High Speed ADC Products Brochure

11.2007
High Speed ADC Products
High Performance Analog ICs
High Speed ADC Products
Contents
01
Introduction
02
LTC2209 High Performance ADC Family
03
LTC2207 High Performance ADC Family
04
LTC2220-1 and LTC2242-12 High IF Sampling ADC Families
05
LTC2224 High IF Sampling ADC Family
06
LTC2255 Lowest Power ADC Family
07
LTC2285 Lowest Power Dual ADC Family
08
LTC1750 High Performance 5V ADC Family
09
Fully Differential ADC Drivers
11
CMOS vs LVDS Outputs
Lowest Power High Speed ADCs
16-Bit
14-Bit
12-Bit
10-Bit
125Msps to
135Msps
160Msps to
185Msps
10Msps
25Msps
40Msps
65Msps
80Msps
105Msps
2202
2203
2204
2205
2206
2207
2208
2209
2205-14
2206-14
2207-14
2208-14
2209-14
2245
2246
2247
2248
2249
2254
2255
2295
2296
2297
2298
2299
2284
2285
2223
2222
2224
2225
2226
2227
2228
2229
2252
2253
2290
2291
2292
2293
2294
2282
2283
2233
2232
2240-12
2220-1
2221
2237
2238
2239
2250
2251
2285
2287
2288
2289
2280
2281
2241-12
2242-10
2240-10
2231
2242-12
2220
2234
2236
210Msps and
250Msps
2230
2241-10
Lowest Power High Speed ADCs
Introduction
Our high speed ADCs offer the lowest power consumption and highest AC performance in the industry.
They incorporate many unique features to ease system design and space constraints, including integrated
bypass capacitance and integrated series resistance for 50ohm digital outputs. Each ADC family offers the
ability to operate the digital output supply as low as 0.5V, helping to reduce digital feedback and allowing
direct connection to low voltage digital interfaces.
The entire ADC portfolio is supported with evaluation boards and a free Quick Eval-II PScope software tool
that can be downloaded from our website at www.linear.com/company/software.jsp. The following tables
provide a demo board selection guide for ADC evaluation.
DC 851
DC 996
DC 890
02
LT C 2 2 0 9 A D C Family
LTC2209 High Performance ADC Family
•
Internal Transparent Dither
•
Data Output Randomizer
•
Single 3.3V Supply
•
PGA Front End (1.5VP-P or 2.25VP-P )
•
LVDS or CMOS Outputs
•
Data Ready Output Clock
•
Optional Clock Duty Cycle Stabilizer
•
64-pin 9mm × 9mm QFN Package
left: DC 996, right: DC 890
The LTC2209 is our highest performance ADC family, targeting the most demanding wideband, low noise, signal acquisition applications,
while still offering the lowest power in the industry.
The LTC2209 and LTC2207 families incorporate two unique features that simplify digital receiver design and improve system performance.
The first is an internal transparent dither circuit that improves the ADC’s spurious free dynamic range (SFDR) response well beyond
100dBc for low level input signals. The second feature is a digital output randomizer that dramatically reduces unwanted tones caused by
digital feedback.
The DC854D demo board family offers CMOS output evaluation, while DC996B is configured for LVDS outputs.
