LMH7324
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SNOSAZ2G – SEPTEMBER 2007 – REVISED MARCH 2013
LMH7324 Quad 700 ps High Speed Comparator with RSPECL Outputs
Check for Samples: LMH7324
FEATURES
DESCRIPTION
•
•
•
•
•
•
The LMH7324 is a quad comparator with 700 ps
propagation delay and low dispersion of 20 ps for a
supply voltage of 5V. The input voltage range
extends 200 mV below the negative supply. This
enables the LMH7324 to ground sense even when
operating on a single power supply. The device
operates from a wide supply voltage range from 5V to
12V, which allows for a wide input voltage range.
However, if a wide input voltage range is not
required, operating from a single-ended 5V supply
results in a significant power savings, and less heat
dissipation.
1
2
•
(VCCI = VCCO = +5V, VEE = 0V.)
Propagation Delay 700 ps
Overdrive Dispersion 20 ps
Fast Rise and Fall Times 150 ps
Supply Range 5V to 12V
Input Common Mode Range Extends 200 mV
Below Negative Rail
RSPECL outputs
APPLICATIONS
•
•
•
•
•
•
The outputs of the LMH7324 are RSPECL compatible
and can also be configured to create LVDS levels.
The LMH7324 operates over the industrial
temperature range of −40°C to 125°C. The LMH7324
is available in a 32-Pin WQFN package.
Digital Receivers
High Speed Signal Restoration
Zero-crossing Detectors
High Speed Sampling
Window Comparators
High Speed Signal Triggering
Typical Application
VCCI
VCCO
10 nF
10 nF
VCCI
VCCO
IN-
Q
DEVICE WITH
RS(P)ECL INPUTS
¼ LMH7324
IN+
51:
51:
Q
VEE
VEE
51:
51:
VT
VREF
GND
10 nF
+
10 PF
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 © 2007–2013, Texas Instruments Incorporated
LMH7324
SNOSAZ2G – SEPTEMBER 2007 – REVISED MARCH 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 (1)
ESD Tolerance
(2)
Human Body Model
2.5 kV
Machine Model
250V
Output Short Circuit Duration
See
(3) (4)
Supply Voltages (VCCx –VEE)
13.2V
Voltage at Input/Output Pins
±13V
Soldering Information
Infrared or Convection (20 sec.)
235°C
Wave Soldering (10 sec.)
260°C
−65°C to +150°C
Storage Temperature Range
Junction Temperature
(1)
(3)
+150°C
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 ensured specifications and the test
conditions, see the Electrical Characteristics.
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)
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.
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.
(2)
(3)
(4)
Operating Ratings (1)
Supply Voltage (VCCx – VEE)
5V to 12V
−40°C to +125°C
Temperature Range
Package Thermal Resistance
32-Pin WQFN
(1)
36°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 ensured specifications and the test
conditions, see the Electrical Characteristics.
12V DC Electrical Characteristics
Unless otherwise specified, all limits are ensured for TJ = 25°C. VCCI = VCCO = 12V, VEE = 0V, RL = 50Ω to VCCO-2V, VCM = 300
mV. (1)
Symbol
Parameter
Min (2)
Typ (3)
VIN Differential = 0V
−5
−2.5
VIN Differential = 0V
−250
40
Conditions
Max (2)
Units
Input Characteristics
(4)
IB
Input Bias Current
IOS
Input Offset Current
TC IOS
Input Offset Current TC
VOS
Input Offset Voltage
TC VOS
Input Offset Voltage TC
VRI
Input Voltage Range
(1)
(2)
(3)
(4)
(5)
2
(5)
VIN Differential = 0V
VCM = 0V
(5)
0.15
−9.5
VCM = 0V
CMRR > 50 dB
µA
250
+9.5
mV
μV/°C
7
VEE
nA
nA/°C
VCCI−2
V
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.
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.
Positive current corresponds to current flowing into the device.
Average Temperature Coefficient is determined by dividing the change in a parameter at temperature extremes by the total temperature
change.
