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LTC6401-8 2.2GHz Low Noise, Low Distortion Differential ADC Driver for DC-140MHz FEATURES

DESCRIPTION
The LTC(R)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 x 3mm QFN package and operates over the -40C to 85C temperature range.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.
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/Hz Internal Op Amp Noise 12.1dB Noise Figure Differential Inputs and Outputs 400 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 x 3mm x 0.75mm QFN Package
APPLICATIONS

Differential ADC Driver Differential Driver/Receiver Single Ended to Differential Conversion IF Sampling Receivers SAW Filter Interfacing
TYPICAL APPLICATION
3.3V 3.3V 60 C2 0.1F C1 1000pF C3 0.1F VIN R1 59.0 C4 0.1F -IN R2 27.4 V- 1.25V C5 0.1F R3 100 V+ +IN +OUT LTC6401-8 -OUT VOCM COILCRAFT 0603CS L1 RS2 24nH 15 CF1 33pF RS4 10 CF3 33pF RS1 15 CF2 33pF OUTPUT IP3 (dBm) RS3 10 50 40 30 20 10
64018 TA01a
Equivalent Output IP3 vs Frequency
(NOTE 7)
AIN+
VDD LTC2208
AIN-
VCM
LTC2208 130Msps 16-Bit ADC
0 0 20 40 60 80 100 120 140 160 180 200 FREQUENCY (MHz)
64018 TA01b
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LTC6401-8 ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW -IN -IN +IN +IN 12 V- 17 11 ENABLE 10 V+ 9 V- 5 -OUT 6 -OUTF 7 +OUTF 8 +OUT 16 15 14 13 V+ 1 VOCM 2 V+ 3 V- 4
Supply Voltage (VCC - VEE) ......................................3.6V Input Current (Note 2)..........................................10mA Operating Temperature Range (Note 3) ............................................... -40C to 85C Specified Temperature Range (Note 4) ............................................... -40C to 85C Storage Temperature Range................... -65C to 150C Maximum Junction Temperature .......................... 150C
UD PACKAGE 16-LEAD (3mm x 3mm) PLASTIC QFN TJMAX = 150C, JA = 68C/W, JC = 4.2C/W EXPOSED PAD (PIN 17) IS V-, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH LTC6401CUD-8#PBF LTC6401IUD-8#PBF TAPE AND REEL LTC6401CUD-8#TRPBF LTC6401IUD-8#TRPBF PART MARKING* LCCY LCCY PACKAGE DESCRIPTION SPECIFIED TEMPERATURE RANGE 16-Lead (3mm x 3mm) Plastic QFN 0C to 70C 16-Lead (3mm x 3mm) Plastic QFN -40C to 85C
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 LTC6401-8 LTC6401-20 LTC6401-26 LTC6400-20 LTC6400-26 GAIN (dB) 8 20 26 20 26 GAIN (V/V) 2.5 10 20 10 20
Please check each datasheet for complete details.
ZIN (DIFFERENTIAL) () 400 200 50 200 50 ICC (mA) 45 50 45 90 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.
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LTC6401-8 DC ELECTRICAL CHARACTERISTICS + The -denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25C. V = 3V, V = 0V, +IN = -IN = VOCM = 1.25V, ENABLE = 0V, No RL unless otherwise noted.
