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PDF LTC6401-20 Data sheet ( Hoja de datos )
| Número de pieza | LTC6401-20 | |
| Descripción | Low Distortion Differential ADC Driver | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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Electrical Specifications Subject to Change
LTC6401-20
1.3GHz Low Noise, Low
Distortion Differential ADC
Driver for 140MHz IF
FEATURES
■ 1.3GHz –3dB Bandwidth
■ Fixed Gain of 10V/V (20dB)
■ 40.8dBm OIP3 at 140MHz
■ 0.91nV/√⎯H⎯z Internal Op Amp Noise
■ 2.1nV/√⎯H⎯z Total Input Noise
■ 6.1dB Noise Figure
■ Differential Inputs and Outputs
■ 200Ω Input Impedance
■ 2.85V ~ 3.5V Supply Voltage
■ 50mA Supply Current (150mW)
■ 1V to 1.6V Output Common Mode Voltage,
Adjustable
■ DC- or AC-Coupled Operation
■ Max Differential Output Swing 4.4VP-P
■ Small 16-Lead 3mm × 3mm × 0.75mm QFN Package
APPLICATIONS
■ Differential ADC Driver
■ Differential Driver/Receiver
■ Single Ended to Differential Conversion
■ IF Sampling Receivers
■ SAW Filter Interfacing
DESCRIPTION
The LTC®6401-20 is a high-speed differential amplifier
targeted at processing signals from DC to 140MHz. The
part has been specifically designed to drive 12-, 14- and
16-bit ADCs with low noise and low distortion, but can also
be used as a general-purpose broadband gain block.
The LTC6401-20 is easy to use, with minimal support
circuitry required. The output common mode voltage is
set using an external pin, independent of the inputs, which
eliminates the need for transformers or AC-coupling ca-
pacitors in many applications. The gain is internally fixed
at 20dB (10V/V).
The LTC6401-20 saves space and power compared to
alternative solutions using IF gain blocks and transform-
ers. The LTC6401-20 is packaged in a compact 16-lead
3mm × 3mm QFN package and operates over the –40°C
to 85°C temperature range.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
TYPICAL APPLICATION
Single-Ended to Differential ADC Driver
3V
0.1μF + 1000pF
0.1μF
66.5Ω
29Ω
0.1μF
0.1μF
V+
VOCM
+IN +OUT
+OUTF
LTC6401-20
–OUTF
–IN V––OUT
ENABLE
20dB GAIN
1.25V
10Ω
10Ω
3.3V
AIN+ VCM VDD
LTC2208
AIN–
LTC2208 130Msps
16-Bit ADC
640120 TA01a
Output IP3 vs Frequency
60
50
40
30
20
10
0
70 90 110 130 150 170 190 200
FREQUENCY (MHz)
640120 TA01b
640120p
1
1 page  
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LTC6401-20
AC ELECTRICAL CHARACTERISTICS
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: As long as output current is below 10mA and junction temperature
is below the Absolute Maximum Ratings, no damage to the part will occur.
Note 3: The LTC6401C and LTC6401I are guaranteed functional over the
operating temperature range of –40°C to 85°C.
Note 4: The LTC6401C is guaranteed to meet specified performance from
0°C to 70°C. It is designed, characterized and expected to meet specified
performance from –40°C to 85°C but is not tested or QA sampled at these
temperatures. The LTC6401I is guaranteed to meet specified performance
from –40°C to 85°C.
Note 5: Input balun 1:4, output balun 4:1, differential load resistance
375Ω.
Note 6: Input balun 1:4.
Note 7: Each input AC couple to 50Ω, each output AC couple to 87.5Ω,
input differential shunt resistor 200Ω.
Note 8: Since the LTC6401-20 is a feedback amplifier with low output
impedance, a resistive load is not required when driving an AD converter.
Therefore, typical output power is very small. In order to compare the
LTC6401-20 with amplifiers that require 50Ω output load, the LTC6401-20
output voltage swing driving a given RL is converted to OIP3 and P1dB as
if it were driving a 50Ω load. Using this modified convention, 2VP-P is by
definition equal to 10dBm, regardless of the actual RL.
