LMH7220
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SNOSAL3E – SEPTEMBER 2006 – REVISED MAY 2013
LMH7220 High Speed Comparator with LVDS Output
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FEATURES
DESCRIPTION
•
The LMH7220 is a high speed, low power comparator
with an operating supply voltage range of 2.7V to
12V. The LMH7220 has a differential, LVDS output,
driving 325 mV into a 100Ω symmetrical transmission
line. The LMH7220 has a 2.9 ns propagation delay
and 0.6 ns rise and fall times while the supply current
is only 6.8 mA at 5V (load current excluded).
1
2
•
•
•
•
•
•
(VS = 5V TA = 25°C, Typical Values unless
Otherwise Specified)
Propagation Delay @ 100 mV Overdrive 2.9 ns
Rise and Fall Times 0.6 ns
Supply Voltage 2.7V to 12V
Supply Current 6.8 mA
Temperature Range −40°C to 125°C
LVDS Output
APPLICATIONS
•
•
•
•
•
•
•
The LMH7220 inputs have a voltage range that
extends 200 mV below ground, allowing ground
sensing applications. The LMH7220 is available in the
6-Pin SOT package. This package is ideal where
space is a critical item.
Acquisition Trigger
Fast Differential Line Receiver
Pulse Height Analyzer
Peak Detector
Pulse Width Modulator
Remote Threshold Detection
Oscilloscope Triggering
LVDS input port
on receiver
Sensor
LMH7220
+
-
100:
100:
Transmission Line
Ref
Figure 1. Typical Schematic
1
2
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
All trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
Copyright © 2006–2013, Texas Instruments Incorporated
LMH7220
SNOSAL3E – SEPTEMBER 2006 – REVISED MAY 2013
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These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
ABSOLUTE MAXIMUM RATINGS
ESD Tolerance
(3)
(1) (2)
Human Body Model
Machine Model
Supply Voltage (VCC - GND)
±13V
(4)
Output Shorted Together
Continuous
(4)
Continuous
−65°C to +150°C
Storage Temperature Range
Voltage on any I/O Pin
Junction Temperature
(1)
(2)
(3)
(4)
(5)
GND−0.2V to VCC+0.2V
(5)
150°C max
Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Conditions indicate specifications
for which the device is intended to be functional, but specific performance is not ensured. For specifications and the test conditions, see
the Electrical Characteristics.
If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for availability and
specifications.
Human Body Model, applicable std. MIL-STD-883, Method 3015.7. Machine Model, applicable std. JESD22-A115-A (ESD MM std. of
JEDEC). Field-Induced Charge-Device Model, applicable std. JESD22-C101-C (ESD FICDM std. of JEDEC)
Applies to both single-supply and split-supply operation. Continuous short circuit operation at elevated ambient temperature can result in
exceeding the maximum allowed junction temperature of 150°C.
The maximum power dissipation is a function of TJ(MAX), θJA. The maximum allowable power dissipation at any ambient temperature is
PD = (TJ(MAX) – TA)/ θJA. All numbers apply for packages soldered directly onto a PC Board.
OPERATING RATINGS
Temperature Range
(1)
(2)
−40°C to +125°C
Supply Voltage
Package Thermal Resistance (θJA )
(1)
(2)
2
250V
13.5V
Differential Input Voltage
Output Shorted to GND
2.5 kV
2.7V to 13V
6-Pin SOT
189°C/W
Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Conditions indicate specifications
for which the device is intended to be functional, but specific performance is not ensured. For specifications and the test conditions, see
the Electrical Characteristics.
The maximum power dissipation is a function of TJ(MAX), θJA. The maximum allowable power dissipation at any ambient temperature is
PD = (TJ(MAX) – TA)/ θJA. All numbers apply for packages soldered directly onto a PC Board.
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SNOSAL3E – SEPTEMBER 2006 – REVISED MAY 2013
+12V DC ELECTRICAL CHARACTERISTICS
Unless otherwise specified, all limits are specified for TJ = 25°C, VCM = 300 mV, −50 mV < VID < +50 mV and RL = 100Ω.
Boldface limits apply at the temperature extremes. (1) (2)
Symbol
Parameter
Conditions
Min (1)
Typ (3)
Max (1)
−2.1
−0.5
IB
Input Bias Current
VIN Differential = 0
−5
−7
IOS
Input Offset Current
VIN Differential = 0
−500
TC IOS
Input Offset Current TC
VIN Differential = 0
VOS
Input Offset Voltage
TC VOS
Input Offset Voltage TC
VRI
Input Voltage Range
CMRR > 50 dB
CMRR
Common-Mode Rejection Ratio
VCM = 0 to VCC−2.2V
PSRR
Power Supply Rejection Ratio
AV
Open Loop Gain
VO
Output Offset Voltage
VIN Differential = 50 mV
1125
ΔVO
VO Change Between ‘0’ and ‘1’
VIN Differential = ±50 mV
−25
VOH
Output Voltage High
VIN Differential = 50 mV
VOL
Output Voltage Low
VIN Differential = 50 mV
925
1060
VOD
Output Voltage Differential
VIN Differential = 50 mV
250
330
ΔVOD
VOD Change between ‘0’ to ‘1’
VIN Differential = ±50 mV
−25
ISC
Short Circuit Current Output to GND
Pin (4)
OUT Q to GND Pin
VIN Differential = 50 mV
5
OUT Q to GND Pin
VIN Differential = 50 mV
5
OUT Q to OUT Q
VIN Differential = 50 mV
5
Output Shorted Together
IS
(1)
(2)
(3)
(4)
Units
µA
+500
±2
−9.5
nA
nA/°C
+9.5
μV/°C
± 50
−0.2
mV
VCC−2
70
dB
63
74
dB
59
(4)
Supply Current
Load Current Excluded
VIN Differential = 50 mV
V
60
dB
1225
1325
mV
+25
mV
1390
1475
mV
7.5
mV
400
mV
+25
mV
10.0
14.0
mA
mA
All limits are specified by testing or statistical analysis.
Electrical Table values apply only for factory testing conditions at the temperature indicated. Factory testing conditions result in very
limited self-heating of the device such that TJ = TA. No specification of parametric performance is indicated in the electrical tables under
conditions of internal self heating where TJ > TA. See APPLICATION INFORMATION for information on temperature de-rating of this
device.
Typical values represent the most likely parametric norm as determined at the time of characterization. Actual typical values may vary
over time and will also depend on the application and configuration. The typical values are not tested and are not ensured on shipped
production material.
Applies to both single-supply and split-supply operation. Continuous short circuit operation at elevated ambient temperature can result in
exceeding the maximum allowed junction temperature of 150°C.
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LMH7220
SNOSAL3E – SEPTEMBER 2006 – REVISED MAY 2013
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+12V AC ELECTRICAL CHARACTERISTICS
Unless otherwise specified, all limits specified for TJ = 25°C, VCM = 300 mV, −50 mV < VID < +50 mV and RL = 100Ω.