PARAMETER Gain Gain Temperature Drift Output Swing Low Output Swing High Maximum Differential Output Swing Output Current Drive Input Offset Voltage Input Offset Voltage Drift Input Common Mode Voltage Range, MIN Input Common Mode Voltage Range, MAX Input Resistance (+IN, -IN) Input Capacitance (+IN, -IN) Output Resistance (+OUT, -OUT) Filtered Output Resistance (+OUTF, -OUTF) Filtered Output Capacitance (+OUTF, -OUTF) Common Mode Rejection Ratio Common Mode Gain Output Common Mode Range, MIN
SYMBOL GDIFF TCGAIN VSWINGMIN VSWINGMAX VOUTDIFFMAX IOUT VOS TCVOS IVRMIN IVRMAX RINDIFF CINDIFF ROUTDIFF ROUTFDIFF COUTFDIFF CMRR GCM VOCMMIN VOCMMAX VOSCM TCVOSCM IVOCM ENABLE Pin VIL VIH IIL IIH Power Supply VS IS ISHDN PSRR
CONDITIONS VIN = 400mV Differential VIN = 400mV Differential Each Output, VIN = 1.6V Differential Each Output, VIN = 1.6V Differential 1dB Compressed VOUT > 2VP-P,DIFF Differential Differential

MIN 7.5
TYP 8 -0.5 89
MAX 8.5 170
UNITS dB mdB/C mV V VP-P mA
Input/Output Characteristic
2.3 10 -4
2.42 4.6 4 3 1
mV V/C V V pF pF dB V/V
1.6 Differential Differential, Includes Parasitic Differential Differential Differential, Includes Parasitic Input Common Mode Voltage 1.1V~1.4V VOCM = 1V to 1.6V

340 18 85 36
400 1 25 100 2.7 55 1
460 32 115
Output Common Mode Voltage Control 1 1.1 1.6 1.5 -15 5 3.6 15 0.8 2.4 0.5 1.4 2.85 36 50 3 45 0.8 73.5 4 3.5 60 3 15 V V V V mV V/C A V V A A V mA mA dB
Output Common Mode Range, MAX
Common Mode Offset Voltage Common Mode Offset Voltage Drift VOCM Input Current ENABLE Input Low Voltage ENABLE Input High Voltage ENABLE Input Low Current ENABLE Input High Current Operating Supply Range Supply Current Shutdown Supply Current Power Supply Rejection Ratio (Differential Outputs)
VOCM = 1.1V to 1.5V

ENABLE = 0.8V ENABLE = 2.4V

ENABLE = 0.8V, Input and Output Floating ENABLE = 2.4V, Input and Output Floating V+ = 2.85V to 3.5V

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LTC6401-8 AC ELECTRICAL CHARACTERISTICS
ENABLE = 0V, No RL unless otherwise noted.
SYMBOL -3dBBW 0.5dBBW 0.1dBBW 1/f SR tS1% tOVDR tON tOFF -3dBBWVOCM 10MHz Input Signal HD2,10M/HD3,10M Second/Third Order Harmonic Distortion VOUT = 2VP-P, RL = 200 VOUT = 2VP-P, No RL IMD3,10M OIP3,10M P1dB,10M NF10M eIN,10M eON,10M 70MHz Input Signal HD2,70M/HD3,70M Second/Third Order Harmonic Distortion VOUT = 2VP-P, RL = 200 VOUT = 2VP-P, No RL IMD3,70M OIP3,70M P1dB,70M NF70M eIN,70M eON,70M Third-Order Intermodulation (f1 = 69.5MHz f2 = 70.5MHz) Equivalent Third-Order Output Intercept Point (f1 = 69.5MHz f2 = 70.5MHz) 1dB Compression Point Noise Figure Input Referred Voltage Noise Density Output Referred Voltage Noise Density VOUT = 2VP-P Composite, RL = 200 VOUT = 2VP-P Composite, No RL VOUT = 2VP-P Composite, No RL (Note 7) RL = 375 (Notes 5, 7) RL = 375 (Note 5) Includes Resistors (Short Inputs) Includes Resistors (Short Inputs) -91/-72 -100/-87 -83 -92 50 18.3 12.2 3.2 7.9 dBc dBc dBc dBc dBm dBm dB nV/Hz nV/Hz Third-Order Intermodulation (f1 = 9.5MHz f2 = 10.5MHz) Equivalent Third-Order Output Intercept Point (f1 = 9.5MHz f2 = 10.5MHz) 1dB Compression Point Noise Figure Input Referred Voltage Noise Density Output Referred Voltage Noise Density VOUT = 2VP-P Composite, RL = 200 VOUT = 2VP-P Composite, No RL VOUT = 2VP-P Composite, No RL (Note 7) RL = 375 (Notes 5, 7) RL = 375 (Note 5) Includes Resistors (Short Inputs) Includes Resistors (Short Inputs) -109/-88 -118/-100 -88 -93 50.7 17.8 12.1 3.2 8 dBc dBc dBc dBc dBm dBm dB nV/Hz nV/Hz PARAMETER -3dB Bandwidth Bandwidth for 0.5dB Flatness Bandwidth for 0.1dB Flatness 1/f Noise Corner Slew Rate 1% Settling Time Overdrive Recovery Time Turn-On Time Turn-Off Time VOCM Pin Small Signal -3dB BW VOUT = 2V Step (Note 6) VOUT = 2VP-P (Note 6) VOUT = 1.9VP-P (Note 6) VOUT Within 10% of Final Values ICC Falls to 10% of Nominal 0.1VP-P at VOCM, Measured Single-Ended at Output (Note 6)
Specifications are at TA = 25C. V+ = 3V, V- = 0V, VOCM = 1.25V,
MIN 1 TYP 2.22 0.43 0.22 12.2 3400 2.3 18 79 193 14 MAX UNITS GHz GHz GHz kHz V/s ns ns ns ns MHz
CONDITIONS 200mVP-P,OUT (Note 6) 200mVP-P,OUT (Note 6) 200mVP-P,OUT (Note 6)
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LTC6401-8 AC ELECTRICAL CHARACTERISTICS
ENABLE = 0V, No RL unless otherwise noted.
PARAMETER SYMBOL 140MHz Input Signal HD2,140M/ HD3,140M IMD3,140M OIP3,140M P1dB,140M NF140M eIN,140M eON,140M IMD3,130M/150M Second/Third Order Harmonic Distortion VOUT = 2VP-P, RL = 200 VOUT = 2VP-P, No RL Third-Order Intermodulation (f1 = 139.5MHz f2 = 140.5MHz) Equivalent Third-Order Output Intercept Point (f1 = 139.5MHz f2 = 140.5MHz) 1dB Compression Point Noise Figure Input Referred Voltage Noise Density Output Referred Voltage Noise Density Third-Order Intermodulation (f1 = 130MHz f2 = 150MHz) Measure at 170MHz VOUT = 2VP-P Composite, RL = 200 VOUT = 2VP-P Composite, No RL VOUT = 2VP-P Composite, No RL (Note 7) RL = 375 (Notes 5, 7) RL = 375 (Note 5) Includes Resistors (Short Inputs) Includes Resistors (Short Inputs) VOUT = 2VP-P Composite, RL = 375 -78/-59 -87/-70 -71 -80 44.2 18.7 12.3 3.1 7.9 -75 -67 dBc dBc dBc dBc dBm dBm dB nV/Hz nV/Hz dBc
Specifications are at TA = 25C. V+ = 3V, V- = 0V, VOCM = 1.25V,
MIN TYP MAX UNITS
CONDITIONS
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 -40C to 85C. Note 4: The LTC6401C is guaranteed to meet specified performance from 0C to 70C. It is designed, characterized and expected to meet specified
performance from -40C to 85C but is not tested or QA sampled at these temperatures. The LTC6401I is guaranteed to meet specified performance from -40C to 85C. Note 5: Input and output baluns used. See Test Circuit A. Note 6: Measured using Test Circuit B. RL = 87.5 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 50 output load, the LTC6401-8 output voltage swing driving a given RL is converted to OIP3 and P1dB as if it were driving a 50 load. Using this modified convention, 2VP-P is by definition equal to 10dBm, regardless of actual RL.