Applications: Software Defined Radio, Spectrum Analyzer, Oscilloscope, MRI, GPS, Avionics Instrumentation, Radar and Satellite
Communications, Servowriter, CCD Tester, RFID, Disk Drive Tester, Radiography, RF Power Meter
Part Number
Resolution
Speed
SNR
SFDR
Full Power
Bandwidth
Power
Dissipation
LTC2209
16-Bit
160Msps
77.1dB
100dB
700MHz
1450mW
LTC2208
16-Bit
130Msps
77.7dB
100dB
700MHz
1250mW
LTC2209-14
14-Bit
160Msps
76.8dB
98dB
700MHz
1450mW
LTC2208-14
14-Bit
130Msps
77.1dB
98dB
700MHz
1320mW
CLKOUT
OF
D15
CLKOUT
ANALOG
INPUT
AIN +
AIN -
LTC2208
ANALOG
SUMMATION
Σ
S/H
AMP
16-BIT
PIPELINED
ADC CORE
DIGITAL
SUMMATION
OF
OUTPUT
DRIVERS
D15
CLKOUT
OF
D15/DO
DO
D14
CLOCK/DUTY
CYCLE
CONTROL
PRECISION
DAC
MULTIBIT DEEP
PSEUDO-RANDOM
NUMBER
GENERATOR
D2
RAND = HIGH,
SCRAMBLE,
ENABLED
D1
D14/DO
D2/DO
D1/DO
}
DITH
ENC+
ENC-
DITHER ENABLE
HIGH = DITHER ON
LOW = DITHER OFF
DO
DO
LT C 2 2 0 7 A D C Family
LTC2207 High Performance ADC Family
•
Internal Transparent Dither
•
Data Output Randomizer
•
Single 3.3V Supply
•
PGA Front End
•
CMOS Outputs
•
Data Ready Output Clock
•
Clock Duty Cycle Stabilizer
•
48-pin 7mm × 7mm QFN Package
left: DC 919, right: DC 718
The LTC2202 through LTC2207 family of high speed ADCs use the DC718B USB board to interface between the demo board and a PC.
Currently, two demo boards are available. The DC918C, with its AC-coupled analog inputs and differential clock input, targets the
LTC2204 through the LTC2207. The DC919A offers the choice of DC- or AC-coupled analog inputs and has a single-ended clock input for
the LTC2202/LTC2203. The LTC2205-LTC2207 ADCs can be evaluated using the DC-coupled DC919A, but the single-ended clock
will slightly degrade performance.
Applications: Oscilloscope, MRI, IR and CCD Cameras, Bomb Detection, Spectroscopy, Disk Drive Tester
Part Number
Resolution
Speed
SNR
SFDR
Full Power
Bandwidth
Power
Dissipation
LTC2207
16-Bit
105Msps
77.9dB
100dB
700MHz
900mW
LTC2206
16-Bit
80Msps
77.9dB
100dB
700MHz
725mW
LTC2205
16-Bit
65Msps
79.0dB
100dB
700MHz
610mW
LTC2204
16-Bit
40Msps
79.1dB
100dB
700MHz
480mW
LTC2203
16-Bit
25Msps
81.6dB
100dB
380MHz
220mW
LTC2202
16-Bit
10Msps
81.6dB
100dB
380MHz
140mW
LTC2207-14
14-Bit
105Msps
77.3dB
98dB
700MHz
947mW
LTC2206-14
14-Bit
80Msps
77.3dB
98dB
700MHz
762mW
LTC2205-14
14-Bit
65Msps
78.3dB
98dB
700MHz
600mW
03
04
02
LT C 2 2 2 0 - 1 and LT C 2 2 4 2 - 1 2 Families
LTC2220-1 and LTC2242-12 High IF Sampling ADC Families
•
Single 3.3V or 2.5V Supply
•
LVDS or CMOS Outputs
•
Data Ready Output Clock
•
Selectable 1VP-P or 2VP-P Input Range
•
Clock Duty Cycle Stabilizer
•
64-pin 9mm × 9mm QFN Package
left: DC 997, right: DC 890
The 3.3V LTC2220-1 family is pin-compatible with the 2.5V LTC2242-12 family of ADCs, sampling up to 250Msps at 12 bits.
The LTC2242-12 offers higher sampling rates at lower power dissipation for multichannel applications. With a 1.2GHz full power
bandwidth, undersampling at IFs as high as 1GHz are achievable. The CMOS outputs can be demultiplexed by a factor of two to
reduce the data rate for data collection and processing in FPGAs.