Boldface limits apply at the temperature extremes. (1) (2)
Symbol
Parameter
Conditions
Min (1)
Typ (3)
860
1080
Mb/s
ps
TR
Toggle Rate
Overdrive = ±50 mV; CL = 2 pF @
50% Output Swing
tjitter_RMS
RMS-Random Jitter
Overdrive = 100 mV; CL = 2 pF
Center Frequency = 70 MHz
Bandwidth = 10 Hz – 20 MHz
4.29
tPDLH
Propagation Delay
tPDLH = (tPDH + tPDL ) / 2
(see Figure 21 application
information)
Input SR = Constant
VID start value = −100 mV
Overdrive 20 mV
3.56
Overdrive 50 mV
2.98
Overdrive 100 mV
2.7
Overdrive 1V
2.24
Input Overdrive Dispersion
@Overdrive 20 - 100 mV
0.86
@Overdrive 100 mV - 1V
0.46
tOD-disp
Max (1)
7
Units
ns
ns
tSR-disp
Input Slew Rate Dispersion
0.05 V/ns to 1 V/ns
Overdrive 100 mV
0.24
ns
tCM-disp
Input Common Mode dispersion
SR = 4 V/ns; Overdrive 100 mV
VCM = 0 to 10V
0.55
ns
ΔtPDLH
Q to Q Time Skew
| tPDH - tPDL | (4)
Overdrive = 100 mV; CL = 2 pF
0
ns
ΔtPDHL
Q to Q Time Skew
| tPDL - tPDH | (4)
Overdrive = 100 mV; CL = 2 pF
0.06
ns
tr
Output Rise Time (20% - 80%)
Overdrive = 100 mV; CL = 2 pF
0.56
ns
Overdrive = 100 mV; CL = 2 pF
0.49
ns
tf
(1)
(2)
(3)
(4)
(5)
4
Output Fall Time (20% - 80%)
(5)
(5)
All limits are specified by testing or statistical analysis.
Electrical Table values apply only for factory testing conditions at the temperature indicated. Factory testing conditions result in very
limited self-heating of the device such that TJ = TA. No specification of parametric performance is indicated in the electrical tables under
conditions of internal self heating where TJ > TA. See APPLICATION INFORMATION for information on temperature de-rating of this
device.
Typical values represent the most likely parametric norm as determined at the time of characterization. Actual typical values may vary
over time and will also depend on the application and configuration. The typical values are not tested and are not ensured on shipped
production material.
Propagation Delay Skew, ΔtPD, is defined as the average of ΔtPDLH and ΔtPDHL.
The rise or fall time is the average of the Q and Q rise or fall time.
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SNOSAL3E – SEPTEMBER 2006 – REVISED MAY 2013
+5V DC ELECTRICAL CHARACTERISTICS
Unless otherwise specified, all limits specified for TJ = 25°C, VCM = 300 mV, −50 mV < VID < +50 mV and RL = 100Ω.
Boldface limits apply at the temperature extremes. (1) (2)
Symbol
Parameter
Conditions
Min (1)
Typ (3)
Max (1)
−1.5
−0.5
IB
Input Bias Current
VIN Differential = 0
−5
−7
IOS
Input Offset Current
VIN Differential = 0
−500
TC IOS
Input Offset Current TC
VIN Differential = 0
VOS
Input Offset Voltage
TC VOS
Input Offset Voltage TC
VRI
Input Voltage Range
CMRR > 50 dB
CMRR
Common-Mode Rejection Ratio
VCM = 0 to VCC−2.2V
PSRR
Power Supply Rejection Ratio
AV
Open Loop Gain
VO
Output Offset Voltage
VIN Differential = 50 mV
1125
ΔVO
VO Change Between ‘0’ and ‘1’
VIN Differential = ±50 mV
−25
VOH
Output Voltage High
VIN Differential = 50 mV
VOL
Output Voltage Low
VIN Differential = 50 mV
925
1060
VOD
Output Voltage Differential
VIN Differential = 50 mV
250
320
ΔVOD
VOD Change between ‘0’ to ‘1’
VIN Differential = ±50 mV
−25
ISC
Short Circuit Current Output to GND
Pin (4)
OUT Q to GND Pin
VIN Differential = 50 mV
5
OUT Q to GND Pin
VIN Differential = 50 mV
5
OUT Q to OUT Q
VIN Differential = 50 mV
5
Output Shorted Together
IS
(1)
(2)
(3)
(4)
Units
µA
+500
±2
−9.5
nA
nA/°C
+9.5
μV/°C
± 50
−0.2
mV
VCC−2
70
dB
63
74
dB
59
(4)
Supply Current
Load Current Excluded
VIN Differential = 50 mV
V
60
dB
1217
1325
mV
+25
mV
1380
1475
mV
6.8
mV
400
mV
+25
mV
9
12.6
mA
mA
All limits are specified by testing or statistical analysis.
Electrical Table values apply only for factory testing conditions at the temperature indicated. Factory testing conditions result in very
limited self-heating of the device such that TJ = TA. No specification of parametric performance is indicated in the electrical tables under
conditions of internal self heating where TJ > TA. See APPLICATION INFORMATION for information on temperature de-rating of this
device.
Typical values represent the most likely parametric norm as determined at the time of characterization. Actual typical values may vary
over time and will also depend on the application and configuration. The typical values are not tested and are not ensured on shipped
production material.
Applies to both single-supply and split-supply operation. Continuous short circuit operation at elevated ambient temperature can result in
exceeding the maximum allowed junction temperature of 150°C.
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LMH7220
SNOSAL3E – SEPTEMBER 2006 – REVISED MAY 2013
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+5V AC ELECTRICAL CHARACTERISTICS
Unless otherwise specified, all limits specified for TJ = 25°C, VCM = 300 mV, −50 mV < VID < +50 mV and RL = 100Ω.
Boldface limits apply at the temperature extremes. (1) (2)
Symbol
Parameter
Conditions
Min (1)
Typ (3)
750
940
Mb/s
ps
TR
Toggle Rate
Overdrive = ±50 mV; CL = 2 pF @
50% Output Swing
tjitter_RMS
RMS-Random Jitter
Overdrive = 100 mV; CL = 2 pF
Center Frequency = 70 MHz
Bandwidth = 10 Hz – 20 MHz
4.44
tPDLH
Propagation Delay
tPDLH = (tPDH + tPDL ) / 2
(see Figure 21 application
information)
Input SR = Constant
VID start value = -100mV
Overdrive 20 mV
3.63
Overdrive 50 mV
3.09
Overdrive 100 mV
2.9
Overdrive 1V
2.41
Input Overdrive Dispersion
@Overdrive 20 - 100 mV
0.79
@Overdrive 100 mV - 1V
0.43
tOD-disp
Max (1)
7
Units
ns
ns
tSR-disp
Input Slew Rate Dispersion
0.05 V/ns to 1 V/ns
Overdrive 100 mV
0.20
ns
tCM-disp
Input Common Mode Dispersion
SR = 4 V/ns; Overdrive 100 mV
VCM = 0 to 3V
0.21
ns
ΔtPDLH
Q to Q Time Skew
| tPDH - tPDL | (4)
Overdrive = 100 mV; CL = 2 pF
0.09
ns
ΔtPDHL
Q to Q Time Skew
| tPDL - tPDH | (4)
Overdrive = 100 mV; CL = 2 pF
0.07
ns
tr
Output Rise Time (20% - 80%)
Overdrive = 100 mV; CL = 2 pF
0.59
ns
Overdrive = 100 mV; CL = 2 pF
0.55
ns
tf
(1)
(2)
(3)
(4)
(5)
6
Output Fall Time (20% - 80%)
(5)
(5)
All limits are specified by testing or statistical analysis.