TYPICAL PERFORMANCE CHARACTERISTICS
Frequency Response
14 12 10 8 GAIN (dB) 6 4 2 0 -2 -4 -6 10 100 1000 FREQUENCY (MHz) 3000
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Gain 0.1dB Flatness
TEST CIRCUIT B 1.0 0.8 0.6 GAIN FLATNESS (dB) PHASE (DEGREE) 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1.0 10 100 1000 FREQUENCY (MHz) 3000
64018 G02
S21 Phase and Group Delay vs Frequency
0 TEST CIRCUIT B 0.7
TEST CIRCUIT B
-50
0.6 GROUP DELAY (ns)
-100
0.5
-150 PHASE GROUP DELAY 0 200 400 600 FREQUENCY (MHz) 800
0.4
-200
0.3 1000
64018 G03
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LTC6401-8 TYPICAL PERFORMANCE CHARACTERISTICS
Input and Output Reflection and Reverse Isolation vs Frequency
0 -10 S PARAMETERS (dB) -20 -30 -40 -50 -60 S12 -70 -80 10 100 1000 FREQUENCY (MHz) 3000
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Input and Output Impedance vs Frequency
500 450 IMPEDANCE MAGNITUDE () 400 350 300 250 200 150 100 50 0 10 100 FREQUENCY (MHz) ZOUT ZIN ZOUT ZIN PHASE IMPEDANCE MAGNITUDE 100 80 60 PSRR, CMRR (dB) 40 20 0 -20 -40 -60 -80 -100 1000
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PSRR and CMRR vs Frequency
80 PSRR 70 60 PHASE (DEGREES) 50 40 30 20 10 0 1 10 100 FREQUENCY (MHz) 1000
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TEST CIRCUIT B
S11
CMRR
S22
Noise Figure and Input Referred Noise Voltage vs Frequency
20 5 1.35 INPUT REFERRED NOISE VOLTAGE (nV/Hz)
Small Signal Transient Response
RL = 87.5 PER OUTPUT TEST CIRCUIT B 2.5
Large Signal Transient Response
RL = 87.5 PER OUTPUT TEST CIRCUIT B
18 NOISE FIGURE (dB) EN 16
4
2.0 OUTPUT VOLTAGE (V) +OUT OUTPUT VOLTAGE (V) 1.30 +OUT 1.5
3
1.25
14 NOISE FIGURE 12
2
1.0 -OUT 0.5
1.20
1
-OUT
10 10 100 FREQUENCY (MHz)
0 1000
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1.15 0 2 4 6 TIME (ns) 8 10
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0 0 4 8 12 TIME (ns) 16 20
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Overdrive Recovery Response
4 3 2 INPUT VOLTAGE (V) 1 0 -1 R = 87.5 PER OUTPUT L -2 -3 -4 -5 -6 TEST CIRCUIT B 0 25 50 75 TIME (ns) -OUT 100 -IN +OUT +IN 5.0 4.5 4.0 OUTPUT VOLTAGE (V) 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 125
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1% Settling Time for 2V Output Step
5 4 3 2 SETTLING (%) 1 0 -1 -2 -3 -4 -5 0 1 2 3 TIME (ns) 4 5
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Harmonic Distortion vs Frequency
-40 -50 -60 -70 -80 -90 -100 -110 0 DIFFERENTIAL INPUT VOUT = 2VP-P
RL = 87.5 PER OUTPUT TEST CIRCUIT B HARMONIC DISTORTION (dBc)
HD2 NO RL HD2 200 RL HD3 NO RL HD3 200 RL 20 40 60 80 100 120 140 160 180 200 FREQUENCY (MHz)
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LTC6401-8 TYPICAL PERFORMANCE CHARACTERISTICS
Third Order Intermodulation Distortion vs Frequency
-40 -50 THIRD ORDER IMD (dBc) -60 200 RL -70 -80 NO RL -90 -100 -110 0 20 40 60 80 100 120 140 160 180 200 FREQUENCY (MHz)
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Harmonic Distortion vs Frequency
-40 -50 -60 -70 -80 -90 -100 -110 0 SINGLE-ENDED INPUT VOUT = 2VP-P THIRD ORDER IMD (dBc) -40 -50 -60 -70 -80 -90 -100 -110
Third Order Intermodulation Distortion vs Frequency
SINGLE-ENDED INPUT VOUT = 2VP-P COMPOSITE
DIFFERENTIAL INPUT VOUT = 2VP-P COMPOSITE HARMONIC DISTORTION (dBc)
200 RL
NO RL
HD2 NO RL HD2 200 RL HD3 NO RL HD3 200 RL 20 40 60 80 100 120 140 160 180 200 FREQUENCY (MHz)
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0