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12V DC Electrical Characteristics (continued)
Unless otherwise specified, all limits are ensured for TJ = 25°C. VCCI = VCCO = 12V, VEE = 0V, RL = 50Ω to VCCO-2V, VCM = 300
mV. (1)
Symbol
Min (2)
Typ (3)
Max (2)
Units
+12
V
Parameter
Conditions
VRID
Input Differential Voltage Range
VEE ≤ INP or INM ≤ VCCI
CMRR
Common Mode Rejection Ratio
0V ≤ VCM ≤ VCC−2V
83
dB
PSRR
Power Supply Rejection Ratio
VCM = 0V, 5V ≤ VCC ≤ 12V
75
dB
AV
Active Gain
Hyst
Hysteresis
−12
Fixed Internal Value
54
dB
20.8
mV
Output Characteristics
VOH
Output Voltage High
VIN Differential = 25 mV
10.78
10.85
10.93
V
VOL
Output Voltage Low
VIN Differential = 25 mV
10.43
10.50
10.58
V
VOD
Output Voltage Differential
VIN Differential = 25 mV
300
345
400
mV
Power Supplies
IVCCI
VCCI Supply Current/Channel
VIN Differential = 25 mV
5.6
8
IVCCO
VCCO Supply Current/Channel
VIN Differential = 25 mV
11.6
17
mA
12V AC Electrical Characteristics
Unless otherwise specified, all limits are ensured for TJ = 25°C. VCCI = VCCO = 12V, VEE = 0V, RL = 50Ω to VCCO-2V, VCM =
300 mV. (1)
Symbol
TR
Parameter
Conditions
Min (2)
Typ (3)
Max (2)
Units
Maximum Toggle Rate
Overdrive = ±50 mV, CL = 2 pF
@ 50% Output Swing
3.84
Gb/s
Minimum Pulse Width
Overdrive = ±50 mV, CL = 2 pF
@ 50% Output Swing
280
ps
tjitter-RMS
RMS Random Jitter
Overdrive = ±100 mV, CL = 2 pF
Center Frequency = 140 MHz
Bandwidth = 10 Hz–20 MHz
615
fs
tPDH
Propagation Delay
(see Figure 15 application note)
Overdrive 20 mV
737
Overdrive 50 mV
720
Input SR = Constant
VIN Startvalue = VREF −100 mV
Overdrive 100 mV
706
Overdrive 1V
731
Input Overdrive Dispersion
tPDH @ Overdrive 20 mV ↔ 100 mV
31
tPDH @ Overdrive 100 mV ↔ 1V
25
tOD-disp
ps
ps
tSR-disp
Input Slew Rate Dispersion
0.1 V/ns to 1 V/ns
Overdrive 100 mV
40
ps
tCM-disp
Input Common Mode Dispersion
SR = 1 V/ns, Overdrive 100 mV,
0V ≤ VCM ≤ VCCI – 2V
28
ps
ΔtPDLH
Q to Q Time Skew
| tPDH - tPDL | (4)
Overdrive = 50 mV, CL = 2 pF
55
ps
ΔtPDHL
Q to Q Time Skew
| tPDL - tPDH | (4)
Overdrive = 50 mV, CL = 2 pF
40
ps
tr
Output Rise Time (20% - 80%) (5)
Overdrive = 50 mV, CL = 2 pF
140
ps
tf
Output Fall Time (20% - 80%) (5)
Overdrive = 50 mV, CL = 2 pF
140
ps
(1)
(2)
(3)
(4)
(5)
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.
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.