Electrical Table values apply only for factory testing conditions at the temperature indicated. Factory testing conditions result in very
limited self-heating of the device such that TJ = TA. No specification of parametric performance is indicated in the electrical tables under
conditions of internal self heating where TJ > TA. See APPLICATION INFORMATION for information on temperature de-rating of this
device.
Typical values represent the most likely parametric norm as determined at the time of characterization. Actual typical values may vary
over time and will also depend on the application and configuration. The typical values are not tested and are not ensured on shipped
production material.
Propagation Delay Skew, ΔtPD, is defined as the average of ΔtPDLH and ΔtPDHL.
The rise or fall time is the average of the Q and Q rise or fall time.
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SNOSAL3E – SEPTEMBER 2006 – REVISED MAY 2013
+2.7V DC ELECTRICAL CHARACTERISTICS
Unless otherwise specified, all limits specified for TJ = 25°C, VCM = 300 mV, −50 mV < VID < +50 mV and RL = 100Ω.
Boldface limits apply at the temperature extremes. (1)
Symbol
Parameter
Conditions
Min (1)
Typ (2)
Max (1)
−1.3
−0.5
IB
Input Bias Current
VIN Differential = 0
−5
−7
IOS
Input Offset Current
VIN Differential = 0
−500
TC IOS
Input Offset Current TC
VIN Differential = 0
VOS
Input Offset Voltage
TC VOS
Input Offset Voltage TC
VRI
Input Voltage Range
CMRR > 50 dB
CMRR
Common-Mode Rejection Ratio
VCM = 0 to VCC−2.2V
PSRR
Power Supply Rejection Ratio
AV
Open Loop Gain
VO
Output Offset Voltage
VIN Differential = 50 mV
1125
ΔVO
VO Change Between ‘0’ and ‘1’
VIN Differential = ± 50 mV
−25
VOH
Output Voltage High
Average of ‘0’ to ‘1’
VIN Differential = 50 mV
VOL
Output Voltage Low
Average of ‘0’ to ‘1’
VIN Differential = 50 mV
VOD
Output Voltage Differential
ΔVOD
VOD Change between ‘0’ to ‘1’
ISC
Short Circuit Current Output to GND
Pin (3)
OUT Q to GND Pin
VIN Differential = 50 mV
5
OUT Q to GND Pin
VIN Differential = 50 mV
5
OUT Q to OUT Q
VIN Differential = 50 mV
5
Output Shorted Together
IS
(1)
(2)
(3)
Units
µA
+500
±2
−9.5
nA
nA/°C
+9.5
μV/°C
± 50
−0.2
mV
VCC−2
70
dB
63
74
dB
59
(3)
Supply Current
dB
1213
1325
mV
+25
mV
1370
1475
mV
925
1060
VIN Differential = 50 mV
250
315
VIN Differential = ±50 mV
−25
Load Current Excluded
VIN Differential = 50 mV
V
56
6.6
mV
400
mV
+25
mV
9
12.6
mA
mA
All limits are specified by testing or statistical analysis.
Typical values represent the most likely parametric norm as determined at the time of characterization. Actual typical values may vary
over time and will also depend on the application and configuration. The typical values are not tested and are not ensured on shipped
production material.
Applies to both single-supply and split-supply operation. Continuous short circuit operation at elevated ambient temperature can result in
exceeding the maximum allowed junction temperature of 150°C.
Submit Documentation Feedback
Copyright © 2006–2013, Texas Instruments Incorporated
Product Folder Links: LMH7220
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LMH7220
SNOSAL3E – SEPTEMBER 2006 – REVISED MAY 2013
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+2.7V AC ELECTRICAL CHARACTERISTICS
Unless otherwise specified, all limits specified for TJ = 25°C, VCM = 300 mV, −50 mV < VID < +50 mV and RL = 100Ω.
Boldface limits apply at the temperature extremes. (1) (2)
Symbol
Parameter
Conditions
Min (1)
Typ (3)
700
880
Mb/s
ps
TR
Toggle Rate
Overdrive = ±50 mV; CL = 2 pF @
50% Output Swing
tjitter_RMS
RMS-Random Jitter
Overdrive = 100 mV; CL = 2 pF
Center Frequency = 70 MHz
Bandwidth = 10 Hz – 20 MHz
4.82
tPDLH
Propagation Delay
tPDLH = (tPDH + tPDL ) / 2
(see Figure 21 application
information)
Input SR = Constant
VID start value = -100mV
Overdrive 20 mV
3.80
Overdrive 50 mV
3.29
Overdrive 100 mV
3.0
Overdrive 1V
2.60
Input Overdrive Dispersion
@Overdrive 20 - 100 mV
0.83
@Overdrive 100 mV - 1V
0.37
tOD-disp
Max (1)
7
Units
ns
ns
tSR-disp
Input Slew Rate Dispersion
0.05 V/ns to 1 V/ns
Overdrive 100 mV
0.23
ns
tCM-disp
Input Common Mode dispersion
SR = 4 V/ns; Overdrive 100 mV
VCM = 0 to 1.5V
0.16
ns
ΔtPDLH
Q to Q Time Skew
| tPDH - tPDL | (4)
Overdrive = 100 mV; CL = 2 pF
0.09
ns
ΔtPDHL
Q to Q Time Skew
| tPDL - tPDH | (4)
Overdrive = 100 mV; CL = 2 pF
0.09
ns
tr
Output Rise Time (20% - 80%)
Overdrive = 100 mV; CL = 2 pF
0.64
ns
Overdrive = 100 mV; CL = 2 pF
0.59
ns
tf
(1)
(2)
(3)
(4)
(5)
Output Fall Time (20% - 80%)
(5)
(5)
All limits are specified by testing or statistical analysis.
Electrical Table values apply only for factory testing conditions at the temperature indicated. Factory testing conditions result in very
limited self-heating of the device such that TJ = TA. No specification of parametric performance is indicated in the electrical tables under
conditions of internal self heating where TJ > TA. See APPLICATION INFORMATION for information on temperature de-rating of this
device.
Typical values represent the most likely parametric norm as determined at the time of characterization. Actual typical values may vary
over time and will also depend on the application and configuration. The typical values are not tested and are not ensured on shipped
production material.
Propagation Delay Skew, ΔtPD, is defined as the average of ΔtPDLH and ΔtPDHL.
The rise or fall time is the average of the Q and Q rise or fall time.
VCC
IN+
IN-
+
OUT Q
-
OUT Q
GND
Figure 2. Schematic Diagram
8
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SNOSAL3E – SEPTEMBER 2006 – REVISED MAY 2013
Connection Diagram
6
5
4
IN+
VCC
OUT Q
IN-
GND
OUT Q
1
2
3
Figure 3. 6-Pin SOT Top View
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TYPICAL PERFORMANCE CHARACTERISTICS
At TJ = 25°C; unless otherwise specified: VCM = 0.3V, VOVERDRIVE = 100 mV, RL = 100Ω.
Input Current
vs.
Differential Input Voltage
Bias Current
vs.