Applications: Cable Headend Systems, CATV Return Path Digitizer, Power Amplifier Linearization, Data Acquisition, Disk Drive Tester,
Multichannel Receivers, Radar and Jammers, GPS, High Definition Video, Laser Range Finders, Telemetry, Frequency Synthesizer,
BTS Microwave Link, Wafer Defect Detection
Part Number
Resolution
Speed
SNR
SFDR
Full Power
Bandwidth
Power
Dissipation
LTC2220-1
12-Bit
185Msps
67.7dB
80dB
775MHz
910mW
LTC2220
12-Bit
170Msps
67.7dB
84dB
775MHz
890mW
LTC2221
12-Bit
135Msps
67.8dB
84dB
775MHz
660mW
LTC2230
10-Bit
170Msps
61.2dB
78dB
775MHz
890mW
LTC2231
10-Bit
135Msps
61.2dB
78dB
775MHz
660mW
Part Number
Resolution
Speed
SNR
SFDR
Full Power
Bandwidth
Power
Dissipation
LTC2242-12
12-Bit
250Msps
65.4dB
78dB
1.2GHz
740mW
LTC2241-12
12-Bit
210Msps
65.5dB
78dB
1.2GHz
585mW
LTC2240-12
12-Bit
170Msps
65.6dB
80dB
1.2GHz
445mW
LTC2242-10
10-Bit
250Msps
60.6dB
78dB
1.2GHz
740mW
LTC2241-10
10-Bit
210Msps
60.6dB
78dB
1.2GHz
585mW
LTC2240-10
10-Bit
170Msps
60.6dB
80dB
1.2GHz
445mW
LT C 2 2 2 4 A D C Family
LTC2224 High IF Sampling ADC Family
•
Single 3.3V Supply
•
CMOS Outputs
•
Data Ready Output Clock
•
Selectable 1VP-P or 2VP-P Input Range
•
Clock Duty Cycle Stabilizer
•
48-pin 7mm × 7mm QFN Package
left: DC 751, right: DC 718
This high speed family is optimized for multicarrier wireless basestation transceiver applications for all major air interfaces including GSM,
CDMA, WCDMA (UMTS), TD-SCDMA and WiMAX.
Applications: PA Linearization, WiMAX Radio, Satellite Communications, CATV Return Path, Wireless Modem, Flow and Jitter Meters,
Missile Guidance System, Plasma Generator, DVD Equipment, Battery Monitor, Fiber Optic Gyro, Blood Analyzer, Digital Oscilloscope
Part Number
Resolution
Speed
SNR
SFDR
Full Power
Bandwidth
Power
Dissipation
LTC2224
12-Bit
135Msps
67.6dB
84dB
775MHz
630mW
LTC2222
12-Bit
105Msps
68.4dB
84dB
775MHz
475mW
LTC2223
12-Bit
80Msps
68.5dB
84dB
775MHz
366mW
LTC2222-11
11-Bit
105Msps
65.7dB
84dB
775MHz
475mW
LTC2234
10-Bit
135Msps
61.2dB
78dB
775MHz
630mW
LTC2232
10-Bit
105Msps
61.3dB
78dB
775MHz
475mW
LTC2233
10-Bit
80Msps
61.3dB
78dB
775MHz
366mW
03
05
06
02
LT C 2 2 5 5 A D C Family
LTC2255 Lowest Power ADC Family
•
Single 3V Supply
•
CMOS Outputs
•
Single-Ended Clock Input
•
Selectable 1VP-P or 2VP-P Input Range
•
Clock Duty Cycle Stabilizer
•
32-pin 5mm × 5mm QFN Package
left: DC 782, right: DC 718
The combination of high performance and low power makes this family ideal for battery-powered, test and measurement equipment.