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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5V DC Electrical Characteristics
Unless otherwise specified, all limits are ensured for TJ = 25°C. VCCI = VCCO = 5V, VEE = 0V, RL = 50Ω to VCCO-2V, VCM = 300
mV. (1)
Symbol
Parameter
Conditions
(4)
IB
Input Bias Current
IOS
Input Offset Current
TC IOS
Input Offset Current TC
VOS
Input Offset Voltage
(5)
Min
Typ
VIN Differential = 0V
−5
−2.2
VIN Differential = 0V
−250
30
VIN Differential = 0V
(3)
Max
(2)
VCM = 0V
Units
µA
+250
0.1
−9.5
VCM = 0V
(5)
(2)
nA
nA/°C
+9.5
mV
μV/°C
TC VOS
Input Offset Voltage TC
VRI
Input Voltage Range
CMRR > 50 dB
VEE
7
VCCI−2
V
VRID
Input Differential Voltage Range
VEE ≤ INP or INM ≤ VCCI
−5
+5
V
CMRR
Common Mode Rejection Ratio
0V ≤ VCM ≤ VCC−2V
80
dB
PSRR
Power Supply Rejection Ratio
VCM = 0V, 5V ≤ VCC ≤ 12V
75
dB
AV
Active Gain
54
dB
Hyst
Hysteresis
22.5
mV
Fixed Internal Value
Output Characteristics
VOH
Output Voltage High
VIN Differential = 25 mV
3.8
3.87
3.95
V
VOL
Output Voltage Low
VIN Differential = 25 mV
3.45
3.52
3.60
V
VOD
Output Voltage Differential
VIN Differential = 25 mV
300
345
400
mV
Power Supplies
IVCCI
VCCI Supply Current/Channel
VIN Differential = 25 mV
5.4
7.5
mA
IVCCO
VCCO Supply Current/Channel
VIN Differential = 25 mV
11
15
mA
(1)
(2)
(3)
(4)
(5)
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.
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.
Positive current corresponds to current flowing into the device.
Average Temperature Coefficient is determined by dividing the change in a parameter at temperature extremes by the total temperature
change.
5V AC Electrical Characteristics
Unless otherwise specified, all limits are ensured for TJ = 25°C. VCCI = VCCO = 5V, VEE = 0V, RL = 50Ω to VCCO-2V, VCM = 300
mV. (1)
Symbol
TR
tjitter-RMS
(1)
(2)
(3)
4
Parameter
Conditions
Min
(2)
Typ
(3)
Max
(2)
Units
Maximum Toggle Rate
Overdrive = ±50 mV, CL = 2 pF
@ 50% Output Swing
3.72
Gb/s
Minimum Pulse Width
Overdrive = ±50 mV, CL = 2 pF
@ 50% Output Swing
290
ps
RMS Random Jitter
Overdrive = ±100 mV, CL = 2 pF
Center Frequency = 140 MHz
Bandwidth = 10 Hz–20 MHz
602
fs
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.
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.
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5V AC Electrical Characteristics (continued)
Unless otherwise specified, all limits are ensured for TJ = 25°C. VCCI = VCCO = 5V, VEE = 0V, RL = 50Ω to VCCO-2V, VCM = 300
mV. (1)
Symbol
tPDH
tOD-disp
Parameter
Conditions
Min
(2)
Typ
(3)
Propagation Delay
(see Figure 15 application note)
Overdrive 20 mV
740
Overdrive 50 mV
731
Input SR = Constant
VIN Startvalue = VREF −100 mV
Overdrive 100 mV
722
Overdrive 1V
740
Input Overdrive Dispersion
TPDH @ Overdrive 20 mV ↔ 100 mV
18
TPDH @ Overdrive 100 mV ↔ 1V
19
Max
(2)
Units
ps
ps
tSR-disp
Input Slew Rate Dispersion
0.1 V/ns to 1 V/ns,
Overdrive = 100 mV
40
ps
tCM-disp
Input Common Mode Dispersion
SR = 1 V/ns, Overdrive 100 mV,
0V ≤ VCM ≤ VCCI – 2V
24
ps
ΔtPDLH-disp
Q to Q Time Skew
| tPDH - tPDL | (4)
Overdrive = 50 mV, CL = 2 pF
60
ps
ΔtPDHL
Q to Q Time Skew
| tPDL - tPDH | (4)
Overdrive = 50 mV, CL = 2 pF
40
ps
tr
Output Rise Time (20% - 80%)
Overdrive = 50 mV, CL = 2 pF
145
ps
tf
Output Fall Time (20% - 80%)
Overdrive = 50 mV, CL = 2 pF
145
ps
(4)
(5)
(5)
(5)
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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Connection Diagram
VCCI
VCCO
IN-
Q
¼
IN+
LMH7324
Q
INA-
INA+
VEEA
VEED
IND+
IND-
VCCID
VEE
VCCIA
VEE
32
31
30
29
28
27
26
25
VCCOA
1
24
VCCOD
QA
2
23
QD
QA
3
22
QD
VEEA
4
21
VEED
LMH7324
19
QC
QB
7
18
QC
VCCOB
8
17
VCCOC
11
12
13
14
15
16
VCCIC
10
VEEC
9
INC-
6
INC+
QB
VEEB
VEEC
INB+
20
INB-
5
VCCIB
VEEB
Figure 1. 32-Pin WQFN
Top View
6
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Typical Performance Characteristics
At TJ = 25°C, V+ = +5V, V− = 0V, unless otherwise specified.
