LTC6401-8
2.2GHz Low Noise, Low
Distortion Differential ADC
Driver for DC-140MHz
FEATURES
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DESCRIPTION
2.2GHz –3dB Bandwidth
Fixed Gain of 2.5V/V (8dB)
–92dBc IMD3 at 70MHz (Equivalent OIP3 = 50dBm)
–80.5dBc IMD3 at 140MHz (Equivalent OIP3 = 44dBm)
1nV/√⎯H⎯z Internal Op Amp Noise
12.1dB Noise Figure
Differential Inputs and Outputs
400Ω Input Impedance
2.85V to 3.5V Supply Voltage
45mA Supply Current (135mW)
1V to 1.6V Output Common Mode, Adjustable
DC- or AC-Coupled Operation
Max Differential Output Swing 4.6VP-P
Small 16-Lead 3mm × 3mm × 0.75mm QFN Package
APPLICATIONS
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Differential ADC Driver
Differential Driver/Receiver
Single Ended to Differential Conversion
IF Sampling Receivers
SAW Filter Interfacing
The LTC®6401-8 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 LTC6401-8 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 capacitors in many applications. The gain is internally fixed
at 8dB (2.5V/V).
The LTC6401-8 saves space and power compared to alternative solutions using IF gain blocks and transformers.
The LTC6401-8 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
3.3V
3.3V
Equivalent Output IP3 vs Frequency
60
C1
1000pF
CF2
33pF
C3
0.1μF
V+
RS1
15Ω
R1
59.0Ω
C4
0.1μF
LTC6401-8
–IN
R2
27.4Ω
RS3
10Ω
+OUT
+IN
VIN
L1
RS2 24nH
15Ω
–OUT
VOCM
V–
COILCRAFT
0603CS
1.25V
CF1
33pF RS4
10Ω
CF3
33pF
AIN+
VDD
LTC2208
AIN–
VCM
LTC2208
130Msps
16-Bit ADC
R3
100Ω
40
30
20
10
64018 TA01a
C5
0.1μF
(NOTE 7)
50
OUTPUT IP3 (dBm)
C2
0.1μF
0
0
20 40 60 80 100 120 140 160 180 200
FREQUENCY (MHz)
64018 TA01b
64018f
1
LTC6401-8
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
+IN
+IN
–IN
–IN
TOP VIEW
Supply Voltage (VCC – VEE) ......................................3.6V
Input Current (Note 2)..........................................±10mA
Operating Temperature Range
(Note 3) ............................................... –40°C to 85°C
Specified Temperature Range
(Note 4) ............................................... –40°C to 85°C
Storage Temperature Range................... –65°C to 150°C
Maximum Junction Temperature .......................... 150°C
16 15 14 13
V+ 1
12 V–
VOCM 2
10 V+
9 V–
7
8
+OUT
6
+OUTF
5
–OUT
4
–OUTF
V–
11 ENABLE
17
V+ 3
UD PACKAGE
16-LEAD (3mm × 3mm) PLASTIC QFN
TJMAX = 150°C, θJA = 68°C/W, θJC = 4.2°C/W
EXPOSED PAD (PIN 17) IS V–, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
SPECIFIED TEMPERATURE RANGE
LTC6401CUD-8#PBF
LTC6401CUD-8#TRPBF
LCCY
16-Lead (3mm × 3mm) Plastic QFN 0°C to 70°C
LTC6401IUD-8#PBF
LTC6401IUD-8#TRPBF
LCCY
16-Lead (3mm × 3mm) Plastic QFN –40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
Consult LTC Marketing for information on non-standard lead based finish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
LTC6400 AND LTC6401 SELECTOR GUIDE
PART NUMBER
Please check each datasheet for complete details.
GAIN
(dB)
GAIN
(V/V)
ZIN (DIFFERENTIAL)
(Ω)
ICC
(mA)
LTC6401-8
8
2.5
400
45
LTC6401-20
20
10
200
50
LTC6401-26
26
20
50
45
LTC6400-20
20
10
200
90
LTC6400-26
26
20
50
85
In addition to the LTC6401 family of amplifiers, a lower distortion LTC6400 family is available. The LTC6400 is pin compatible to the LTC6401, and has the
same low noise performance. The LTC6400 shows higher linearity especially at input frequency above 140MHz at the expense of higher supply current.
Please refer to the separate LTC6400 data sheets for complete details. Other gain versions from 8dB to 14dB will follow.