20 40 60 80 100 120 140 160 180 200 FREQUENCY (MHz)
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Equivalent Output 1dB Compression Point vs Frequency
20 OUTPUT 1dB COMPRESSION POINT (dBm) DIFFERENTIAL INPUT RL = 375 TEST CIRCUIT A (NOTE 7) OUTPUT IP3 (dBm) 60 50 40 30 20 10 0 0 20 40 60 80 100 120 140 160 180 200 FREQUENCY (MHz)
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Equivalent Output Third Order Intercept Point vs Frequency
19
NO RL
18
200 RL
17
16
15
DIFFERENTIAL INPUT (NOTE 7) 0 20 40 60 80 100 120 140 160 180 200 FREQUENCY (MHz)
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Turn-On Time
3.5 3.0 2.5 VOLTAGE (V) 2.0 1.5 1.0 0.5 0 -0.5 -100 RL = 87.5 PER OUTPUT 0 100 200 300 TIME (ns) ENABLE 400 -10 500 +OUT -OUT ICC 70 60 50 40 30 20 10 0 VOLTAGE (V) ICC (mA) 3.5 3.0 2.5 2.0
Turn-Off Time
RL = 87.5 PER OUTPUT 70 60 50 40 +OUT 1.5 1.0 0.5 0 ENABLE 0 100 200 300 TIME (ns) ICC 400 -OUT 30 20 10 0 -10 500 ICC (mA)
-0.5 -100
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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 0.1F capacitors as close to the pins as possible. VOCM (Pin 2): This pin sets the output common mode voltage. A 0.1F 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 12.5. -OUTF, +OUTF (Pins 6, 7): Filtered Outputs. These pins have 50 series resistors and a 2.7pF shunt capacitor. ENABLE (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 11 ENABLE 10 V+ 9 V-
BIAS CONTROL +IN 13 +IN 14 -IN 15 -IN 16 RG 200 RG 200 RF 500 ROUT 12.5 8 RFILT 50 IN+ OUT- RFILT 50 IN- OUT+ RF 500 2k 5.3pF ROUT 12.5 5 COMMON MODE CONTROL CFILT 2.7pF 6 -OUT 7 -OUTF +OUTF +OUT
1
V+
2 VOCM
3
V+
4
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V-
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LTC6401-8 APPLICATIONS INFORMATION
Circuit Operation The LTC6401-8 is a low noise and low distortion fully differential op amp/ADC driver with: * Operation from DC to 2.2GHz -3dB bandwidth * Fixed gain of 2.5V/V (8dB) * Differential input impedance 400 * Differential output impedance 25 * Differential impedance of output filter 100 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 200/500 resistors in the feedback network. Small output resistors of 12.5 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. Filter resistors of 50 are available for additional filtering. Lowpass/bandpass filters are easily implemented with just a couple of external components. Moreover, they offer single-ended 50 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. Input Impedance and Matching The differential input impedance of the LTC6401-8 is 400. Usually the differential inputs need to be terminated to a lower value impedance, e.g. 50, 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.
LTC6401-8 25 13 +IN 50 IN+ OUT- 50 IN- 200 16 -IN OUT+ 500 12.5 -OUT 5
64018 F01
200
500
12.5 +OUT 8
+ -
VIN
14 +IN 57.6 15 -IN
+OUTF 7 2.7pF -OUTF 6
25
Figure 1. Input Termination for Differential 50 Input Impedance Using Shunt Resistor
LTC6401-8 25 13 +IN 1:4 50 IN+ OUT- 50 IN- 200 16 -IN MINI CIRCUITS TCM4-19 OUT+ 500 12.5 -OUT 5
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200
500
12.5 +OUT 8
25
Figure 2. Input Termination for Differential 50 Input Impedance Using a Balun
*
+ -
VIN
*
14 +IN 402 15 -IN
+OUTF 7 2.7pF -OUTF 6
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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 50, it is found that the single-ended input impedance is 322 and RT is 59 in order to match to a 50 source impedance.