Supply Voltage
10
0.0
VCM = 300V
5
IIN+
IIN0
-5
VCM = 2.5V
VS = 5V
-10
IBIAS = (IIN+ + IIN-)/2
BIAS CURRENT (PA)
INPUT CURRENT (PA)
VIN_DIFF = 0
-0.5
-1.0
-1.5
-2.0
-2.5
VIN+ SWEEP = 1.6 to 3.4V
VIN- SWEEP = 3.4V to 1.6V
-15
-2
-1.5
-1
-0.5
0
0.5
1
1.5
-3.0
2
2
3
4
DIFFERENTIAL INPUT VOLTAGE (V)
5 6 7 8 9 10 11 12 13
SUPPLY VOLTAGE (V)
Figure 4.
Figure 5.
Bias Current
vs.
Temperature
Output Offset Voltage
vs.
Supply Voltage
1.3
0.0
BIAS CURRENT (PA)
OUTPUT OFFSET VOLTAGE (V)
VCM = 300 mV
-1.0
VS = 2.7V
-2.0
-3.0
VS = 5V
-4.0
VS = 12V
-5.0
-6.0
VCM = 300 mV
-7.0
VIN_DIFF = 0
VIN_DIFF = 50 mV
1.28
1.26
125°C
1.24
85°C
1.22
25°C
IBIAS = (IIN+ + IIN-)/2
-8.0
-40 -25 -10 5 20 35 50 65 80 95 110 125
-40°C
1.2
2.5
4.5
TEMPERATURE (°C)
10.5
Figure 7.
Output Offset Voltage
vs.
Temperature
Differential Output Voltage
vs.
Supply Voltage
12.5
0.35
VIN_DIFF = 50 mV
1.28
1.26
VS = 12V
1.24
VS = 5V
1.22
VS = 2.7V
1.2
-40 -25 -10 -5 20 35 50 65 80 95 110 125
DIFFERENTIAL OUTPUT VOLTAGE (V)
VCM = 300 mV
OUTPUT OFFSET VOLTAGE (V)
8.5
Figure 6.
1.3
VCM = 300 mV
0.345 VIN_DIFF = 50 mV
0.34
125°C
0.335
-40°C
0.33
25°C
85°C
0.325
TEMPERATURE (°C)
0.32
2.5
125°C
4.5
6.5
8.5
10.5
12.5
SUPPLY VOLTAGE (V)
Figure 8.
10
6.5
SUPPLY VOLTAGE (V)
Figure 9.
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SNOSAL3E – SEPTEMBER 2006 – REVISED MAY 2013
TYPICAL PERFORMANCE CHARACTERISTICS (continued)
At TJ = 25°C; unless otherwise specified: VCM = 0.3V, VOVERDRIVE = 100 mV, RL = 100Ω.
Differential Output Voltage
vs.
Temperature
14
VCM = 300 mV
VIN_DIFF = 50 mV
12
0.36
SUPPLY CURRENT (mA)
DIFFERENTIAL OUTPUT VOLTAGE (V)
0.38
Supply Current
vs.
Supply Voltage
VS = 12V
0.34
0.32
VS = 5V
VS = 2.7V
0.30
125°C
8
6
25°C
LOAD CURRENT EXCLUDED
Figure 11.
Supply Current
vs.
Temperature
Rise & Fall Time
vs.
Temperature
1.0
11 VIN_DIFF = 50 mV
10
RL = 100:
VIN+ = 0.2V to 0.4V PULSE
VS = 12V
VIN- = 300 mV
INPUT SLEW RATE: 1 V/ns
0.8
9
VS = 5V
7
6
0.7
FALL TIME
0.6
VS = 2.7V
5
0.5
RISE TIME
4
3
-40 -25 -10 5 20 35 50 65 80 95 110 125
TEMPERATURE (°C)
Figure 12.
Figure 13.
Propagation Delay
vs.
Temperature
Propagation Delay
vs.
Overdrive Voltage
5.0
VS = 12V
RL = 100:
VIN+ = 0.2V to 0.4V PULSE
3.4
VIN- = 300 mV
INPUT SLEW RATE: 1 V/ns
PROPAGATION DELAY (ns)
PROPAGATION DELAY (ns)
0.4
-40 -25 -10 5 20 35 50 65 80 95 110 125
TEMPERATURE (°C)
3.6
3.0
2.8
2.6
2.4
VS = 2.7V
VS = 12V
12.5
VS = 12V
0.9
TIME (ns)
SUPPLY CURRENT (mA)
LOAD CURRENT EXCLUDED
12 V
CM = 300 mV
3.2
10.5
Figure 10.
13
3.8
8.5
SUPPLY VOLTAGE (V)
TEMPERATURE (°)
4.0
-40°C
4
2 VCM = 300 mV
VIN_DIFF = 50 mV
0
2.5
4.5
6.5
0.28
-40 -25 -10 -5 20 35 50 65 80 95 110 125
8
85°C
10
VS = 5V
VCM = 0.3V
4.5 VIN- = VCM
VIN+ = VCM - 100 mV to VCM + VOD
4.0
3.5
VS = 2.7V
3.0
2.5
2.0
VS = 12V
2.2
1.5
2.0
-40 -25 -10 5 20 35 50 65 80 95 110 125
1.0
0
200
400
VS = 5V
600
800
TEMPERATURE (°C)
OVERDRIVE VOLTAGE (mV)
Figure 14.
Figure 15.
1000
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
At TJ = 25°C; unless otherwise specified: VCM = 0.3V, VOVERDRIVE = 100 mV, RL = 100Ω.
Propagation Delay
vs.
Common Mode Voltage
Propagation Delay
vs.
Slew Rate
5.0
6.0
VIN- = VCM
VIN+ = VCM - 50 mV to VCM + 50 mV
PROPAGATION DELAY (ns)
PROPAGATION DELAY (ns)
4.5
4.0
3.5
VS = 2.7V
VS = 5V
3.0
VS = 12V
2.5
2.0
1.5
VCM = 0.3V
5.5 VIN- = VCM
VIN+ = VCM - 100 mV to VCM + 100 mV
5.0
4.5
4.0
VS = 2.7V
3.5
VS = 5V
3.0
2.5
1.0
-1 0 1 2 3 4 5 6 7
VS = 12V
2.0
8 9 10 11 12
0
200
COMMON MODE VOLTAGE (V)
400
600
800
1000
SLEW RATE (V/Ps)
Figure 16.
Figure 17.
Pulse Response Over Temperature
1500
25°C
1400
Q-OUTPUT (mV)
125°C
-40°C
1300
85°C
1200
1100
VS = 5V
VCM = 300 mV
VIN- = VCM
1000
900
VIN+ = VCM - 100 mV to VCM + 100 mV
800
0
2
4
6
8
10 12 14 16 18 20
TIME (ns)
Figure 18.
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APPLICATION INFORMATION
INTRODUCTION
The LMH7220 is a high speed comparator with LVDS outputs. The LVDS (Low Voltage Differential Signaling)
standard uses differential outputs with a voltage swing of approximately 325 mV on each output. The most widely
used setup for LVDS outputs consists of a switched current source of 3.25 mA. The output pins need to be
differentially terminated with an external 100Ω resistor, producing the standardized output voltage swing of 325
mV. The common mode level of both outputs is about 1.2V, and is independent of the power supply voltage. The
use of complementary outputs gives a high level of suppression for common mode noise. The very fast rise and
fall times of the LMH7220 enable data transmission rates up to several hundreds of Megabits per second (Mbps).
Due to the current-nature of the outputs the power consumption remains at a very low level even if the data
transmission rate is rising. Power delivered to a load resistance of 100Ω is only 1.2 mW.