Propagation Delay vs.Supply Voltage
Propagation Delay vs.Temperature
900
900
850
125°C
PROPAGATION DELAY (ps)
PROPAGATION DELAY (ps)
VCM = 300 mV
800
85°C
750
-40°C
0° and 25°C
700
650
VCM = 300 mV
600
5
VIN = ±100 mV
6
7
8
9
10
11
VINDIFF = ±100 mV
850
800
VS = 5V
750
700
VS = 12V
650
600
-50
12
-25
0
50
100 125
75
TEMPERATURE (°C)
SUPPLY VOLTAGE (V)
Figure 2.
Figure 3.
Propagation Delay vs.Overdrive Voltage
Propagation Delay vs.Supply Voltage for Different
Overdrive
900
850
VCM = 300 mV
VIN = (VCM - 100 mV) to
850
PROPAGATION DELAY (ps)
PROPAGATION DELAY (ps)
25
(VCM + VOVERDRIVE)
800
5V
750
700
12V
650
600
VCM = 300 mV
825 V = (V
IN
CM - 100 mV) to
(VCM + VOVERDRIVE)
800
1000 mV
775
20 mV
750
725
700
50 mV
675
0
300
600
900
1200
650
1500
100 mV
5
6
8
9
11
12
SUPPLY VOLTAGE (V)
OVERDRIVE VOLTAGE (mV)
Figure 4.
Figure 5.
Propagation Delay vs.Common Mode Voltage
Propagation Delay vs.Slew Rate
900
800
750
PROPAGATION DELAY (ps)
PROPAGATION DELAY (ps)
VCM = 300 mV
5V
12V
700
650
SR = 2 V/ns
VIN = ±100 mV
600
-1 0 1 2 3 4 5 6 7 8 9 10 11 12
VOD = 100 mV
850
800
5V
12V
750
700
650
600
0
200
400
600
800
1000
SLEW RATE (V/µs)
COMMON MODE VOLTAGE (V)
Figure 6.
Figure 7.
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Typical Performance Characteristics (continued)
−
+
At TJ = 25°C, V = +5V, V = 0V, unless otherwise specified.
Pulse Response and Maximum Toggle Rate
Bias Current vs.Temperature
400
-1.0
140 MHz
fMAX
-1.5
200
BIAS CURRENT ( A)
OUTPUT VOLTAGE (mV)
300
100
0
-100
-200
-2.0
-2.5
VS = 5V
VCM = 2.5V
-3.0
VINDIFF = 0V
-300
VS = 5V
-3.5
-400
-500
0
2
4
6
8
-4.0
-50
10
-25
TIME (ns)
0
25
Figure 8.