64018f
2
LTC6401-8
DC ELECTRICAL CHARACTERISTICS + The ● –denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. V = 3V, V = 0V, +IN = –IN = VOCM = 1.25V, ⎯E⎯N⎯A⎯B⎯L⎯E = 0V, No RL unless
otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
8
8.5
UNITS
Input/Output Characteristic
GDIFF
Gain
VIN = ±400mV Differential
●
TCGAIN
Gain Temperature Drift
VIN = ±400mV Differential
●
–0.5
VSWINGMIN
Output Swing Low
Each Output, VIN = ±1.6V Differential
●
89
VSWINGMAX
Output Swing High
Each Output, VIN = ±1.6V Differential
●
7.5
2.3
VOUTDIFFMAX
Maximum Differential Output Swing
1dB Compressed
●
IOUT
Output Current Drive
VOUT > 2VP-P,DIFF
●
10
VOS
Input Offset Voltage
Differential
●
–4
TCVOS
Input Offset Voltage Drift
Differential
●
IVRMIN
Input Common Mode Voltage Range, MIN
IVRMAX
Input Common Mode Voltage Range, MAX
RINDIFF
Input Resistance (+IN, –IN)
Differential
CINDIFF
Input Capacitance (+IN, –IN)
Differential, Includes Parasitic
170
V
4.6
VP-P
mA
4
3
ROUTDIFF
Output Resistance (+OUT, –OUT)
Differential
ROUTFDIFF
Filtered Output Resistance (+OUTF, –OUTF)
Differential
●
COUTFDIFF
Filtered Output Capacitance (+OUTF, –OUTF)
Differential, Includes Parasitic
CMRR
Common Mode Rejection Ratio
Input Common Mode Voltage 1.1V~1.4V
●
V
V
400
460
1
●
mV
μV/°C
1
340
mV
2.42
1.6
●
dB
mdB/°C
Ω
pF
18
25
32
Ω
85
100
115
Ω
36
2.7
pF
55
dB
1
V/V
Output Common Mode Voltage Control
GCM
Common Mode Gain
VOCM = 1V to 1.6V
VOCMMIN
Output Common Mode Range, MIN
VOCMMAX
Output Common Mode Range, MAX
VOSCM
Common Mode Offset Voltage
TCVOSCM
Common Mode Offset Voltage Drift
●
5
IVOCM
VOCM Input Current
●
3.6
VIL
⎯E⎯N⎯A⎯B⎯L⎯E Input Low Voltage
●
VIH
⎯E⎯N⎯A⎯B⎯L⎯E Input High Voltage
IIL
⎯E⎯N⎯A⎯B⎯L⎯E Input Low Current
⎯E⎯N⎯A⎯B⎯L⎯E = 0.8V
●
IIH
⎯E⎯N⎯A⎯B⎯L⎯E Input High Current
⎯E⎯N⎯A⎯B⎯L⎯E = 2.4V
●
1
1.1
●
VOCM = 1.1V to 1.5V
●
1.6
1.5
●
–15
V
V
V
V
15
mV
μV/°C
15
μA
0.8
V
⎯E⎯N⎯A⎯B⎯L⎯E Pin
●
2.4
V
0.5
μA
1.4
4
μA
3
3.5
V
45
60
mA
0.8
3
mA
Power Supply
VS
Operating Supply Range
●
2.85
●
36
IS
Supply Current
⎯E⎯N⎯A⎯B⎯L⎯E = 0.8V, Input and Output Floating
ISHDN
Shutdown Supply Current
⎯E⎯N⎯A⎯B⎯L⎯E = 2.4V, Input and Output Floating
●
PSRR
Power Supply Rejection Ratio
(Differential Outputs)
V+ = 2.85V to 3.5V
●
50
73.5
dB
64018f
3
LTC6401-8
AC ELECTRICAL CHARACTERISTICS
⎯E⎯N⎯A⎯B⎯L⎯E = 0V, No RL unless otherwise noted.
Specifications are at TA = 25°C. V+ = 3V, V– = 0V, VOCM = 1.25V,
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
–3dBBW
–3dB Bandwidth
200mVP-P,OUT (Note 6)
1
2.22
GHz
0.5dBBW
Bandwidth for 0.5dB Flatness
200mVP-P,OUT (Note 6)
0.43
GHz
0.1dBBW
Bandwidth for 0.1dB Flatness
200mVP-P,OUT (Note 6)
1/f
1/f Noise Corner
SR
Slew Rate
VOUT = 2V Step (Note 6)
tS1%
1% Settling Time
VOUT = 2VP-P (Note 6)
2.3
ns
tOVDR
Overdrive Recovery Time
VOUT = 1.9VP-P (Note 6)
18
ns
tON
Turn-On Time
VOUT Within 10% of Final Values
79
ns
tOFF
Turn-Off Time
ICC Falls to 10% of Nominal
193
ns
–3dBBWVOCM
VOCM Pin Small Signal –3dB BW
0.1VP-P at VOCM, Measured Single-Ended at
Output (Note 6)
14
MHz
0.22
GHz
12.2
kHz
3400
V/μs
10MHz Input Signal
