LTC6401-14 Linear Technology, LTC6401-14 Datasheet - Page 10

• Manufacturer Part Number: LTC6401-14
• Description: Low Distortion Differential ADC Driver
• Manufacturer: Linear Technology

www.DataSheet4U.com
APPLICATIONS INFORMATION
LTC6401-14
Referring to Figure 3, LTC6401-14 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
of R
that the single-ended input impedance is 185Ω and R
68.5Ω in order to match to a 50Ω source impedance.
The LTC6401-14 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:
The noise performance of the LTC6401-14 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
10
A
S
Figure 3. Input Termination for Single-Ended 50Ω Input
Impedance
+
–
V
, R
0.1μF
68.5Ω
V
50Ω
=
R
IN
29Ω
G
S
R
T
V
and R
V
0.1μF
0.1μF
OUT
IN
13
14
15
16
F
=
. For example, when R
+IN
+IN
–IN
–IN
R
S
1000
100Ω
100Ω
+
T
are determined by the combination
200 25
IN+
IN–
•
500Ω
500Ω
R
+
OUT–
OUT+
L
R
L
S
is 50Ω, it is found
12.5Ω
12.5Ω
50Ω
50Ω
LTC6401-14
2.7pF
+OUTF
–OUTF
640114 F03
+OUT
–OUT
8
7
6
5
T
is
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 LTC6401-14 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. LTC6401-14 Internal Filter Topology Modifi ed for Low
Filter Bandwidth (Three External Capacitors)
13
14
15
16
+
–
+IN
+IN
–IN
–IN
1/2 R
1/2 R
V
IN
S
S
100Ω
100Ω
13
14
15
16
Figure 4. Calculate Differential Gain
IN+
IN–
+IN
+IN
–IN
–IN
100Ω
100Ω
500Ω
500Ω
OUT–
OUT+
IN+
IN–
500Ω
500Ω
12.5Ω
12.5Ω
50Ω
50Ω
OUT–
OUT+
LTC6401-14
2.7pF
+OUTF
–OUTF
640114 F05
+OUT
–OUT
12.5Ω
12.5Ω
50Ω
50Ω
8
7
6
5
LTC6401-14
2.7pF
+OUTF
–OUTF
12pF
640114 F04
+OUT
–OUT
8pF
8pF
FILTERED OUTPUT
(87.5MHz)
8
7
6
5
1/2 R
1/2 R
V
OUT
L
L
640114f