Applications: WiMAX, Software Defined Radio, CCD and Thermal Imaging, MRI Equipment, Industrial Camera, Ultrasound, Radar,
Spectroscopy, Handheld Instrumentation, RFID
Part Number
Resolution
Speed
SNR
SFDR
Full Power
Bandwidth
Power
Dissipation
LTC2255
14-Bit
125Msps
72.5dB
88dB
640MHz
395mW
LTC2254
14-Bit
105Msps
72.4dB
88dB
640MHz
320mW
LTC2249
14-Bit
80Msps
73.0dB
90dB
575MHz
222mW
LTC2248
14-Bit
65Msps
74.3dB
90dB
575MHz
204mW
LTC2247
14-Bit
40Msps
74.4dB
90dB
575MHz
120mW
LTC2246
14-Bit
25Msps
74.5dB
90dB
575MHz
75mW
LTC2245
14-Bit
10Msps
74.4dB
90dB
575MHz
60mW
LTC2253
12-Bit
125Msps
70.2dB
88dB
640MHz
395mW
LTC2252
12-Bit
105Msps
70.2dB
88dB
640MHz
320mW
LTC2229
12-Bit
80Msps
70.6dB
90dB
575MHz
211mW
LTC2228
12-Bit
65Msps
71.3dB
90dB
575MHz
204mW
LTC2227
12-Bit
40Msps
71.4dB
90dB
575MHz
120mW
LTC2226
12-Bit
25Msps
71.4dB
90dB
575MHz
75mW
LTC2225
12-Bit
10Msps
71.3dB
90dB
575MHz
60mW
LTC2251
10-Bit
125Msps
61.6dB
85dB
640MHz
395mW
LTC2250
10-Bit
105Msps
61.6dB
85dB
640MHz
320mW
LTC2239
10-Bit
80Msps
61.6dB
85dB
575MHz
211mW
LTC2238
10-Bit
65Msps
61.8dB
85dB
575MHz
204mW
LTC2237
10-Bit
40Msps
61.8dB
85dB
575MHz
120mW
LTC2236
10-Bit
25Msps
61.8dB
85dB
575MHz
75mW
LT C 2 2 8 5 D ual A D C Family
LTC2285 Lowest Power Dual ADC Family
•
Single 3V Supply
•
-110dB Channel-to-Channel Crosstalk
•
CMOS Outputs
•
Single-Ended Clock Input
•
Selectable 1VP-P or 2VP-P Input Range
•
Clock Duty Cycle Stabilizer
•
64-pin 9mm × 9mm QFN Package
left: DC 851, right: DC 890
The LTC2285 dual ADC family offers exceptionally low crosstalk between channels of -110dB, the best performance in the industry.
Each ADC has a separate single-ended input clock, which is combined on the evaluation boards for ease of evaluation. The duals can be
evaluated in their multiplexed output bus configuration using the DC816 family, but the MUX feature is limited to 65Msps per channel.
The DC851 family demultiplexed (parallel) output demo board allows for evaluation of the entire family of dual ADCs.
Applications: Antenna Diversity, RFID, WiMAX Basestation, Phased Array Multichannel Radio, Non-Destructive Testing, Software Defined
Radio, Collision Avoidance Systems, Wireless Microphone, Night Vision Goggles, NTSC Video Digitizer
Part Number
Resolution
Speed
SNR
SFDR
Full Power
Bandwidth
Power
Dissipation
LTC2285
14-Bit
125Msps
72.4dB
88dB
640MHz
790mW
LTC2284
14-Bit
105Msps
72.4dB
88dB
575MHz
540mW
LTC2299
14-Bit
80Msps
73.0dB
