thus symmetrical
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 322Ω and R
T
is
59Ω in order to match to a 50Ω source impedance.
Figure 4. Calculate Differential Gain
APPLICATIONS INFORMATION
Figure 3. Input Termination for Single-Ended 50Ω Input
Impedance
and noise is obvious when constant noise fi gure circle
and constant gain circle are plotted within the input Smith
Chart, based on which users can choose the optimal source
impedance for a given gain and noise requirement.
Output Impedance Match and Filter
The LTC6401-8 can drive an ADC directly without external
output impedance matching. Alternatively, the differential
output impedance of 25Ω can be made larger, e.g. 50Ω,
by series resistors or LC network.
The internal low pass fi lter outputs at +OUTF/–OUTF
have a –3dB bandwidth of 590MHz. External capacitors
can reduce the lowpass fi lter bandwidth as shown in
Figure 5. A bandpass fi lter is easily implemented with
Figure 5. LTC6401-8 Internal Filter Topology Modifi ed for Low
Filter Bandwidth (Three External Capacitors)
64018 F03
+OUT
+OUTF
–OUTF
–OUT
+IN
IN+ OUT–
IN– OUT+
+IN
–IN
–IN
200Ω
R
T
59.0Ω
0.1μF
0.1μF
12.5Ω
500Ω
LTC6401-8
200Ω
R
S
50Ω
V
IN
500Ω
12.5Ω
50Ω
50Ω
2.7pF
13
14
15
16
7
5
6
8
+
–
0.1μF
27.4Ω
64018 F04
+OUT
+OUTF
–OUTF
–OUT
+IN
IN+ OUT–
IN– OUT+
+IN
–IN
–IN
200Ω 12.5Ω
500Ω
LTC6401-8
200Ω
1/2 R
S
1/2 R
S
V
IN
V
OUT
500Ω
12.5Ω
50Ω
50Ω
2.7pF
13
14
15
16
7
5
6
8
+
–
1/2 R
L
1/2 R
L
64018 F05
+OUT
+OUTF
–OUTF
–OUT
+IN
IN+ OUT–
IN– OUT+
+IN
–IN
–IN
200Ω 12.5Ω
500Ω
LTC6401-8
200Ω
500Ω
12.5Ω
50Ω
50Ω
2.7pF
13
14
15
16
7
5
6
8
8pF
8pF
12pF
FILTERED OUTPU
(87.5MHz)
The LTC6401-8 is unconditionally stable, i.e. differential
stability factor Kf>1 and stability measure B1>0. However,
the overall differential gain is affected by both source
impedance and load impedance as shown in Figure 4:
A
V
VR
R
R
V
OUT
IN S
L
L
==
++
1000
400 25
•
The noise performance of the LTC6401-8 also depends
upon the source impedance and termination. For example,
an input 1:4 transformer in Figure 2 improves SNR by
adding 6dB gain at the inputs. A trade-off between gain