The LMH7220 inputs have a common mode voltage range that extends 200 mV below the negative supply
voltage thus allowing ground sensing in case of single supply. The rise and fall times of the LMH7220 are about
0.6 ns, while the propagation delay time is about 2.7 ns. The LMH7220 can operate over the full supply voltage
range of 2.7V to 12V, while using single or dual supply voltages. The LVDS outputs refer to the negative supply
rail. The supply current is 6.8 mA at 5V (load current excluded). The LMH7220 is available in the 6-Pin SOT
package.
In
•
•
•
•
•
•
•
the next sections the following issues are discussed:
In- and output topology
Definition of terms of used specifications
Propagation delay and dispersion
Hysteresis and oscillations
The output
Applying transmission lines
PCB layout
INPUT & OUTPUT TOPOLOGY
All input and output pins are protected against excessive voltages by ESD diodes. These diodes are connected
from the negative supply to the positive supply. As can be seen in Figure 19, both inputs are connected to these
diodes. Protection against excessive supply voltages is provided by a power clamp between VCC and GND. Both
inputs are also connected to the bases of the input transistors of the differential pair via 1.5 kΩ resistors. The
input transistors cannot withstand high reverse voltages between bases and emitter, due to their high frequency
properties. To protect the input stage against damage, both bases are connected together by a string of antiparallel diodes. Be aware of situations in which differential input voltage level is such that these diodes are
conducting. In this case the input current is raised far above the normal value stated in the datasheet tables.
VCC
VCC
VCC
1.5 k:
VCC
1.5 k:
IN+
IN-
Power
Clamp
Equivalent Input Circuitry
Figure 19. Equivalent Input Circuitry
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The output can be seen as a bridge configuration in which switches are crosswise closed, producing the
differential LVDS logic high and low levels (see Figure 20). The output switches are fed at top and bottom by two
current sources. The top one is fixed and determines the differential voltage across the external load resistor.
The other one is regulated and determines the common-mode voltage on the outputs. It is essential to keep the
output common-mode voltage at the defined standardized LVDS level under all circumstances. To realize this,
both outputs are internally connected together via two equal resistors. At the midpoint this produces the common
mode output voltage, which is made equal to VREF (1.2V) by means of the CM feedback loop.
VCC
Current 3.25 mA
Output Q
Output Q
Current 3.25 mA
+
CM loop
+
-
VREF
Equivalent Output Circuitry
Figure 20. Equivalent Output Circuitry
DEFINITIONS
For a good understanding of many parameters of the LMH7220 it is necessary to perform a lot of measurements.
All of those parameters are listed in the data tables in the first part of the datasheet. There are different tables for
several supply voltages containing a separate set of data per supply voltage. In the table below is a list of
abbreviations of the measured parameters and a short description of the conditions which are applied for
measuring them . Following this table several parameters are highlighted to explain more clearly what it means
exactly and what effects such a phenomena can have for any applied electronic circuit.
Symbol
Text
Description
IB
Input Bias Current
Current flowing in or out the input pins, when both biased at 0.3 Volt above
GND
IOS
Input Offset Current
Difference between the positive- and the negative input currents needed to
make the outputs change state, averaged for H to L and L to H transitions
TC IOS
Average Input Offset Current Drift
Temperature Coefficient of IOS
VOS
Input Offset Voltage
Voltage difference needed between IN+ and IN− to make the outputs
change state, averaged for H to L and L to H transitions
TC VOS
Average Input Offset Voltage Drift
Temperature Coefficient of VOS
CMRR
Common Mode Rejection Ratio
Ratio of input offset voltage change and input common mode voltage
change
VRI
Input Voltage Range
Upper and lower limits of the input voltage are defined as where CMRR
drops below 50 dB.
PSRR
Power Supply Rejection Ratio
Ratio of input offset voltage change and supply voltage change from VS-MIN
to VS-MAX
VO
Output Offset Voltage
Output Common Mode Voltage averaged for logic ‘0’ and logic ‘1’ levels
(See Figure 30)
14
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Symbol
Text
Description
ΔVO
Change in Output Offset Voltage
Difference in Output Common Mode Voltage between logic ‘0’ and logic ‘1’
levels (See Figure 31)
VOH
Output Voltage High
High state single ended output voltage (Q or Q) (See Figure 30)
VOL
Output Voltage Low
Low state single ended output voltage (Q or Q) (See Figure 30)
VODH
Output Differential Voltage logic ‘1’
VOH(Q) – VOL(Q) (logic level ‘1’) (See Figure 31)
VODL
Output Differential Voltage logic ‘0’
VOH(Q) – VOL(Q) (logic level ‘0’) (See Figure 31)
VOD
Average of VODH and VODL
(VODH + VODL) / 2
ΔVOD
Change in VOD between ‘0’ and ‘1’
|VODH – VODL| (See Figure 31)
Hyst
Hysteresis
Difference in input switching levels for L to H and H to L transitions. (See
Figure 29)
ISQG, ISQG
Short Circuit Current one output to GND
Current that flows from one output to GND if shorted single ended
ISQQ
Short Circuit Current outputs together
Current flowing between output Q and output Q if shorted differentially
TR
Maximum Toggle Rate
Maximum frequency at which the outputs can toggle before VOD drops
under 50% of the nominal value.
PW
Pulse Width
Time from 50% of the rising edge of a signal to 50% of the falling edge
tPDH resp tPDL
Propagation Delay
Delay time between the moment the input signal crosses the switching
level L to H and the moment the output signal crosses 50% of the rising
edge of Q output (tPDH), or delay time between the moment the input signal
crosses the switching level H to L and the moment the output signal
crosses 50% of the falling edge of Q output (tPDL)
tPDLresp tPDH
Delay time between the moment the input signal crosses the switching
level L to H and the moment the output signal crosses 50% of the falling
edge of Q output (tPDL), or delay time between the moment the input signal
crosses the switching level H to L and the moment the output signal
crosses 50% of the rising edge of Q output (tPDH)
tPDLH
Average of tPDH and tPDL
tPDHL
Average of tPDL and tPDH
tPD
Average of tPDLH and tPDHL
tPDHd resp tPDLd
Delay time between the moment the input signal crosses the switching
level L to H and the zero crossing of the rising edge of the differential
output signal (tPDHd), or delay time between the moment the input signal
crosses the switching level H to L and the zero crossing of the falling edge
of the differential output signal (tPDLd)
ΔtPDLH resp ΔtPDHL
Q to Q time skew
Time skew between 50% levels of rising edge of Q output and falling edge
of Q output (ΔtPDLH ), or time skew between 50% levels of falling edge of Q
output and rising edge of Q output (ΔtPDHL)
ΔtPD
Average Q to Q time skew
Average of tPDLH and tPDHL for L to H and H to L transients
ΔtPDd
Average diff. time skew
Average of tPDHd and tPDLd for L to H and H to L transients
tOD-disp
Input overdrive dispersion
Change in tPD for different overdrive voltages at the input pins
tSR-disp
Input slew rate dispersion
Change in tPD for different slew rates at the input pins
tCM-disp
Input Common Mode dispersion
Change in tPD for different common mode voltages at the input pins
tr / trd
Output rise time (20% - 80%)
Time needed for the (single ended or differential) output voltage to change
from 20% of its nominal value to 80%
tf / tfd
Output fall time (20% - 80%)
Time needed for the (single ended or differential) output voltage to change
from 80% of its nominal value to 20%
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PW
Voverdrive
Differential
Input Signal
0
'tPDLH
tPDHL = (tPDL + tPDH)/ 2
'tPDHL
tf
tPD = (tPDLH + tPDHL)/ 2
tr
tPDH
80% or 90%
tPDL
Output Q
tPDLH = (tPDH + tPDL)/ 2
VO
10% or 20%
'tPDLH = | tPDH - tPDL |
'tPDHL = | tPDL - tPDH |
tPDH
VO
Output Q
'tPD = 'tPDLH + 'tPDHL)/ 2
tPDL
'tPDQ = | tPDH - tPDL |
trd
'tPDQ = | tPDL - tPDH |
tPDHd
80% or 90%
Differential
Output Signal
0
tPDLd
10% or 20%
tPDd = (tPDHd + tPDLd)/ 2
'tPDd = | tPDHd - tPDLd |
tfd
Figure 21. Propagation Delay Definition
DELAY AND DISPERSION
Comparators are widely used to connect the analog world to the digital one. The accuracy of a comparator is
dictated by its DC properties such as offset voltage and hysteresis and by its timing aspects such as rise and fall
times and delay. For low frequency applications most comparators are much faster than the analog input signals
they handle. The timing aspects are less important here than the accuracy of the input switching levels. The
higher the frequency, the more important the timing properties of the comparator become, because the response
of the comparator can give e.g. a noticeable change in time frame or duty cycle. A designer has to know these
effects in order to deal with them. In order to predict what the output signal will do compared to the input signal,
several parameters are defined which describe the behavior of the comparator. For a good understanding of the
timing parameters discussed in the following section, a brief explanation is given and several timing diagrams are
shown for clarification.