90dB
575MHz
444mW
LTC2298
14-Bit
65Msps
74.3dB
90dB
575MHz
400mW
LTC2297
14-Bit
40Msps
74.4dB
90dB
575MHz
235mW
LTC2296
14-Bit
25Msps
74.5dB
90dB
575MHz
150mW
LTC2295
14-Bit
10Msps
74.4dB
90dB
575MHz
120mW
LTC2283
12-Bit
125Msps
70.2dB
88dB
640MHz
790mW
LTC2282
12-Bit
105Msps
70.1dB
88dB
575MHz
540mW
LTC2294
12-Bit
80Msps
70.6dB
90dB
575MHz
422mW
LTC2293
12-Bit
65Msps
71.3dB
90dB
575MHz
400mW
LTC2292
12-Bit
40Msps
71.4dB
90dB
575MHz
235mW
LTC2291
12-Bit
25Msps
71.4dB
90dB
575MHz
150mW
LTC2290
12-Bit
10Msps
71.3dB
90dB
575MHz
120mW
LTC2281
10-Bit
125Msps
61.6dB
85dB
640MHz
790mW
LTC2280
10-Bit
105Msps
61.6dB
85dB
575MHz
540mW
LTC2289
10-Bit
80Msps
61.6dB
85dB
575MHz
422mW
LTC2288
10-Bit
65Msps
61.8dB
85dB
575MHz
400mW
LTC2287
10-Bit
40Msps
61.8dB
85dB
575MHz
235mW
LTC2286
10-Bit
25Msps
61.8dB
85dB
575MHz
150mW
07
03
08
02
LT C 1 7 5 0 A D C Family
LTC1750 High Performance 5V ADC Family
•
Single 5V Supply
•
CMOS Outputs
•
Data Ready Output Clock
•
Selectable Input Ranges (Varied)
•
48-pin TSSOP Package
left: DC 520, right: DC 718
The LTC1750 and LTC1749 are high IF sampling ADCs up to 500MHz IF, while the rest of the family is best suited for IFs below 250MHz.
This pin-compatible family provides excellent SNR performance from a 5V supply.
Applications: PA Linearization, ATE, Instrumentation, Video Equipment, Particle Monitoring Equipment, Cardiac Ultrasound,
CMOS Imaging, Digital Communications Simulator, Cell Sorter, Broadcast Camera, Data Acquisition
Part Number
Resolution
Speed
SNR
SFDR
Full Power
Bandwidth
Power
Dissipation
LTC1750
14-Bit
80Msps
75.5dB
90dB
500MHz
1450mW
LTC1748
14-Bit
80Msps
76.3dB
90dB
240MHz
1400mW
LTC1742
14-Bit
65Msps
76.5dB
90dB
240MHz
1275mW
LTC1746
14-Bit
25Msps
77.5dB
91dB
240MHz
390mW
LTC1749
12-Bit
80Msps
71.8dB
87dB
500MHz
1450mW
LTC1747
12-Bit
80Msps
72.0dB
85dB
240MHz
1400mW
LTC1741
12-Bit
65Msps
72.0dB
85dB
240MHz
1275mW
LTC1743
12-Bit
50Msps
72.5dB
85dB
150MHz
1000mW
LTC1745
12-Bit
25Msps
72.5dB
96dB
240MHz
380mW
F ully D ifferential A D C D rivers
Fully Differential ADC Drivers
Linear Technology offers a wide range of fully differential
amplifiers optimized for driving high-speed ADCs.
They provide a wide variety of features, including
built-in gain-setting resistors and integrated single pole
filters, 4th order integrated filters, and the wide common
mode input range. These amplifiers can also easily convert
single-ended inputs to differential outputs.