HD2,10M/HD3,10M Second/Third Order Harmonic Distortion VOUT = 2VP-P, RL = 200Ω
VOUT = 2VP-P, No RL
IMD3,10M
OIP3,10M
–109/–88
dBc
–118/–100
dBc
Third-Order Intermodulation
(f1 = 9.5MHz f2 = 10.5MHz)
VOUT = 2VP-P Composite, RL = 200Ω
–88
dBc
VOUT = 2VP-P Composite, No RL
–93
dBc
Equivalent Third-Order Output Intercept
Point (f1 = 9.5MHz f2 = 10.5MHz)
VOUT = 2VP-P Composite, No RL (Note 7)
50.7
dBm
P1dB,10M
1dB Compression Point
RL = 375Ω (Notes 5, 7)
17.8
dBm
NF10M
Noise Figure
RL = 375Ω (Note 5)
12.1
dB
eIN,10M
Input Referred Voltage Noise Density
Includes Resistors (Short Inputs)
3.2
nV/√⎯H⎯z
eON,10M
Output Referred Voltage Noise Density
Includes Resistors (Short Inputs)
8
nV/√⎯H⎯z
70MHz Input Signal
HD2,70M/HD3,70M Second/Third Order Harmonic Distortion VOUT = 2VP-P, RL = 200Ω
VOUT = 2VP-P, No RL
–91/–72
dBc
–100/–87
dBc
dBc
IMD3,70M
Third-Order Intermodulation
(f1 = 69.5MHz f2 = 70.5MHz)
VOUT = 2VP-P Composite, RL = 200Ω
–83
VOUT = 2VP-P Composite, No RL
–92
dBc
OIP3,70M
Equivalent Third-Order Output Intercept
Point (f1 = 69.5MHz f2 = 70.5MHz)
VOUT = 2VP-P Composite, No RL (Note 7)
50
dBm
P1dB,70M
1dB Compression Point
RL = 375Ω (Notes 5, 7)
18.3
dBm
NF70M
Noise Figure
RL = 375Ω (Note 5)
12.2
dB
eIN,70M
Input Referred Voltage Noise Density
Includes Resistors (Short Inputs)
3.2
nV/√⎯H⎯z
eON,70M
Output Referred Voltage Noise Density
Includes Resistors (Short Inputs)
7.9
nV/√⎯H⎯z
64018f
4
LTC6401-8
AC ELECTRICAL CHARACTERISTICS
⎯E⎯N⎯A⎯B⎯L⎯E = 0V, No RL unless otherwise noted.
SYMBOL
PARAMETER
Specifications are at TA = 25°C. V+ = 3V, V– = 0V, VOCM = 1.25V,
CONDITIONS
MIN
TYP
MAX
UNITS
140MHz Input Signal
HD2,140M/
HD3,140M
Second/Third Order Harmonic Distortion VOUT = 2VP-P, RL = 200Ω
–78/–59
–87/–70
dBc
IMD3,140M
Third-Order Intermodulation
(f1 = 139.5MHz f2 = 140.5MHz)
VOUT = 2VP-P Composite, RL = 200Ω
–71
dBc
VOUT = 2VP-P Composite, No RL
–80
dBc
OIP3,140M
Equivalent Third-Order Output Intercept
Point (f1 = 139.5MHz f2 = 140.5MHz)
VOUT = 2VP-P Composite, No RL (Note 7)
44.2
dBm
P1dB,140M
1dB Compression Point
RL = 375Ω (Notes 5, 7)
18.7
dBm
NF140M
Noise Figure
RL = 375Ω (Note 5)
12.3
dB
eIN,140M
Input Referred Voltage Noise Density
Includes Resistors (Short Inputs)
3.1
nV/√⎯H⎯z
eON,140M
Output Referred Voltage Noise Density
Includes Resistors (Short Inputs)
7.9
nV/√⎯H⎯z
IMD3,130M/150M
Third-Order Intermodulation
(f1 = 130MHz f2 = 150MHz) Measure at
170MHz
VOUT = 2VP-P Composite, RL = 375Ω
–75
VOUT = 2VP-P, No RL
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: Input pins (+IN, –IN) are protected by steering diodes to either
supply. If the inputs go beyond either supply rail, the input current should
be limited to less than 10mA.
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
dBc
–67
dBc
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 and output baluns used. See Test Circuit A.
Note 6: Measured using Test Circuit B. RL = 87.5Ω per output.
Note 7: Since the LTC6401-8 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
LTC6401-8 with amplifiers that require 50Ω output load, the LTC6401-8
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 actual RL.