PROPAGATION DELAY
The propagation delay parameter is defined as the time it takes for the comparator to change the output level
halfway in its transition from L to H or H to L, in reaction to the moment the input signal crosses the switching
level. Due to this definition there are two parameters, tPDH and tPDL (Figure 22). Both parameters don’t
necessarily have the same value. It is possible that differences will occur due to a different response of the
internal circuitry. As a result of this effect another parameter is defined: ΔtPD. This parameter is defined as the
absolute value of the difference between tPDH and tPDL.
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PW
80%
80%
50%
50%
VIN
20%
20%
tPDH
tPDL
80%
80%
Output Q
50%
50%
20%
20%
tr
tf
Figure 22. Pulse Parameter
Input Signal
If ΔtPD isn’t zero, duty cycle distortion will occur. For example when applying a symmetrical waveform (e.g. a
sinewave) at the input, it is expected that the comparator produces a symmetrical square wave at the output with
a duty cycle of 50%. In case of different tPDH and tPDL the duty cycle of the output signal will not remain at 50%,
but will be lower or higher. In addition to the propagation delay parameters for single ended outputs discussed
before, there are other parameters in case of complementary outputs. These parameters describe the delay from
input to each of the outputs and the difference between both delay times (see Figure 23). When the differential
input signal crosses the reference level from L to H, both outputs will switch to their new state with some delay.
This is defined as tPDH for the Q output and tPDL for the Q output, while the difference between both signals is
defined as ΔtPDLH. similar definitions for the falling slope of the input signal can be seen in Figure 21.
time
VREF
Output Q
tPDH
time
VO
Output Q
'tPDLH
time
VO
tPDL
Propagation Delay
Figure 23. Propagation Delay
Both output circuits should be symmetrical. At the moment one output is switching ‘on’ the other is switching ‘off’
with ideally no skew between them. The design of the LMH7220 is optimized to minimize this timing difference.
Propagation delay tPD is defined as the average delay of both outputs at both slopes: (tPDLH + tPDHL) / 2.
DISPERSION
There are several circumstances that will produce a variation of the propagation delay time. This effect is called
dispersion.
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Amplitude Overdrive Dispersion
One of the parameters that causes dispersion is the amplitude variation of the input signal. Figure 24 shows the
dispersion due to a variation of the input overdrive voltage. The overdrive is defined as the ‘goto’ differential
voltage applied to the inputs. Figure 24 shows the impact it has on the propagation delay time if overdrive is
varied from 10 millivolts to 100 millivolts. This parameter is measured with a constant slew rate of the input
signal.
Output Differential Signal
Input Differential Signal
Overdrive 100 mV
+
Overdrive 10 mV
0
time
-100 mV
-
Overdrive Dispersion
+
Dispersion
0
time
-
Figure 24. Overdrive Dispersion
The overdrive dispersion is caused by the fact that switching currents in the input stage depend on the level of
the differential input signal.
Slew Rate Dispersion
Output Differential Signal
Input Differential Signal
The slew rate is another parameter that affects propagation delay. The higher the input slew rate, the faster the
input stage switches (Figure 25).
+
0
time
-
Slew Rate Dispersion
+
Dispersion
0
time
-
Figure 25. Slew Rate Dispersion
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A combination of overdrive- and slew rate dispersion occurs when applying signals with different amplitude at
constant frequency. A small amplitude will produce a small voltage change per time unit (dV/dt) but also a small
maximum switching current (overdrive) in the input transistors. High amplitudes produce a high dV/dt and a
bigger overdrive.
Common Mode Dispersion
Dispersion will also occur when changing the common mode level of the input signal (Figure 26). When VREF is
swept through the CMVR (Common Mode Voltage Range), this results in a variation of the propagation delay
time. This variation is called Common Mode Dispersion.
Output Differential Signal
Input Differential Signal
Vin cm
+
Vin cm
0
time
-
Common Mode Dispersion
+
Dispersion
0
time
-
Figure 26. Common Mode Dispersion
All of the dispersion effects discussed before influence the propagation delay. In practice the dispersion is often
caused by a combination of more than one varied parameter. It is good to realize this if there is the need to
predict how much dispersion a circuit will show.
HYSTERESIS & OSCILLATIONS
In contrast to an op amp, the output of a comparator has only two defined states ‘0’ or ‘1’. Due to finite
comparator gain however, there will be a small band of input differential voltage where the output is in an
undefined state. An input signal with fast slopes will pass this band very quickly without problems. During slow
slopes however, passing the band of uncertainty can be relatively long. This enables the comparator outputs to
switch back and forth several times between ‘0’ and ‘1’ on a single slope. The comparator will switch on its input
noise, ground bounce (possible oscillations), ringing etc. Noise in the input signal will also contribute to these
undesired switching effects.
In the next sections an explanation follows about these phenomena in situations where no hysteresis is applied,
and the possible improvement hysteresis can give.
Using No Hysteresis
In Figure 27 can be seen what happens when the input signal rises from just under the threshold VREF to a level
just above it. From the moment the input reaches the lowest dotted line around VREF at t=0, the output toggles on
noise etc. Toggling ends when the input signal leaves the undefined area at t=1. In this example the output was
fast enough to toggle three times. Due to this behavior digital circuitry connected to the output will count a wrong
number of pulses. One way to prevent this is to choose a very slow comparator with an output that is not able to
switch more than once between ‘0’ and ‘1’ during the time the input state is undefined.
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slow output
fast output
Input Signal
mV
Vref
time
1
time
0
1
time
0
t=0
t=1
Oscillations & Noise
Figure 27. Oscillations & Noise
In most circumstances this is not an option because the slew rate of the input signal will vary.
