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LTC6400IUD-8#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
LTC6400-8
10
64008f
Figure 4. Calculate Differential Gain
APPLICATIONS INFORMATION
Figure 3. Input Termination for Single-Ended 50Ω Input
Impedance
The LTC6400-8 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 LTC6400-8 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 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.
64008 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
LTC6400-8
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
64008 F04
+OUT
+OUTF
–OUTF
–OUT
+IN
IN+ OUT–
IN– OUT+
+IN
–IN
–IN
200Ω 12.5Ω
500Ω
LTC6400-8
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
order to minimize the refl ection due to input mismatch.
Alternatively, one could apply a narrowband impedance
match at the inputs of the LTC6400-8 for frequency selec-
tion and/or noise reduction.
Referring to Figure 3, LTC6400-8 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 1. Input Termination for Differential 50Ω Input Impedance
Using Shunt Resistor
Figure 2. Input Termination for Differential 50Ω Input Impedance
Using a Balun
64008 F01
+OUT
+OUTF
–OUTF
–OUT
+IN
IN+ OUT–
IN– OUT+
+IN
–IN
–IN
200Ω
57.6Ω
12.5Ω
500Ω
LTC6400-8
200Ω
25Ω
25Ω
V
IN
500Ω
12.5Ω
50Ω
50Ω
2.7pF
13
14
15
16
7
5
6
8
+
–
64008 F02
+OUT
+OUTF
–OUTF
–OUT
+IN
IN+ OUT–
IN– OUT+
+IN
–IN
MINI CIRCUITS
TCM4-19
–IN
200Ω 12.5Ω
500Ω
LTC6400-8
200Ω
25Ω
402Ω
25Ω
V
IN
500Ω
12.5Ω
50Ω
50Ω
2.7pF
13
14
15
16
7
5
6
8
+
–
1:4
•
•
LTC6400-8
11
64008f
Figure 5. LTC6400-8 Internal Filter Topology Modifi ed for Low
Filter Bandwidth (Three External Capacitors)
64008 F05
+OUT
+OUTF
–OUTF
–OUT
+IN
IN+ OUT–
IN– OUT+
+IN
–IN
–IN
200Ω 12.5Ω
500Ω
LTC6400-8
200Ω
500Ω
12.5Ω
50Ω
50Ω
2.7pF
13
14
15
16
7
5
6
8
8pF
8pF
12pF
FILTERED OUTPUT
(87.5MHz)
Figure 6. LTC6400-8 Modifi ed 165MHz for Bandpass Filtering
(Three External Capacitors, One External Inductor)
Output Impedance Match and Filter
The LTC6400-8 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 7. Single-Ended Input to LTC6400-8 and LTC2208
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 LTC6400-8’s 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 1.6V. Bandwidth of V
OCM
control is
typically 14MHz, which is dominated by a low pass fi lter
connected to the V
OCM
pin and is aimed to reduce com-
mon mode noise generation at the outputs. The internal
common mode feedback loop has a –3dB bandwidth
around 400MHz, allowing fast rejection of any common
mode output voltage disturbance. The V
OCM
pin should
be tied to a DC bias voltage with a 0.1μF bypass capaci-
tor. When interfacing with 3V A/D converters such as the
LT22xx families, the V
OCM
pin can be connected to the
V
CM
pin of the ADC.
Driving A/D Converters
The LTC6400-8 has been specifi cally designed to interface
directly with high speed A/D converters. Figure 7 shows the
LTC6400-8 with single-ended input driving the LTC2208,
which is a 16-bit, 130Msps ADC. Two external 5Ω resistors
help eliminate potential resonance associated with bond
wires of either the ADC input or the driver output. V
OCM
of the LTC6400-8 is connected to V
CM
of the LTC2208 at
1.25V. Alternatively, an input single-ended signal can be
converted to differential signal via a balun and fed to the
input of the LTC6400-8.
APPLICATIONS INFORMATION
64008 F06
IN+ OUT–
IN– OUT+
200Ω 12.5Ω
500Ω
LTC6400-8
LTC2208
200Ω
500Ω
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
2.7pF
39pF
27.4Ω
59.0Ω
0.1μF
0.1μF
64008 F07
LTC6400-8
V
OCM
ENABLE
LTC2208
4.99Ω
0.1μF
LTC2208 130Msps
16-Bit ADC
1.25V
4.99Ω
8dB GAIN
+OUT
+OUTF
–OUTF
–OUT
AIN
–
AIN
+
V
CM
–IN
+IN
IF IN
LTC6400-8
12
64008f
Figure 8 summarizes the IMD3 performance of the whole
system as shown in Figure 7.
APPLICATIONS INFORMATION
Figure 9. Top Silkscreen for DC987B. Test Circuit A
FREQUENCY (MHz)
0
IMD3 (dBc)
–40
–90
–80
–70
–60
–50
–100
–110
50 100 150 200 250
64008 F08
300
SINGLE-ENDED INPUT
F
S
= 122.8Msps
DRIVER V
OUT
= 2V
P-P
COMPOSITE
Figure 8. IMD3 for the Combination of LTC6400-8 and LTC2208
Test Circuits
Due to the fully-differential design of the LTC6400 and
its usefulness in applications with differing characteristic
specifi cations, two test circuits are used to generate the
information in this datasheet. Test Circuit A is DC987B,
a two-port demonstration circuit for the LTC6400 family.
The silkscreen is shown in Figure 9. This circuit includes
input and output transformers (baluns) for single-ended-
to-differential conversion and impedance transformation,
allowing direct hook-up to a 2-port network analyzer.
There are also series resistors at the output to present
the LTC6400 with a 375Ω differential load, optimizing
distortion performance. Due to the input and output trans-
formers, the –3dB bandwidth is reduced from 2.2GHz to
approximately 1.46GHz.
Test Circuit B uses a 4-port network analyzer to measure
S-parameters and gain/phase response. This removes the
effects of the wideband baluns and associated circuitry,
for a true picture of the >1GHz S-parameters and AC
characteristics.
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LTC6400IUD-8#TRPBF

Mfr. #:
Buy LTC6400IUD-8#TRPBF
Manufacturer:
Analog Devices Inc.
Description:
High Speed Operational Amplifiers 3GHz Low Noise/Low Distortion Differential Amp
Lifecycle:
New from this manufacturer.
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