TYPICAL PERFORMANCE CHARACTERISTICS
Frequency Response
10
0.6
6
4
2
0
0.4
0.2
0
–0.2
–0.4
–2
–0.6
–4
–0.8
–6
–1.0
10
100
1000
FREQUENCY (MHz)
3000
64018 G01
0
TEST CIRCUIT B
10
100
1000
FREQUENCY (MHz)
3000
64018 G02
TEST CIRCUIT B
0.7
–50
0.6
–100
0.5
–150
0.4
PHASE
GROUP DELAY
–200
0
200
400
600
FREQUENCY (MHz)
800
GROUP DELAY (ns)
GAIN FLATNESS (dB)
0.8
8
GAIN (dB)
1.0
TEST CIRCUIT B
12
PHASE (DEGREE)
14
S21 Phase and Group Delay vs
Frequency
Gain 0.1dB Flatness
0.3
1000
64018 G03
64018f
5
LTC6401-8
TYPICAL PERFORMANCE CHARACTERISTICS
Input and Output Reflection and
Reverse Isolation vs Frequency
500
TEST CIRCUIT B
IMPEDANCE MAGNITUDE (Ω)
–20
S11
–30
S22
–40
–50
–60
S12
–70
ZIN
350
3000
ZOUT
300
250
150
–40
100
–60
ZOUT
EN
3
14
2
NOISE FIGURE
1
12
+OUT
1.25
1.20
–OUT
2
4.0
3
1
3.5
2
4
6
TIME (ns)
8
2.5
2.0
1.5
–4
1.0
25
0.5
–OUT
50
75
TIME (ns)
100
10
0
0
125
64018 G10
4
8
12
TIME (ns)
1
0
–1
–2
–3
DIFFERENTIAL INPUT
VOUT = 2VP-P
–50
–60
–70
–80
–90
HD2 NO RL
HD2 200Ω RL
HD3 NO RL
HD3 200Ω RL
–100
–4
–5
–110
1
2
3
TIME (ns)
20
Harmonic Distortion vs Frequency
–40
RL = 87.5Ω PER OUTPUT
TEST CIRCUIT B
0
16
64018 G09
2
SETTLING (%)
INPUT VOLTAGE (V)
3.0
–3
0
–OUT
0.5
HARMONIC DISTORTION (dBc)
4
OUTPUT VOLTAGE (V)
4.5
–6
1.0
64018 G08
3
TEST CIRCUIT B
+OUT
1.5
0
0
5
+OUT
RL = 87.5Ω PER OUTPUT
TEST CIRCUIT B
2.0
5.0
–5
Large Signal Transient Response
2.5
1% Settling Time for 2V
Output Step
–IN
1000
64018 G06
1.15
0
1000
+IN
10
100
FREQUENCY (MHz)
1
RL = 87.5Ω PER OUTPUT
TEST CIRCUIT B
1.30
Overdrive Recovery Response
0
0
–100
1000
64018 G07
–1 R = 87.5Ω PER OUTPUT
L
–2
30
10
OUTPUT VOLTAGE (V)
4
1.35
OUTPUT VOLTAGE (V)
18
4
40
Small Signal Transient Response
5
16
CMRR
20
–80
100
FREQUENCY (MHz)
10
INPUT REFERRED NOISE VOLTAGE (nV/√Hz)
NOISE FIGURE (dB)
–20
ZIN
50
64018 G05
20
100
FREQUENCY (MHz)
60
0
200
Noise Figure and Input Referred
Noise Voltage vs Frequency
10
PSRR
20
64018 G04
10
70
40
0
100
1000
FREQUENCY (MHz)
10
80
60
400
50
–80
80
PHASE (DEGREES)
S PARAMETERS (dB)
PHASE
IMPEDANCE MAGNITUDE
450
–10
PSRR and CMRR vs Frequency
100
PSRR, CMRR (dB)
0
Input and Output Impedance vs
Frequency
4
5
64018 G11
0
20 40 60 80 100 120 140 160 180 200
FREQUENCY (MHz)
64018 G12
64018f
6
LTC6401-8
TYPICAL PERFORMANCE CHARACTERISTICS
Third Order Intermodulation
Distortion vs Frequency
–60
200Ω RL
–70
–80
NO RL
–90
–100
–50
–70
–80
–90
HD2 NO RL
HD2 200Ω RL
HD3 NO RL
HD3 200Ω RL
–110
20 40 60 80 100 120 140 160 180 200
FREQUENCY (MHz)
0
NO RL
–90
–100
–110
0
20 40 60 80 100 120 140 160 180 200
FREQUENCY (MHz)
64018 G15
60
50
OUTPUT IP3 (dBm)
OUTPUT 1dB COMPRESSION POINT (dBm)
–80
Equivalent Output Third Order
Intercept Point vs Frequency
DIFFERENTIAL INPUT
RL = 375Ω
TEST CIRCUIT A (NOTE 7)
19
200Ω RL
–70
64018 G14
Equivalent Output 1dB
Compression Point vs Frequency
20
–60
20 40 60 80 100 120 140 160 180 200
FREQUENCY (MHz)
64018 G13
18
17
16
NO RL
40
200Ω RL
30
20
10
DIFFERENTIAL INPUT
(NOTE 7)
0
15
0
0
20 40 60 80 100 120 140 160 180 200
FREQUENCY (MHz)
20 40 60 80 100 120 140 160 180 200
FREQUENCY (MHz)
64018 G17
64018 G16
Turn-On Time
Turn-Off Time
3.5
70
3.5
3.0
60
3.0
60
50
2.5
50
ICC
2.5
–OUT
1.5
30
20
1.0
+OUT
0.5
10
0
0
RL = 87.5Ω PER OUTPUT
–0.5
–100
0
100
200
300
TIME (ns)
ENABLE
400
–10
500
64018 G18
70
RL = 87.5Ω PER OUTPUT
40
2.0
+OUT
30
1.5
–OUT
1.0
20
10
0.5
0
ICC (mA)
40
2.0
ICC (mA)
VOLTAGE (V)
0
SINGLE-ENDED INPUT
VOUT = 2VP-P COMPOSITE
–50
–60
–100
–110
–40
SINGLE-ENDED INPUT
VOUT = 2VP-P
THIRD ORDER IMD (dBc)
HARMONIC DISTORTION (dBc)
–50
THIRD ORDER IMD (dBc)
–40
DIFFERENTIAL INPUT
VOUT = 2VP-P COMPOSITE
VOLTAGE (V)
–40
Third Order Intermodulation
Distortion vs Frequency
Harmonic Distortion vs Frequency
–0.5
–100
0
0
ICC
ENABLE
100
200
300
TIME (ns)
400
–10
500
64018 G19
64018f
7
LTC6401-8
PIN FUNCTIONS
V+ (Pins 1, 3, 10): Positive Power Supply (Normally tied
to 3V or 3.3V). All three pins must be tied to the same
voltage. Bypass each pin with 1000pF and 0.1μF capacitors as close to the pins as possible.