RS 50 0.1F 13 +IN 50 IN+ 14 +IN 0.1F 15 -IN 0.1F 16 -IN 27.4 200 IN- OUT+ 500 12.5 -OUT 5
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LTC6401-8 1/2 RS 13 +IN 50 IN+ OUT- 50 15 -IN 1/2 RS 16 -IN 200 IN- OUT+ 500 12.5 -OUT 5
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200
500
12.5 +OUT 8
1/2 RL
+ -
VIN
14 +IN
+OUTF 7 VOUT 2.7pF -OUTF 6 1/2 RL
Figure 4. Calculate Differential Gain
LTC6401-8 200 500 12.5 +OUT 8 OUT- 50
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. Output Impedance Match and Filter The LTC6401-8 can drive an ADC directly without external output impedance matching. Alternatively, the differential output impedance of 25 can be made larger, e.g. 50, by series resistors or LC network. 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
LTC6401-8 200 13 +IN 50 IN+ 14 +IN 50 15 -IN 200 IN- OUT+ 500 12.5 -OUT 5
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+ -
VIN RT 59.0
+OUTF 7 2.7pF -OUTF 6
Figure 3. Input Termination for Single-Ended 50 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 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
500
12.5 +OUT 8 8pF +OUTF 7 2.7pF -OUTF 6 FILTERED OUTPUT 12pF (87.5MHz) 8pF
OUT-
16 -IN
Figure 5. LTC6401-8 Internal Filter Topology Modified for Low Filter Bandwidth (Three External Capacitors)
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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.
LTC6401-8 12.5 +OUT 8 50 IN+ 14 +IN 50 15 -IN 200 16 -IN IN- OUT+ 500 12.5 -OUT 5
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1.25V 0.1F
0.1F IF IN 59.0 0.1F +IN
VOCM +OUT +OUTF LTC6401-8 -OUTF -IN -OUT ENABLE 8dB GAIN
4.99
AIN-
VCM LTC2208
39pF 10 4.99
27.4
AIN+ 4.99 LTC2208 130Msps 16-Bit ADC
64018 F07
200 13 +IN
500
OUT-
+OUTF 7 16nH 1.7pF -OUTF 6 10 39pF 4.99 39pF LTC2208
Figure 7. Single-Ended Input to LTC6401-8 and LTC2208
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 0.1F 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 The LTC6401-8 has been specifically designed to interface directly with high speed A/D converters. Figure 7 shows the
LTC6401-8 with single-ended input driving the LTC2208, which is a 16-bit, 130Msps ADC. Two external 5 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) -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
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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 375 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
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12
LTC6401-8 TYPICAL APPLICATIONS
Demo Circuit 987B Schematic (Test Circuit A)
VCC ENABLE 1 3 DIS 2 JP1 R16 0 VCC C17 1000pF C18 0.1F
R2 (1) R6 0 T1 (2) R4 (2) C2 0.1F R24 (1) C1 0.1F SL1 (2)
12 11 V- ENABLE 13 6 1 2 +IN
10 V+
9 V- +OUT 8