DIFFERENTIAL OUTPUT VOLTAGE (V)
INPUT CURRENT ( A)
125
Output Voltage vs.Input Voltage
0
VCM = 2.5V
VS = 5V
VIN+ = 0 to 5V
VIN- = 5 to 0V
-3
IB+
IB-
-4
-5
-5
100
0.4
1
-2
75
Figure 9.
Input Current vs.Differential Input Voltage
-1
50
TEMPERATURE (°C)
-4
-3
-2
-1
0
1
2
3
4
0.3
25°C & -40°C
0.2
-40°C
25°C
0.1
0
-0.2 VS = 5V
VCM = 300 mV
-0.3
-0.4
-40 -30 -20
5
125°C
125°C
-0.1
DIFFERENTIAL INPUT VOLTAGE (V)
-10
0
10
20
30
40
DIFFERENTIAL INPUT VOLTAGE (mV)
Figure 10.
Figure 11.
Hysteresis Voltage vs. Temperature
30
HYSTERESIS VOLTAGE (mV)
29
28
VS = 5V
27
VCM = 300 mV
26
25
24
23
22
21
20
-50
-25
0
25
50
75
100
125
TEMPERATURE (°C)
Figure 12.
8
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APPLICATION INFORMATION
INTRODUCTION
The LMH7324 is a high speed comparator with RS(P)ECL (Reduced Swing Positive Emitter Coupled Logic)
outputs, and is compatible with LVDS (Low Voltage Differential Signaling) if VCCO is set to 2.5V. The use of
complementary outputs gives a high level of suppression for common mode noise. The very fast rise and fall
times of the LMH7324 enable data transmission rates up to several Gigabits per second (Gbps). The LMH7324
inputs have a common mode voltage range that extends 200 mV below the negative supply voltage thus allowing
ground sensing when used with a single supply. The rise and fall times of the LMH7324 are about 150 ps, while
the propagation delay time is about 700 ps. The LMH7324 can operate over the supply voltage range of 5V to
12V, while using single or dual supply voltages. This is a flexible way to interface between several high speed
logic families. Several configurations are described in the section INTERFACE BETWEEN LOGIC FAMILIES.
The outputs are referenced to the positive VCCO supply rail. The supply current is 17 mA at 5V (per comparator,
load current excluded.) The LMH7324 is offered in a 32-Pin WQFN package. This small package is ideal where
space is an important issue.
INPUT & OUTPUT TOPOLOGY
All input and output pins are protected against excessive voltages by ESD diodes. These diodes are conducting
from the negative supply to the positive supply. As can be seen in Figure 13, both inputs are connected to these
diodes. Protection against excessive supply voltages is provided by two power clamps per comparator: one
between the VCCI and the VEE and one between the VCCO and the VEE.
VCCI
VCCI
VCCI
VCCI
VCCO
IN-
IN+
VEE
VEE
VEE
Power
Clamp
2X per Comparator
Figure 13. Equivalent Input Circuitry
The output stage of the LMH7324 is built using two emitter followers, which are referenced to the VCCO. (See
Figure 14.) Each of the output transistors is active when a current is flowing through any external output resistor
connected to a lower supply rail. Activating the outputs is done by connecting the emitters to a termination
voltage which lies 2V below the VCCO. In this case a termination resistor of 50Ω can be used and a transmission
line of 50Ω can be driven. Another method is to connect the emitters through a resistor to the most negative
supply by calculating the right value for the emitter current in accordance with the datasheet tables. Both
methods are useful, but they each have good and bad aspects.
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VCCO
Output Q
Output Q
VEE
Figure 14. Equivalent Output Circuitry
The output voltages for ‘1’ and ‘0’ have a difference of approximately 400 mV and are respectively 1.1V (for the
‘1’) and 1.5V (for the ‘0’) below the VCCO. This swing of 400 mV is enough to drive any LVDS input but can also
be used to drive any ECL or PECL input, when the right supply voltage is chosen, especially the right level for
the VCCO.
DEFINITIONS
This table provides a short description of the parameters used in the datasheet and in the timing diagram of
Figure 15.