VOCM (Pin 2): This pin sets the output common mode
voltage. A 0.1μF external bypass capacitor is recommended.
V– (Pins 4, 9, 12, 17): Negative Power Supply. All four
pins must be connected to same voltage/ground.
–OUT, +OUT (Pins 5, 8): Unfiltered Outputs. These pins
have series resistors, ROUT 12.5Ω.
–OUTF, +OUTF (Pins 6, 7): Filtered Outputs. These pins
have 50Ω series resistors and a 2.7pF shunt capacitor.
⎯E⎯N⎯A⎯B⎯L⎯E (Pin 11): This pin is a logic input referenced to
VEE. If low, the part is enabled. If high, the part is disabled
and draws very low standby current while the internal op
amp has high output impedance.
+IN (Pins 13, 14): Positive Input. Pins 13 and 14 are
internally shorted together.
–IN (Pins 15, 16): Negative Input. Pins 15 and 16 are
internally shorted together.
Exposed Pad (Pin 17): V–. The Exposed Pad must be
connected to same voltage/ground as pins 4, 9, 12.
BLOCK DIAGRAM
V–
12
V–
V+
ENABLE
11
10
9
BIAS CONTROL
+IN
13
ROUT
12.5Ω
+OUT
8
RFILT
50Ω
+IN
14
IN+
+OUTF
7
OUT–
CFILT
2.7pF
RFILT
50Ω
–IN
15
–IN
16
RF
500Ω
RG
200Ω
IN–
OUT+
RF
500Ω
RG
200Ω
–OUTF
6
ROUT
12.5Ω
–OUT
5
2k
COMMON
MODE CONTROL
5.3pF
1
V+
2
3
VOCM
V+
4
64018 BD
V–
64018f
8
LTC6401-8
APPLICATIONS INFORMATION
Circuit Operation
Input Impedance and Matching
The LTC6401-8 is a low noise and low distortion fully
differential op amp/ADC driver with:
The differential input impedance of the LTC6401-8 is 400Ω.
Usually the differential inputs need to be terminated to a
lower value impedance, e.g. 50Ω, in order to provide an
impedance match for the source. Several choices are available. One approach is to use a differential shunt resistor
(Figure 1). Another approach is to employ a wideband
transformer and shunt resistor (Figure 2). Both methods
provide a wideband match. The termination resistor or
the transformer must be placed close to the input pins in
order to minimize the reflection due to input mismatch.
Alternatively, one could apply a narrowband impedance
match at the inputs of the LTC6401-8 for frequency selection and/or noise reduction.
• Fixed gain of 2.5V/V (8dB)
• Differential input impedance 400Ω
• Differential output impedance 25Ω
• Differential impedance of output filter 100Ω
The LTC6401-8 is composed of a fully differential amplifier
with on chip feedback and output common mode voltage
control circuitry. Differential gain and input impedance
are set by 200Ω/500Ω resistors in the feedback network.
Small output resistors of 12.5Ω improve the circuit stability
over various load conditions. They also provide a possible
external filtering option, which is often desirable when the
load is an ADC.
LTC6401-8
25Ω
12.5Ω
+OUT 8
50Ω
+
–
Filter resistors of 50Ω are available for additional filtering.
Lowpass/bandpass filters are easily implemented with just
a couple of external components. Moreover, they offer
single-ended 50Ω matching in wideband applications and
no external resistor is needed.
The LTC6401-8 is very flexible in terms of I/O coupling.
It can be AC- or DC-coupled at the inputs, the outputs or
both. Due to the internal connection between input and
output, users are advised to keep input common mode
voltage between 1V and 1.6V for proper operation. If the
inputs are AC-coupled, the input common mode voltage
is automatically biased approximately 250mV above VOCM
and thus no external circuitry is needed for bias. The
LTC6401-8 provides an output common mode voltage
set by VOCM, which allows driving ADC directly without
external components such as transformer or AC coupling
capacitors. The input signal can be either single-ended
or differential with only minor difference in distortion
performance.
500Ω
200Ω
13 +IN
VIN
IN+
OUT–
IN–
OUT+
14 +IN
57.6Ω
25Ω
+OUTF 7
50Ω
15 –IN
500Ω
200Ω
2.7pF
–OUTF 6
12.5Ω
–OUT 5
16 –IN
64018 F01
Figure 1. Input Termination for Differential 50Ω Input Impedance
Using Shunt Resistor
LTC6401-8
25Ω
500Ω
200Ω
12.5Ω
13 +IN
+OUT 8
50Ω
1:4
+
–
VIN
IN+
•
•
• Operation from DC to 2.2GHz –3dB bandwidth
OUT–
14 +IN
402Ω
15 –IN
25Ω
MINI CIRCUITS
TCM4-19
IN–
200Ω
16 –IN
+OUTF 7
50Ω
500Ω
2.7pF
–OUTF 6
OUT+
12.5Ω
–OUT 5
64018 F02
Figure 2. Input Termination for Differential 50Ω Input Impedance
Using a Balun
64018f
9
LTC6401-8
APPLICATIONS INFORMATION
Referring to Figure 3, LTC6401-8 can be easily configured
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 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 322Ω and RT is
59Ω in order to match to a 50Ω source impedance.