R10 86.6 R8 (1) R7 (1) R9 86.6 C3 0.1F C22 0.1F VCC C4 0.1F SL2 (2) T2 TCM 4:19 1:4
R14 (1) 3 2 1 4 R12 0
*
J2 -IN
R5 0dB (1)
LTC6401-8 15 -IN -OUTF 6
*
J1 +IN
C21 0.1F
14
+IN
+OUTF
7
J4 +OUT SL3 (2) J5 -OUT
4
3
R3 (2)
6
R11 (1)
*
*
16 R1 0 VCC
-IN V+ 1 VOCM 2 V+ 3
-OUT V- 4
5
R13 0
VCC R19 1.5k TP5 VOCM R20 1k R17 0 T3 TCM 4:19 1:4
C10 0.1F
C9 1000pF
C12 1000pF
C13 0.1F
C7 0.1F T4 TCM 4:19 1:4 R18 0
*
R25 0
2
4
3
C24 0.1F
R21 (1)
C6 0.1F
R22 (1)
2 1
*
J6 TEST IN
6
1 C19 0.1F
C23 0.1F
C5 0.1F
3 C20 0.1F
4
J7 TEST OUT
6
R26 0
*
VCC
*
TP2 VCC 2.85V TO 3.5V TP3 GND
C14 4.7F
C15 1F (2) VERSION -E SL = SIGNAL LEVEL IC LTC6401CUD-8 R3 R4 T1 MINI-CIRCUITS TCM4-19 (1:4) SL1 6dB SL2 8dB SL3 2dB 200 200
NOTE: UNLESS OTHERWISE SPECIFIED. (1) DO NOT STUFF.
64018 TA02
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13
LTC6401-8 TYPICAL APPLICATIONS
Test Circuit B, 4-Port Analysis
V+ 1000pF 0.1F
V- 12 11
ENABLE 10
V+ 9
V-
BIAS CONTROL +IN 13 0.1F +IN 14 133 -IN 15 -IN 16 0.1F COMMON MODE CONTROL 1 2 VOCM 0.1F VOCM 0.1F 3 4
64018 TA03
PORT 1 (50)
RG 200
RF 500
ROUT 12.5 8 RFILT 50
+OUT 37.4 +OUTF 7 CFILT 2.7pF 6 0.1F
PORT 3 (50)
IN+
OUT- RFILT 50
1/2 AGILENT E5O71A
-OUTF
1/2 AGILENT E5O71A
IN- RG 200
OUT+ RF 500 ROUT 12.5
PORT 2 (50)
-OUT 37.4 5 0.1F
PORT 4 (50)
V+
V+
V-
1000pF
V+
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14
LTC6401-8 PACKAGE DESCRIPTION
UD Package 16-Lead Plastic QFN (3mm x 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 0.75 0.05 BOTTOM VIEW--EXPOSED PAD R = 0.115 TYP 15 16 0.40 0.10 1 1.45 0.10 (4-SIDES) 2 PIN 1 NOTCH R = 0.20 TYP OR 0.25 x 45 CHAMFER
3.00 0.10 (4 SIDES) PIN 1 TOP MARK (NOTE 6)
(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
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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 LT(R)1993-2 LT1993-4 LT1993-10 LT1994 LT5514 LT5524 LTC6400-20 LTC6400-26 LTC6401-20 LTC6401-26 LT6402-6 LT6402-12 LT6402-20 LTC6404-1 LTC6406 LT6411 DESCRIPTION 800MHz Differential Amplifier/ADC Driver 900MHz Differential Amplifier/ADC Driver 700MHz Differential Amplifier/ADC Driver Low Noise, Low Distortion Differential Op Amp Ultralow Distortion IF Amplifier/ADC Driver with Digitally Controlled Gain Low Distortion IF Amplifier/ADC Driver with Digitally Controlled Gain 1.8GHz Low Noise, Low Distortion, Differential ADC Driver 1.9GHz Low Noise, Low Distortion, Differential ADC Driver 1.3GHz Low Noise, Low Distortion, Differential ADC Driver 1.6GHz Low Noise, Low Distortion, Differential ADC Driver 300MHz Differential Amplifier/ADC Driver 300MHz Differential Amplifier/ADC Driver 300MHz Differential Amplifier/ADC Driver 600MHz Low Noise Differential ADC Driver 3GHz Rail-to-Rail Input Differential Op Amp Low Power Differential ADC Driver/Dual Selectable Gain Amplifier COMMENTS