TYPICAL PERFORMANCE CHARACTERISTICS
Frequency Response
25
50SE/50SE NORMALIZED TO NO RL
20
15
BALUN/BALUN
10
5
0
10 100 1000 3000
FREQUENCY (MHz)
640120 G01
Gain 0.1dB Flatness
1.0
0.8
50SE/50SE
0.6
0.4
0.2
0 BALUN/BALUN
–0.2
–0.4
–0.6
–0.8
–1.0
10
100
FREQUENCY (MHz)
1000
640120 G02
S21 Phase and Group Delay vs
Frequency
100
50SE/50SE
PHASE
0
BALUN/BALUN
GROUP DELAY
1.5
1.2
–100
0.9
–200
0.6
–300
–400
0
50SE/50SE
GROUP DELAY
BALUN/BALUN PHASE
0.3
200 400 600
FREQUENCY (MHz)
0
800 1000
640120 G03
640120p
5
5 Page 
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LTC6401-20
APPLICATIONS INFORMATION
RS 0.1μF
50Ω
–+
VIN
RT
66.5Ω
0.1μF
13 +IN
14 +IN
100Ω
IN+
15 –IN
RS 0.1μF
50Ω
RT
66.5Ω
16 –IN
IN–
100Ω
1000Ω
OUT–
OUT+
1000Ω
LTC6401-20
12.5Ω
+OUT 8
50Ω
+OUTF 7
50Ω 1.7pF
–OUTF 6
12.5Ω
–OUT 5
640120 F03
Figure 3. Input Termination for Single-Ended 50Ω Input
Impedance
two outputs have the same gain and thus symmetrical
swing. In general, the single-ended input impedance and
termination resistor RT are determined by the combination
of RS, RG and RF. For example, when RS is 50Ω, it is found
that the single-ended input impedance is 200Ω and RT is
66.5Ω in order to match to a 50Ω source impedance.
The LTC6401-20 is unconditionally stable. However, the
overall differential gain is affected by both source imped-
ance and load impedance as shown in Figure 4:
AV =
VOUT
VIN
= 2000 • RL
RS + 200 25 + RL
The noise performance of the LTC6401-20 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 gain at the inputs. A trade-off between
gain and noise is obvious when constant noise figure
circle and constant gain circle are plotted within the same
1/2 RS
13 +IN
100Ω
1000Ω
–+ VIN
14 +IN
15 –IN
IN+ OUT–
IN– OUT+
LTC6401-20
12.5Ω
1/2 RL
+OUT 8
50Ω
+OUTF 7
50Ω 1.7pF
VOUT
–OUTF 6
1/2 RS
16 –IN
100Ω
1000Ω
12.5Ω
1/2 RL
–OUT 5
640120 F04
Figure 4. Calculate Differential Gain
input Smith Chart, based on which users can choose the
optimal source impedance for a given gain and noise
requirement.
Output Match and Filter
The LTC6401-20 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 filter outputs at +OUTF/–OUTF
have a –3dB bandwidth of 590MHz. External capacitor
can reduce the low pass filter bandwidth as shown in
Figure 5. A bandpass filter is easily implemented with
only a few components as shown in Figure 6. Three
39pF capacitors and a 16nH inductor create a bandpass
filter with 165MHz center frequency, –3dB frequencies at
138MHz and 200MHz.
100Ω
13 +IN
1000Ω
14 +IN
IN+ OUT–
15 –IN
IN– OUT+
100Ω
16 –IN
1000Ω
LTC6401-20
12.5Ω
+OUT 8
50Ω
+OUTF 7
50Ω 1.7pF
–OUTF 6
12.5Ω
–OUT 5
640120 F05
8.2pF
12pF
FILTERED OUTPUT
(87.5MHz)
8.2pF
Figure 5. LTC6401-20 Internal Filter Topology Modified for Low
Filter Bandwidth (Three External Capacitors)
100Ω
13 +IN
1000Ω
14 +IN
IN+ OUT–
15 –IN
IN– OUT+
100Ω
16 –IN
1000Ω
LTC6401-20
12.5Ω
39pF
10Ω
+OUT 8
50Ω
+OUTF 7
50Ω 1.7pF
16nH
–OUTF 6
12.5Ω
10Ω
–OUT 5
39pF
640120 F06
4.99Ω
39pF
4.99Ω
LTC2208
Figure 6. LTC6401-20 Internal Filter Topology Modified
for Bandpass Filtering (Three External Capacitors, One
External Inductor)
640120p
11
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LTC6401-20.PDF ] | |
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