16-bit
LT1994
LT6404-X
LT6600-X
Resolution
LT6402-X
LT6411
LTC6406
LT1993-X
LTC6401-X
LTC6400-X
DC
100MHz
200MHz
300MHz
Input Frequency Range
Product
Input Frequency
Range
Gain Range
Distortion
HD2/HD3
Noise
Is
Vs
Price
(1,000 pcs)
Features
LT1994
DC to 2MHz
Resistor-set
-96dBc @ 1MHz
3 nV/√Hz
13.3mA
2.4V to 12.6V
$1.65
RR Output
LT6600-2.5
DC to 2.5MHz
Resistor-set
-88dBc @ 1MHz
23 nV/√Hz
26mA
3V to 11V
$2.95
Integrated
4th order filter
fc = 2.5MHz
LT6600-5
DC to 5MHz
Resistor-set
-93dBc @ 1MHz
16 nV/√Hz
28mA
3V to 11V
$2.95
Integrated
4th order filter
fc = 5MHz
LTC6403-1
DC to 5MHz
Resistor-set
-95dBc @ 3MHz
2.8 nV/√Hz
11mA
2.7V to 5.25V
TBD
RR Output
LTC6404-1
DC to 10MHz
Resistor-set
-92dBc @ 10MHz
1.5 nV/√Hz
27mA
2.7V to 5.5V
$3.44
RR Output
LTC6404-2
DC to 15MHz
Resistor-set
-98dBc @ 10MHz
1.5 nV/√Hz
30mA
2.7V to 5.25V
$3.44
Min Gain = 2V/V
LT6600-10
DC to 10MHz
Resistor-set
-88dBc @ 1MHz
14 nV/√Hz
35mA
3V to 11V
$2.95
Integrated
4th order filter
fc = 10MHz
LT6600-15
DC to 15MHz
Resistor-set
-86dBc @ 1MHz
19 nV/√Hz
35mA
3V to 11V
$2.95
Integrated
4th order filter
fc = 15MHz
LT6600-20
DC to 20MHz
Resistor-set
-83dBc @ 2.5MHz
15 nV/√Hz
42mA
3V to 11V
$2.95
Integrated
4th order filter
fc = 20MHz
LT6402-6
DC to 30MHz
6dB
-84dBc @ 25MHz
3.9 nV/√Hz
30mA
4V to 5.5V
$2.39
1st order filter
LT6402-12
DC to 30MHz
12dB
-82dBc @ 25MHz
2.6 nV/√Hz
30mA
4V to 5.5V
$2.39
1st order filter
LT6402-20
DC to 30MHz
20dB
-80dBc @ 25MHz
1.9 nV/√Hz
30mA
4V to 5.5V
$2.39
1st order filter
LT6411
DC to 50MHz
0dB, 6dB
-76dBc @ 10MHz
7.7 nV/√Hz
8mA
4.5V to 12.5V
$2.39
Fixed Gain
LTC6406
DC to 50MHz
Resistor-set
-72dBc @ 50MHz
1.6 nV/√Hz
18mA
2.7V to 3.5V
$3.44
RR Input
LT1993-2
DC to 70MHz
6dB
-70dBc @ 70MHz
3.8 nV/√Hz
100mA
4V to 5.5V
$2.95
1st order filter
LT1993-4
DC to 70MHz
12dB
-73dBc @ 70MHz
2.4 nV/√Hz
100mA
4V to 5.5V
$2.95
1st order filter
LT1993-10
DC to 70MHz
20dB
-70dBc @ 70MHz
1.9 nV/√Hz
100mA
4V to 5.5V
$2.95
1st order filter
LTC6401-8
DC to 140MHz
8dB
-87dBc @ 70MHz
3.2 nV/√Hz
45mA
2.85V to 3.5V
$2.95
1st order filter
LTC6401-20
DC to 140MHz
20dB
-88dBc @ 70MHz
2.1 nV/√Hz
50mA
2.85V to 3.5V
$2.95
1st order filter
LTC6401-26
DC to 140MHz
26dB
-81dBc @ 70MHz
1.5 nV/√Hz
45mA
2.85V to 3.5V
$2.95
1st order filter
LTC6400-20
DC to 300MHz
20dB
-76dBc @ 140MHz
2.1 nV/√Hz
90mA
2.85V to 3.5V
$3.20
1st order filter
LTC6400-26
DC to 300MHz
26dB
-81dBc @ 140MHz
1.5 nV/√Hz
85mA
2.85V to 3.5V
$3.20
1st order filter
09
03
10
02
F ully D ifferential A D C D rivers
LTC6400 High Speed Fully Differential ADC Driver Family
Integrated Gain-Setting Resistors:
• LTC6400-8
8dB
• LTC6400-14
14dB
• LTC6400-20
20dB
• LTC6400-26
26dB
High Performance (LTC6400-20):
• 1.8GHz GBW
• 4500V/us Slew Rate
• 41 dBm OIP3 @ 240MHz
• -74dBc IMD3 @ 240MHz
• 1.9nV/√Hz Op Amp Noise
•
Integrated First-Order Filter
•
Independent Output Common Mode Control
•
3mm × 3mm QFN package
•
Lower power, Lower Speed LTC6401 available
LTC6400
The LTC6400 high-speed fully differential amplifier family achieves high performance while simplifying system design. Capable of driving
up to 300MHz signals with low noise and distortion, the LTC6400 has integrated gain-setting resistors, adjustable output filtering, and
flexible I/O coupling. This integration maximizes performance and simplifies board design. Fully specified at 3V supply, the LTC6400
can often share the same supply rail with the ADC.