RS
50Ω
+
–
LTC6401-8
0.1μF
500Ω
200Ω
12.5Ω
13 +IN
VIN
12.5Ω
13 +IN
1/2 RL
+OUT 8
50Ω
IN+
+
–
VIN
OUT–
+OUTF 7
14 +IN
VOUT
50Ω
15 –IN
1/2 RS
IN–
–OUTF 6
OUT+
500Ω
200Ω
2.7pF
1/2 RL
12.5Ω
16 –IN
–OUT 5
64018 F04
Figure 4. Calculate Differential Gain
and noise is obvious when constant noise figure 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.
+OUT 8
50Ω
RT
59.0Ω
IN+
OUT–
IN–
OUT+
+OUTF 7
14 +IN
0.1μF
50Ω
15 –IN
0.1μF
LTC6401-8
500Ω
200Ω
1/2 RS
200Ω
500Ω
16 –IN
27.4Ω
2.7pF
–OUTF 6
12.5Ω
–OUT 5
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.
64018 F03
Figure 3. Input Termination for Single-Ended 50Ω Input
Impedance
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:
V
RL
1000
A V = OUT =
•
VIN
RS + 400 25 + RL
The internal low pass filter outputs at +OUTF/–OUTF
have a –3dB bandwidth of 590MHz. External capacitors
can reduce the lowpass filter bandwidth as shown in
Figure 5. A bandpass filter is easily implemented with
LTC6401-8
12.5Ω
13 +IN
+OUT 8
8pF
50Ω
IN+
OUT–
+OUTF 7
14 +IN
50Ω
15 –IN
IN–
200Ω
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
500Ω
200Ω
16 –IN
–OUTF 6
OUT+
500Ω
2.7pF
FILTERED OUTPUT
12pF (87.5MHz)
8pF
12.5Ω
–OUT 5
64018 F05
Figure 5. LTC6401-8 Internal Filter Topology Modified for Low
Filter Bandwidth (Three External Capacitors)
64018f
10
LTC6401-8
APPLICATIONS INFORMATION
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.
1.25V
0.1μF
0.1μF
59.0Ω
500Ω
200Ω
LTC6401-8
12.5Ω
13 +IN
0.1μF
39pF
10Ω
4.99Ω
VOCM
4.99Ω
+OUT
+OUTF
LTC6401-8
–OUTF
–IN
–OUT
+IN
IF IN
LTC2208
AIN+
4.99Ω
ENABLE
27.4Ω
8dB GAIN
+OUT 8
VCM
AIN–
LTC2208 130Msps
16-Bit ADC
50Ω
OUT–
64018 F07
+OUTF 7
14 +IN
16nH
50Ω
15 –IN
IN–
200Ω
OUT+
500Ω
16 –IN
1.7pF
LTC2208
–OUTF 6
12.5Ω
Figure 7. Single-Ended Input to LTC6401-8 and LTC2208
39pF
10Ω
–OUT 5
64018 F06
4.99Ω
39pF
Figure 6. LTC6401-8 Modified 165MHz for Bandpass Filtering
(Three External Capacitors, One External Inductor)
Output Common Mode Adjustment
The LTC6401-8’s output common mode voltage is set
by the VOCM pin, which is a high impedance input. The
output common mode voltage is capable of tracking VOCM
in a range from 1V to 1.6V. Bandwidth of VOCM control is
typically 14MHz, which is dominated by a low pass filter
connected to the VOCM pin and is aimed to reduce common 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 VOCM pin should
be tied to a DC bias voltage with a 0.1μF bypass capacitor. When interfacing with 3V A/D converters such as the
LT22xx families, the VOCM pin can be connected to the
VCM pin of the ADC.
Driving A/D Converters
LTC6401-8 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. VOCM
of the LTC6401-8 is connected to VCM 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 LTC6401-8.
Figure 8 summarizes the IMD3 performance of the whole
system as shown in Figure 7.
–40
SINGLE-ENDED INPUT
FS = 122.8Msps
–50 DRIVER V
OUT = 2VP-P COMPOSITE
–60
IMD3 (dBc)
IN+
–70
–80
–90
–100
–110
0
20 40 60 80 100 120 140 160 180 200
FREQUENCY (MHz)
64018 F08
Figure 8. IMD3 for the Combination of LTC6401-8 and LTC2208
The LTC6401-8 has been specifically designed to interface
directly with high speed A/D converters. Figure 7 shows the
64018f
11
LTC6401-8
APPLICATIONS INFORMATION
Test Circuits
Due to the fully-differential design of the LTC6401 and
its usefulness in applications with differing characteristic
specifications, two test circuits are used to generate the
information in this datasheet. Test Circuit A is DC987B,
a two-port demonstration circuit for the LTC6401 family.