Symbol
Text
Description
IB
Input Bias Current
Current flowing in or out of the input pins, when both are biased at the VCM
voltage as specified in the tables.
IOS
Input Offset Current
Difference between the input bias current of the inverting and non-inverting
inputs.
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.
VRI
Input Voltage Range
Voltage which can be applied to the input pin maintaining normal operation.
VRID
Input Differential Voltage Range
Differential voltage between positive and negative input at which the input clamp
is not working. The difference can be as high as the supply voltage but excessive
input currents are flowing through the clamp diodes and protection resistors.
CMRR
Common Mode Rejection Ratio
Ratio of input offset voltage change and input common mode voltage change.
PSRR
Power Supply Rejection Ratio
Ratio of input offset voltage change and supply voltage change from VS-MIN to VSMAX.
AV
Active Gain
Overall gain of the circuit.
Hyst
Hysteresis
Difference between the switching point ‘0’ to ‘1’ and vice versa.
VOH
Output Voltage High
High state single ended output voltage (Q or Q). (See Figure 29)
VOL
Output Voltage Low
Low state single ended output voltage (Q or Q). (See Figure 29)
VOD
Average of VODH and VODL
(VODH + VODL)/2
IVCCI
Supply Current Input Stage
Supply current into the input stage.
IVCCO
Supply Current Output Stage
Supply current into the output stage while current through the load resistors is
excluded.
IVEE
Supply Current VEE Pin
Current flowing out of the negative supply pin.
TR
Maximum Toggle Rate
Maximum frequency at which the outputs can toggle at 50% of the nominal VOH
and VOL.
PW
Pulse Width
Time from 50% of the rising edge of a signal to 50% of the falling edge.
10
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Symbol
Text
tPDH resp tPDL
Description
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).
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.
ΔtPDLH resp
ΔtPDHL
Q to Q Time Skew
Time skew between 50% levels of the rising edge of Q output and the 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.
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%.
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
tPDHd
'tPDQ = | tPDL - tPDH |
80% or 90%
Differential
Output Signal
0
tPDLd
10% or 20%
tPDd = (tPDHd + tPDLd)/ 2
'tPDd = | tPDHd - tPDLd |
tfd
Figure 15. Timing Definitions
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PIN DESCRIPTIONS
Pin
Name
Description
Part
Comment
1.
VCCO
Positive Supply Output Stage
A
This supply pin is independent of the supply for the input stage. This
allows output levels of different logic families.
2.
Q
Inverted Output
A
Output levels are determined by the choice of VCCOA.
3.
Q
Output
A
Output levels are determined by the choice of VCCOA.
4.
VEE
Negative Supply
A
All four VEE pins are circular connected together via two antiparallel
diodes. (See Figure 16)
5.
VEE
Negative Supply
B
All four VEE pins are circular connected together via two antiparallel
diodes. (See Figure 16)
6.
Q
Output
B
Output levels are determined by the choice of VCCOB.
7.
Q
Inverted Output
B
Output levels are determined by the choice of VCCOB.
8.
VCCO
Positive Supply Output Stage
B
This supply pin is independent of the supply for the input stage. This
allows output levels of different logic families.
9.
VCCI
Positive Supply for Input Stage
B
This supply pin is independent of the supply for the output stage.
VCCIand VCCO share the same ground pin VEE.
10.
IN−
Negative Input
B
Input for analog voltages between 200 mV below VEE and 2V below
VCCI.
11.
IN+
Positive Input
B
Input for analog voltages between 200 mV below VEE and 2V below
VCCI.
12.
VEE
Negative Supply
B
All four VEE pins are circular connected together via two antiparallel
diodes. (See Figure 16)
13.
VEE
Negative Supply
C
All four VEE pins are circular connected together via two antiparallel
diodes. (See Figure 16)
14.
IN+
Positive Input
C
Input for analog voltages between 200 mV below VEE and 2V below
VCCI.
15.