LTC6404-1 High-Resolution Fully Differential ADC Driver Family
•
Low Noise: 1.5nV/√Hz
•
Very Low Distortion: < -90dBc @ 10MHz
•
Closed Loop -3dB Bandwidth: 600MHz
•
450V/us Slew Rate
•
Independent Output Common Mode Control
•
3mm × 3mm QFN package
•
Lower power, Lower Speed LTC6403 available
•
2.7V to 5.25V Supply Voltage Range
LTC6404
The LTC6404-1 is capable of driving 10MHz input signals into today’s state-of-the-art high-speed 14- to 18-bit ADCs while operating on
the same supply voltage. It draws only 27mA and features a hardware shutdown mode which reduces current consumption to 0.25mA.
C M O S vs LV D S O utputs
CMOS vs LVDS Outputs
Low-Voltage Differential Signaling (LVDS) is often preferred as a means to reduce ground currents and radiated noise when sampling at
speeds greater than 100Msps. LVDS allows for good common-mode noise rejection at the digital receiver and faster switching rates.
The lower signaling levels allow properly terminated transmission paths which are of paramount importance in maintaining good
signal integrity.
CMOS obliges you to use source termination (provided internally on Linear Technology high-speed ADCs) but this is still not ideal as there
is some lumped capacitance after the internal resistor. Single-ended CMOS outputs have longer rise and fall times that limit the maximum
bit rate due to the larger voltage swing and are potentially more sensitive to crosstalk and above all, ground bounce.
However, LVDS is not completely immune to noise. The CMRR of receivers is not more than 20-30dB at higher frequencies, and the
signaling levels are lower. Adjacent lines routed in close proximity will still result in crosstalk. The presence of a high level CMOS signal
routed alongside an LVDS pair will potentially compromise signal integrity. In addition, high speed ADCs using LVDS can require up to a
quarter Watt more power than CMOS.
An advantage of Linear Technology’s high-speed ADCs is the ability to reduce the digital output supply (OVDD ) as low as 0.5V for running
low-voltage CMOS outputs. Lowering the CMOS levels greatly reduces the resulting noise and also helps reduce digital feedback. For
routing short distances, buffering can be eliminated at the ADC output for direct connection to an FPGA or DSP without discernable
performance degradation compared to LVDS. However, if there is ground bounce in the FPGA as a result of activity at asynchronous
or subharmonic frequencies, the elimination of buffers may allow noise to be injected back into the ADC.
Another technique that can be used to reduce digital noise is to configure the part in a Pseudo-LVDS output mode where the outputs are
configured as CMOS but OVDD is biased at 1.4V and OGND at 1V (see Figure 1). The outputs are then run into a standard LVDS receiver.
This requires a bypassed 1.2V reference voltage near the ADC, and needs to represent similar source impedance to the driven line at the
receiver. If these lines are routed over a belt of copper that is biased at the ADC, it may offer many of the advantages of LVDS without
the power consumption in the ADC.
Figure 1 also shows the integrated series resistance included on every digital output so that each output appears as 50ohms.
LTC2284
OVDD
1.4V
0.1uF
LVDS Receiver
1K
43ohm
50ohm Transmission Line
IN+
1.2V
OGND
1.0V
0.1uF
0.1uF
Close and Same Ground
Figure 1. Pseudo-LVDS Interface
IN-
Shared Reference Line
1K
11
03
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