The silkscreen is shown in Figure 9. This circuit includes
input and output transformers (baluns) for single-endedto-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 LTC6401 with a 375Ω differential load, optimizing
distortion performance. Due to the input and output transformers, the –3dB bandwidth is reduced from 2.2GHz to
approximately 1.65GHz.
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.
Figure 9. Top Silkscreen for DC987B. Test Circuit A
64018f
12
LTC6401-8
TYPICAL APPLICATIONS
Demo Circuit 987B Schematic (Test Circuit A)
VCC
3 DIS
2 JP1
13
T1
(2)
1
•
6
2
C21
0.1μF
4
3
R3
(2)
C2
0.1μF
14
R24
(1)
SL1
(2)
9
V–
+IN
+OUT
+IN
+OUTF
8
R10
86.6Ω
7
R8
(1)
LTC6401-8
15
C1
0.1μF
16
R1
0Ω
–IN
–OUTF
–IN
–OUT
V+
VOCM
1
VCC
C10
0.1μF
VCC
10
V+
2
V+
C18
0.1μF
6
5
R14
(1)
C4
0.1μF
SL2
(2)
R7
(1)
4
C9
1000pF
R12
0Ω
1
6
R11
(1)
C22
0.1μF
R13
0Ω
T2
TCM 4:19
1:4
2
C3
0.1μF
R9
86.6Ω
V–
3
4
3
•
R5
0dB (1)
R4
(2)
•
J2
–IN
R6
0Ω
C17
1000pF
12
11
V– ENABLE
R2
(1)
J1
+IN
VCC
R16
0Ω
•
ENABLE 1
J4
+OUT
SL3
(2)
J5
–OUT
VCC
C12
1000pF
C13
0.1μF
R19
1.5k
TP5
R17
0Ω
6
T3
TCM 4:19
1:4
•
J6
TEST IN
C7
0.1μF
R20
1k
2
C23
0.1μF
C19
0.1μF
3
3
C5
0.1μF
C20
0.1μF
R22
(1)
C6
0.1μF
2
T4
TCM 4:19
1:4
4
R18
0Ω
6
R26
0Ω
•
4
R21
(1)
C24
0.1μF
•
R25
0Ω
1
•
VOCM
1
J7
TEST OUT
VCC
TP2
VCC
2.85V TO 3.5V
TP3
GND
C14
4.7μF
NOTE: UNLESS OTHERWISE SPECIFIED.
(1) DO NOT STUFF.
C15
1μF
(2) VERSION
-E
IC
LTC6401CUD-8
SL = SIGNAL LEVEL
R3
R4
200Ω 200Ω
T1
SL1
SL2
SL3
MINI-CIRCUITS TCM4-19 (1:4)
6dB
8dB
2dB
64018 TA02
64018f
13
LTC6401-8
TYPICAL APPLICATIONS
Test Circuit B, 4-Port Analysis
V+
1000pF
V–
0.1μF
11
V–
V+
ENABLE
12
10
9
BIAS CONTROL
RF
500Ω
RG
200Ω
+IN
13
PORT 1
(50Ω)
ROUT
12.5Ω
RFILT
50Ω
0.1μF
1/2
AGILENT
E5O71A
+IN
14
133Ω
IN+
PORT 3
(50Ω)
+OUTF
IN–
CFILT
2.7pF
1/2
AGILENT
E5O71A
–OUTF
6
OUT+
RF
500Ω
RG
200Ω
0.1μF
7
OUT–
RFILT
50Ω
–IN
15
–IN
16
PORT 2
(50Ω)
+OUT 37.4Ω
8
ROUT
12.5Ω
–OUT 37.4Ω
PORT 4
(50Ω)
5
0.1μF
0.1μF
COMMON
MODE CONTROL
1
1000pF
2
V+
3
VOCM
0.1μF
VOCM
V+
4
64018 TA03
V–
V+
0.1μF
64018f
14
LTC6401-8
PACKAGE DESCRIPTION
UD Package
16-Lead Plastic QFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1691)
0.70 ±0.05
3.50 ± 0.05
1.45 ± 0.05
2.10 ± 0.05 (4 SIDES)
PACKAGE OUTLINE
0.25 ±0.05
0.50 BSC
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
3.00 ± 0.10
(4 SIDES)
BOTTOM VIEW—EXPOSED PAD
PIN 1 NOTCH R = 0.20 TYP
OR 0.25 × 45° CHAMFER
R = 0.115
TYP
0.75 ± 0.05
15
16
PIN 1
TOP MARK
(NOTE 6)
0.40 ± 0.10
1
1.45 ± 0.10
(4-SIDES)
2
(UD16) QFN 0904
0.200 REF
0.00 – 0.05
NOTE:
1. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE MO-220 VARIATION (WEED-2)
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION
ON THE TOP AND BOTTOM OF PACKAGE
0.25 ± 0.05
0.50 BSC
64018f
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
15
LTC6401-8
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
High-Speed Differential Amplifiers/Differential Op Amps
LT®1993-2
800MHz Differential Amplifier/ADC Driver
AV = 2V/V, OIP3 = 38dBm at 70MHz
LT1993-4
900MHz Differential Amplifier/ADC Driver
AV = 4V/V, OIP3 = 40dBm at 70MHz
LT1993-10
700MHz Differential Amplifier/ADC Driver
AV = 10V/V, OIP3 = 40dBm at 70MHz
LT1994
Low Noise, Low Distortion Differential Op Amp
16-Bit SNR and SFDR at 1MHz, Rail-to-Rail Outputs