IN−
Negative Input
C
Input for analog voltages between 200 mV below VEE and 2V below
VCCI.
16.
VCCI
Positive Supply for Input Stage
C
This supply pin is independent of the supply for the output stage. VCCI
and VCCO share the same ground pin VEE.
17.
VCCO
Positive Supply Output Stage
C
This supply pin is independent of the supply for the input stage. This
allows output levels of different logic families.
18.
Q
Inverted Output
C
Output levels are determined by the choice of VCCOC.
19.
Q
Output
C
Output levels are determined by the choice of VCCOC.
20.
VEE
Negative Supply
C
All four VEE pins are circular connected together via two antiparallel
diodes. (See Figure 16)
21.
VEE
Negative Supply
D
All four VEE pins are circular connected together via two antiparallel
diodes. (See Figure 16)
22.
Q
Output
D
Output levels are determined by the choice of VCCOD.
23.
Q
Inverted Output
D
Output levels are determined by the choice of VCCOD.
24.
VCCO
Positive Supply Output Stage
D
This supply pin is independent of the supply for the input stage. This
allows output levels of different logic families.
25.
VCCI
Positive Supply for Input Stage
D
This supply pin is independent of the supply for the output stage. VCCI
and VCCO share the same ground pin VEE.
26.
IN−
Negative Input
D
Input for analog voltages between 200 mV below VEE and 2V below
VCCI.
27.
IN+
Positive Input
D
Input for analog voltages between 200 mV below VEE and 2V below
VCCI.
28.
VEE
Negative Supply
D
All four VEE pins are circular connected together via two antiparallel
diodes. (See Figure 16)
29.
VEE
Negative Supply
A
All four VEE pins are circular connected together via two antiparallel
diodes. (See Figure 16)
30.
IN+
Positive Input
A
Input for analog voltages between 200 mV below VEE and 2V below
VCCI.
31.
IN−
Negative Input
A
Input for analog voltages between 200 mV below VEE and 2V below
VCCI.
12
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PIN DESCRIPTIONS (continued)
Pin
Name
Description
Part
Comment
32.
VCCI
Positive Supply for Input Stage
A
This supply pin is independent of the supply for the output stage. VCCI
and VCCO share the same ground pin VEE.
33.
DAP
Central Pad at the Bottom of the
Package
All
The purpose of this pad is to transfer heat outside the part.
TIPS & TRICKS USING THE LMH7324
This section discusses several aspects concerning special applications using the LMH7324.Topics include the
connection of the DAP in conjunction to the VEE pins and the use of this part as an interface between several
logic families. Other sections discuss several widely used definitions and terms for comparators. The final
sections explain some aspects of transmission lines and the choice for the most suitable components handling
very fast pulses.
THE DAP AND THE VEE PINS
To protect the device against damage during handling and production, two antiparallel connected diodes are
placed between the VEE pins. Under normal operating conditions (all VEE pins have the same voltage level) these
diodes are not functioning, as can be seen in Figure 16.
The DAP (Die Attach Paddle) functions as a heat sink which means that heat can be transferred, using vias
below this pad, to any appropriate copper plane. The DAP is isolated from all other electrical connections and
therefore it is possible to connect this pad to any voltage within the allowed voltage range of the part. Using a
DAP connection it is common practice to connect such a pad to the lowest supply voltage. However in high
frequency designs it can be useful to connect this pad to another supply such as e.g. the ground plane, while the
VEE is for example -5 Volt.
A
DAP
VEE
D
VEE
VEE
B
VEE
C
Figure 16. DAP and VEE Configuration
INTERFACE BETWEEN LOGIC FAMILIES
The LMH7324 can be used to interface between different logic families. The feature that facilitates this is the fact
that the input stage and the output stage use different positive power supply pins which can be used at different
voltages. The only restriction is that both input (VCCI) and output (VCCO) supplies require a minimum of 5V
difference relative to VEE. The negative supply pins are connected together for all four parts. Using the power
pins at different supply voltages enables level-translation between two logic families. For example, it is possible
to translate from logic at negative voltage levels , such as ECL, to logic at positive levels, such as RSPECL and
LVDS and vice versa.