Using Hysteresis
A good way to avoid oscillations and noise during slow slopes is the use of hysteresis. For this purpose a
threshold is introduced that pushes the input switching level back at the moment the output switches (See
Figure 28). In this simple setup, a comparator with a single output and a resistive divider to the positive input is
drawn.
CC
CP
Rf
+
CP
Rp
VREF
VIN
Simplified Schematic
Figure 28. Simplified Schematic
20
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The divider RF-RP feeds back a portion of the output voltage to the positive input. Only a small part of the output
voltage is needed, just enough to avoid the area at which the input is in an undefined state. Assuming this is only
a few millivolts, it is sufficient to add (plus or minus) 10 mV to the positive input to prevent the circuit from
oscillations. If the output switches between 0V and 5V and the choice for one of the resistors is done the other
can be calculated. Assume RP is 50Ω then RF is 25 kΩ for 10 mV threshold on the positive input. The situation of
Figure 29 is now created.
Input Signal
mV
Vref
A
B
Output
1
0
t=0
t=1
Hysteresis
Figure 29. Hysteresis
In this picture there are two dotted lines, A and B, both indicating the resulting level at the positive input. When
the signal at the negative input is low, the state of the input stage is well defined with the negative input much
lower than the positive input. As a result the output will be in the high state. The positive input is at level A. With
the input signal sloping up, this situation remains until VIN crosses level A at t=1. Now the output toggles, and the
voltage at the positive input is lowered to level B. So before the output has the possibility to toggle again, the
difference between both inputs is made sufficient to have a stable situation again. When the input signal comes
down from high to low, the situation is stable until level B is reached at t=0. At this moment the output will toggle
back, and the circuit is back in the start situation with the negative input at a much lower level than the positive
one. In the situation without hysteresis, the output would toggle exactly at VREF. With hysteresis this happens at
the introduced levels A and B, as can be seen in Figure 29. Varying the levels A and B will also vary the timing of
t=0 and t=1. When designing a circuit be aware of this effect. Introducing hysteresis will cause some time shifts
between output and input (e.g. duty cycle variations), but eliminates undesired switching of the output.
Parasitic Capacitors
In the simple schematic of Figure 28 some capacitors are drawn. The capacitors CP. represent the parasitic
(board) capacitance at the input of the part. This capacity will slow down the change of the level of the positive
input in reaction to the changing output voltage. As a result of this, the output may have the time to switch over
more than once. Actually the parasitic capacity represented by CP makes the attenuation circuit of RF and RP
frequency dependent. The only action to take is to create a frequency independent circuit. This is simply done by
placing the compensation capacitor CC in parallel with RF. The capacitor CC can be calculated with the formula
RF *CC = RP *CP; this means that both of the time constants must be the same to create a frequency
independent network. A simple example gives the following assuming that CP is in total 2.5 pF and as already
calculated RF = 25 kΩ in combination with RP = 50Ω. These input data gives:
CC = RP * CP/RF
CC = 50*2.5e-12/25e3
CC = 5e-15 = 0.005 pF
(1)
(2)
(3)
This is not a practical value and different conclusions are possible:
• No capacitor CC needed
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•
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Place a capacitor CC of 1 pF and accept a big overshoot at the positive input being sure that the input stage
is in a secure new position
Place an extra CP of such a value that CC has a realistic value of say 1 pF (extra CP = ±500 pF).
Position of Feedback Resistors
Another important issue while using positive feedback is the placement of the resistors RP and RF. These
resistors must be placed as near as possible to the positive input, because this input is most sensitive for picking
up spurious signals, noise etc. This connection must be very clean for the best performance of the overall circuit.
With raising speeds the total PCB design becomes more and more critical, the LMH7220 comparator doesn’t
have built in hysteresis, so the input signal must meet minimum requirements to make the output switch over
properly. In the following sections some aspects concerning the load connected to the outputs and transmission
lines will be discussed.
THE OUTPUT SWING PROPERTIES
LVDS has differential outputs which means that both outputs have the same swing but in opposite direction
(Figure 30). Both outputs swing around a voltage called the common mode output voltage (VO). This voltage can
be measured at the midpoint of two equal resistors connected to both outputs as discussed in INPUT & OUTPUT
TOPOLOGY. The absolute value of the difference between both voltages is called VOD. LVDS outputs cannot be
held at the VO level because of their digital nature. They only cross this level during a transition. Due to the
symmetrical structure of the circuit, both output voltages cross at VO regardless if the output changes from ‘0’ to
‘1’ or vise versa.
VOH
Output Q
VOD
VO
Output Q
VOL
Figure 30. LVDS Output Signals
In case the outputs aren’t symmetrical or are a-symmetrically loaded, the output voltages differ from the situation
of Figure 30. For this non-ideal situation there are two additional parameters defined, ΔVO and ΔVOD, as can be
seen in Figure 31.
Output Q
'VO
VODH
VODL
Output Q
'VOD = | VODH - VODL |
VOD = (VODH + VODL) / 2
'0'
'1'
'0'
Figure 31. LVDS Output Signals with Different Amplitude
ΔVO is the difference in VO between the ‘1’ state and the ‘0’ state. This variation is acceptable if it is below 50 mV
following the ANSI/TIA/EIA-644 LVDS standard. It is also possible that VOD in the ‘1’ state isn’t the same as in
the ‘0’ state. This parameter is specified as ΔVOD, and is calculated as the absolute value of the difference of
VODH and VODL.
22
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LOADING THE OUTPUT
The output structure creates a current (ILOOP see Figure 32) through an external differential load resistor of 100Ω
nominal. This results in a differential output voltage of 325 mV. The outputs of the comparator are connected to
tracks on a PCB. These tracks can be seen as a differential transmission line. The differential load resistor acts
as a high frequency termination at the end of the transmission line. This means that for a proper signal behavior
the PCB tracks have to be dimensioned for a characteristic impedance of 100Ω as well. Changing the load
resistor also implies a change of the transmission line impedance. More about transmission lines and termination
can be found in the next section. The signal across the 100Ω termination resistor is fed into the inputs of
subsequent circuitry that processes the data. Any connection to input circuitry of course draws current from the
comparator’s outputs. In the case of a balanced input connected to the load resistance, current IP is drawn from
both output connection points to ground. Keep in mind that the LMH7220’s ability to source currents is much
higher than to sink them. The connected input circuitry also forms a differential load to the outputs of the
comparator (see Figure 32). This will cause the voltage across the termination resistor to differ from its nominal
value.
IP
CP
VCC
RP
ILOOP
IN+
IN-
+
-
OUT Q
OUT Q
GND
RLOAD
CLOAD
RP
CP
LOAD
IP
Figure 32. Load
In general one single connection only draws a few µA’s, and doesn’t have much effect on the LVDS output
voltage. For multiple inputs on one output pair, load currents must not exceed the specified limits, as described in
the ANSI or IEEE LVDS standards. Below a specified value of VOD, the functioning of subsequent circuitry
becomes uncertain. However under normal conditions there is no need to worry. Another point of practice is load
capacitances. Capacitances are applied differentially (CLOAD) and also to ground (CP). All of these capacitors will
disturb the pulse shape. The edges of the output pulse become slower, and in reaction the detection of the
transition comes at a later moment. Be aware of this effect when measuring with probes. Both single ended and
differential probes have these capacitances. A standard probe commonly has a load capacity of about 8 to 10
pF. This will cause some degradation of the pulse shape and will add some time delay.