LT5514
Ultralow Distortion IF Amplifier/ADC Driver with Digitally
Controlled Gain
OIP3 = 47dBm at 100MHz, Gain Control Range 10.5dB to 33dB
LT5524
Low Distortion IF Amplifier/ADC Driver with Digitally
Controlled Gain
OIP3 = 40dBm at 100MHz, Gain Control Range 4.5dB to 37dB
LTC6400-20
1.8GHz Low Noise, Low Distortion, Differential ADC Driver
AV = 20dB, 90mA Supply Current, IMD3 = –65dBc at 300MHz
LTC6400-26
1.9GHz Low Noise, Low Distortion, Differential ADC Driver
AV = 26dB, 85mA Supply Current, IMD3 = –71dBc at 300MHz
LTC6401-20
1.3GHz Low Noise, Low Distortion, Differential ADC Driver
AV = 20dB, 50mA Supply Current, IMD3 = –74dBc at 140MHz
LTC6401-26
1.6GHz Low Noise, Low Distortion, Differential ADC Driver
AV = 26dB, 45mA Supply Current, IMD3 = –72dBc at 140MHz
LT6402-6
300MHz Differential Amplifier/ADC Driver
AV = 6dB, Distortion < –80dBc at 25MHz
LT6402-12
300MHz Differential Amplifier/ADC Driver
AV = 12dB, Distortion < –80dBc at 25MHz
LT6402-20
300MHz Differential Amplifier/ADC Driver
AV = 20dB, Distortion < –80dBc at 25MHz
LTC6404-1
600MHz Low Noise Differential ADC Driver
en = 1.5nV/√Hz, Rail-to-Rail Outputs
LTC6406
3GHz Rail-to-Rail Input Differential Op Amp
1.6nV/√Hz Noise, –72dBc Distortion at 50MHz, 18mA
LT6411
Low Power Differential ADC Driver/Dual Selectable Gain
Amplifier
16mA Supply Current, IMD3 = –83dBc at 70MHz, AV = 1, –1 or 2
High-Speed Single-Ended Output Op Amps
LT1812/LT1813/ High Slew Rate Low Cost Single/Dual/Quad Op Amps
LT1814
8nV/√Hz Noise, 750V/μs, 3mA Supply Current
LT1815/LT1816/ Very High Slew Rate Low Cost Single/Dual/Quad Op Amps
LT1817
6nV/√Hz Noise, 1500V/μs, 6.5mA Supply Current
LT1818/LT1819
Ultra High Slew Rate Low Cost Single/Dual Op Amps
6nV/√Hz Noise, 2500V/μs, 9mA Supply Current
LT6200/LT6201
Rail-to-Rail Input and Output Low Noise Single/Dual Op Amps
0.95nV/√Hz Noise, 165MHz GBW, Distortion = –80dBc at 1MHz
LT6202/LT6203/ Rail-to-Rail Input and Output Low Noise Single/Dual/Quad
LT6204
Op Amps
1.9nV/√Hz Noise, 3mA Supply Current, 100MHz GBW
LT6230/LT6231/ Rail-to-Rail Output Low Noise Single/Dual/Quad Op Amps
LT6232
1.1nV/√Hz Noise, 3.5mA Supply Current, 215MHz GBW
LT6233/LT6234/ Rail-to-Rail Output Low Noise Single/Dual/Quad Op Amps
LT6235
1.9nV/√Hz Noise, 1.2mA Supply Current, 60MHz GBW
Integrated Filters
LTC1562-2
Very Low Noise, 8th Order Filter Building Block
Lowpass and Bandpass Filters up to 300kHz
LT1568
Very Low Noise, 4th Order Filter Building Block
Lowpass and Bandpass Filters up to 10MHz
LTC1569-7
Linear Phase, Tunable 10th Order Lowpass Filter
Single-Resistor Programmable Cut-Off to 300kHz
LT6600-2.5
Very Low Noise Differential 2.5MHz Lowpass Filter
SNR = 86dB at 3V Supply, 4th Order Filter
LT6600-5
Very Low Noise Differential 5MHz Lowpass Filter
SNR = 82dB at 3V Supply, 4th Order Filter
LT6600-10
Very Low Noise Differential 10MHz Lowpass Filter
SNR = 82dB at 3V Supply, 4th Order Filter
LT6600-15
Very Low Noise Differential 15MHz Lowpass Filter
SNR = 76dB at 3V Supply, 4th Order Filter
LT6600-20
Very Low Noise Differential 20MHz Lowpass Filter
SNR = 76dB at 3V Supply, 4th Order Filter
64018f
16 Linear Technology Corporation
LT 1207 • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
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