AV = 2V/V, OIP3 = 38dBm at 70MHz AV = 4V/V, OIP3 = 40dBm at 70MHz AV = 10V/V, OIP3 = 40dBm at 70MHz 16-Bit SNR and SFDR at 1MHz, Rail-to-Rail Outputs OIP3 = 47dBm at 100MHz, Gain Control Range 10.5dB to 33dB OIP3 = 40dBm at 100MHz, Gain Control Range 4.5dB to 37dB AV = 20dB, 90mA Supply Current, IMD3 = -65dBc at 300MHz AV = 26dB, 85mA Supply Current, IMD3 = -71dBc at 300MHz AV = 20dB, 50mA Supply Current, IMD3 = -74dBc at 140MHz AV = 26dB, 45mA Supply Current, IMD3 = -72dBc at 140MHz AV = 6dB, Distortion < -80dBc at 25MHz AV = 12dB, Distortion < -80dBc at 25MHz AV = 20dB, Distortion < -80dBc at 25MHz en = 1.5nV/Hz, Rail-to-Rail Outputs 1.6nV/Hz Noise, -72dBc Distortion at 50MHz, 18mA 16mA Supply Current, IMD3 = -83dBc at 70MHz, AV = 1, -1 or 2 High-Speed Differential Amplifiers/Differential Op Amps
High-Speed Single-Ended Output Op Amps LT1812/LT1813/ High Slew Rate Low Cost Single/Dual/Quad Op Amps LT1814 LT1815/LT1816/ Very High Slew Rate Low Cost Single/Dual/Quad Op Amps LT1817 LT1818/LT1819 LT6200/LT6201 Ultra High Slew Rate Low Cost Single/Dual Op Amps Rail-to-Rail Input and Output Low Noise Single/Dual Op Amps 8nV/Hz Noise, 750V/s, 3mA Supply Current 6nV/Hz Noise, 1500V/s, 6.5mA Supply Current 6nV/Hz Noise, 2500V/s, 9mA Supply Current 0.95nV/Hz Noise, 165MHz GBW, Distortion = -80dBc at 1MHz 1.9nV/Hz Noise, 3mA Supply Current, 100MHz GBW 1.1nV/Hz Noise, 3.5mA Supply Current, 215MHz GBW 1.9nV/Hz Noise, 1.2mA Supply Current, 60MHz GBW
LT6202/LT6203/ Rail-to-Rail Input and Output Low Noise Single/Dual/Quad LT6204 Op Amps LT6230/LT6231/ Rail-to-Rail Output Low Noise Single/Dual/Quad Op Amps LT6232 LT6233/LT6234/ Rail-to-Rail Output Low Noise Single/Dual/Quad Op Amps LT6235 Integrated Filters LTC1562-2 LT1568 LTC1569-7 LT6600-2.5 LT6600-5 LT6600-10 LT6600-15 LT6600-20 Very Low Noise, 8th Order Filter Building Block Very Low Noise, 4th Order Filter Building Block Linear Phase, Tunable 10th Order Lowpass Filter Very Low Noise Differential 2.5MHz Lowpass Filter Very Low Noise Differential 5MHz Lowpass Filter Very Low Noise Differential 10MHz Lowpass Filter Very Low Noise Differential 15MHz Lowpass Filter Very Low Noise Differential 20MHz Lowpass Filter
Lowpass and Bandpass Filters up to 300kHz Lowpass and Bandpass Filters up to 10MHz Single-Resistor Programmable Cut-Off to 300kHz SNR = 86dB at 3V Supply, 4th Order Filter SNR = 82dB at 3V Supply, 4th Order Filter SNR = 82dB at 3V Supply, 4th Order Filter SNR = 76dB at 3V Supply, 4th Order Filter SNR = 76dB at 3V Supply, 4th Order Filter
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16 Linear Technology Corporation
(408) 432-1900 FAX: (408) 434-0507
LT 1207 * PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2007

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