TRANSMISSION LINES & TERMINATION TECHNOLOGIES
The LMH7220 uses LVDS technology. LVDS is a way to communicate data using low voltage swing and low
power consumption. Nowadays data rates are growing, requiring increasing speed. Data isn’t only connected to
other IC’s on a single PCB board but in many cases there are interconnections from board to board or from
equipment to equipment. Distances can be short or long but it is always necessary to have a reliable connection,
consume low power and to be able to handle high data rates. LVDS is a differential signal protocol. The
advantage over single ended signal transmission is its higher immunity to common mode noise. Common mode
signals are signals that are equally apparent on both lines and because the receiver only looks at the difference
between both lines, this noise is canceled.
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Maximum Bitrates
A very important specification in high speed circuits are the rise and fall times. In fact these determine the
maximum toggle rate (TR) of the part. The LVDS standard specifies them at 0.26 ns to 1.5 ns. Rise and fall times
are normally specified at 20% and 80% of the signal amplitude (60% difference). TR is defined as the bitrate at
which the differential output voltage drops to 50% of its nominal value.
period
1
VOUT
period
2
80%
Decision Level
20%
1
bit
0
0
1
0
1
0
1
0
Ideal Pulse Out
Figure 33. Bitrate
Need for Terminated Transmission Lines
During the ‘80’s and ‘90’s TI fabricated the 100k ECL logic family. The rise and fall time specification was 0.75 ns
which was very fast and will easily introduce errors in digital circuits if insufficient care has been taken to the
transmission lines and terminations used for these signals. To be helpful to designers that use ECL with “old”
PCB-techniques, the 10k ECL family was introduced with a rise and fall time specification of 2 ns. This was much
slower and more easy to use. LVDS signals have transition times that exceed the fastest ECL family. A careful
PCB design is needed using RF techniques for transmission and termination. Transmission lines can be formed
in several ways. The most commonly used types are the coaxial cable and the twisted pair telephony cable
(Figure 34).
D
2h
d
d
Parallel Wire
Coax Cable
Figure 34. Cable Configuration
These cables have a characteristic impedance determined by their geometric parameters. Widely used
impedances for the coaxial cable are 50Ω and 75Ω. Twisted pair cables have impedances of about 120Ω to
150Ω.
Other types of transmission lines are the strip line and the micro strip. These last types are used on PCB boards.
They have the characteristic impedance dictated by the physical dimensions of a track placed over a metal
ground plane (See Figure 35).
24
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xxxx
xx
xxxx
xx
xxxx
xx
xxxx
SNOSAL3E – SEPTEMBER 2006 – REVISED MAY 2013
top copper
signal line
PCB
FR4
bottom copper
stripline
signal line
Top Copper
PCB
FR4
bottom copper
Microstrip
signal lines
Top Copper
x
PCB
FR4
bottom copper
differential microstrip
Figure 35. PCB Transmission Lines
Differential Microstrip Line
The transmission line which is ideally suited for LVDS signals is the differential micro strip line. This is a double
micro strip line with a narrow space in between. This means both lines have a strong coupling and this
determines mainly the characteristic impedance. The fact that they are routed above a copper plane doesn't
affect differential impedance, only CM-capacitance is added. Each of the structures above has its own geometric
parameters so for each structure there is another formula to calculate the right impedance. For calculations of
these transmission lines visit the Texas Instruments website or feel free to order the RAPIDESIGNER. For some
formula’s given in the ‘LVDS owners manual’ see chapter 3 (see INTRODUCTION for the URL). At the end of the
transmission line there must be a termination having the same impedance as of the transmission line itself. It
doesn’t matter what impedance the line has, if the load has the same value no reflections will occur. When
designing a PCB board with transmission lines on it, space becomes an important item especially on high density
boards. With a single micro strip line, line width is fixed for given impedance and a board material. Other line
widths will result in different impedances.
Advantage of Differential Microstrip
Impedances of transmission lines are always dictated by their geometric parameters. This is also true for
differential micro strip lines. Using this type of transmission lines, track distance determines mainly the resulting
impedance. So, if the PCB manufacturer can produce reliable boards with narrow track spacing the track width
for a given impedance is also small. The wider the spacing, the wider tracks are needed for a certain impedance.
For example two tracks of 0.2 mm width and 0.1 mm spacing have the same impedance as two tracks of 0.8 mm
width and 0.4 mm spacing. With high-end PCB processes, it is possible to design very narrow differential
microstrip transmission lines. It is desirable to use these phenomena to create optimal connections to the
receiving part or the terminating resistor, in accordance with their physical dimensions. Seen from the
comparator, the termination resistor must be connected at the far end of the line. Open connections after the
termination resistor (e.g. to an input of a receiver) must be as short as possible. The allowed length of such
connections varies with the received transients. The faster the transients the shorter open lines must be to
prevent signal degradation.
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PCB LAYOUT CONSIDERATIONS AND COMPONENT VALUES SELECTION
High frequency designs require that both active- and passive components are selected that are specially
designed for this purpose. For reliable high speed design it is highly recommended also to use small surface
mount passive components because these packages have low parasitic capacitance and low inductance simply
because they have no leads to connect them to the PCB. It is possible to amplify signals at frequencies of
several hundreds of MHz using standard through- hole resistors. Surface mount devices however are better
suited for this purpose. Another important issue is the PCB itself, which is no longer a simple carrier for all the
parts and a medium to interconnect them. The PCB becomes a real component itself and consequently
contributes its own high frequency properties to the overall performance of the circuit. Practice dictates that a
high frequency design at least has one ground plane, providing a low impedance path for all decoupling
capacitors and other ground connections. Care should be taken especially that on-board transmission lines have
the same impedance as the cables to which they are connected. Most single ended applications have 50Ω
impedance (75Ω for video and cable TV applications). On PCBs, such low impedance single ended microstrip
transmission lines usually require much wider traces (2 to 3 mm) on a standard double sided PCB board than
needed for a ‘normal’ trace. Another important issue is that inputs and outputs shouldn’t ‘see’ each other. This
occurs if input- and output tracks are routed in parallel over the PCB with only a small amount of physical
separation, and particularly when the difference in signal level is high. Furthermore components should be
placed as flat and low as possible on the surface of the PCB. For higher frequencies a long lead can act as a
coil, a capacitor or an antenna. A pair of leads can even form a transformer. Careful design of the PCB
minimizes oscillations, ringing and other unwanted behavior. For ultra high frequency designs only surface mount
components will give acceptable results. (for more information see OA-15 [SNOA367]).
TI suggests the following evaluation boards as a guide for high frequency layout and as an aid in device testing
and characterization.
LMH730220 / 551012993-002 Rev A
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REVISION HISTORY
Changes from Revision D (April 2013) to Revision E
•
Page
Changed layout of National Data Sheet to TI format .......................................................................................................... 26
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PACKAGE OPTION ADDENDUM
www.ti.com
10-Dec-2020
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
Eco Plan
(2)
Lead finish/
Ball material
MSL Peak Temp
Op Temp (°C)
Device Marking
(3)
(4/5)
(6)
LMH7220MK/NOPB
ACTIVE
SOT-23-THIN
DDC
6
1000
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
C29A
(1)
The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2)
RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance
do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may
reference these types of products as "Pb-Free".
RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.
Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of
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