Interface from ECL to RSPECL
The supply pin VCCI can be connected to ground because the input levels are negative and VEE is at −5.2V. With
this setup the minimum requirements for the supply voltage of 5V are obtained. The VCCO pin must operate at
+5V to create the RSPECL levels. (See Figure 17.)
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5V
+
VCCI
ECL Driver
Coupled
Transmission Line
VCCO
Line Termination
IN+
Q
¼
IN-
LMH7324
VEE
RS-PECL Output
VOH = 3.9V
VOL = 3.5V
Q
VEE
-5.2V
+
Figure 17. ECL TO RSPECL
Interface from PECL to (RS) ECL
This setup needs the VCCI pin at +5V because the input logic levels are positive. To obtain the ECL levels at the
output it is necessary to connect the VCCO to the ground while the VEE has to be connected to the −5.2V. The
reason for this is that the minimum requirement for the supply is 5V. The high level of the output of the LMH7324
is normally 1.1V below the VCCO supply voltage, and the low level is 1.5V below this supply. The output levels
are now −1100 mV for the logic ‘1’ and −1500 mV for the logic ‘0’. (See Figure 18.)
5V
+
VCCI
PECL Driver
Coupled
Transmission Line
VCCO
Line Termination
IN+
Q
¼
INPECL levels:
VOH = 3.9V
VOL = 3.5V
RSECL Levels:
VOH = -1100 mV
VOL = -1500 mV
LMH7324
Q
VEE
VEE
-5.2V
+
Figure 18. PECL TO RSECL
Interface from Analog to LVDS
As seen in Figure 19, the LMH7324 can be configured to create LVDS levels. This is done by connecting the
VCCO to 2.5V. As discussed before, the output levels are now at VCCO −1.1V for the logic ‘1’ and at VCCO -1.5V for
the logic ‘0’. These levels of 1000 mV and 1400 mV comply with the LVDS levels. As can be seen in this setup,
an AC coupled signal via a transmission line is used. This signal is terminated with 50Ω to the ground. The input
stage has its supply from +5V to −5V, which means that the input common mode level is midway between the
input stage supply voltages.
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2.5V
5V
+
+
VCCI
VCCO
50:
+
IN+
Q
¼
-
Levels:
VOH = 1.4V
VOL = 1.0V
LMH7324
IN-
Q
Signal Source
50:
50:
VEE
VEE
-5V
+
Figure 19. ANALOG TO LVDS
STANDARD COMPARATOR SETUP
Figure 20 shows a standard comparator setup which creates RSPECL levels because the VCCO supply voltage is
+5V. In this setup the VEE pin is connected to the ground level. The VCCI pin is connected to the VCCO pin
because there is no need to use different positive supply voltages. The input signal is AC coupled to the positive
input. To maintain reliable results, even for signals with larger amplitudes, the input pins IN+ and IN− are biased
at 1.4V through a resistive divider using a resistor of 1 kΩ to ground and a resistor of 2.5 kΩ to the VCC and by
adding two decoupling capacitors. Both inputs are connected to the bias level by the use of a 10 kΩ resistor.
With this input configuration the input stage can work in a linear area with signals of approximately 3 VPP. (See
input level restrictions in the data tables.)
5V
+
VCCI
2.5 k:
VIN
VCCO
IN+
Q
¼
IN-
LMH7324
Q
Levels:
VOH = 3.9V
VOL = 3.5V
10 k:
10 k:
VEE
VEE
VREF
+
1 k:
Figure 20. Standard Setup
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 frequencies, the more important the timing properties of the comparator become, because the
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response of the comparator can make a noticeable change in critical parameters such as time frame or duty
cycle. A designer has to know these effects and has to deal with them. In order to predict what the output signal
will do, 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 described in the definition section. Two delay parameters can be
distinguished, tPDH and tPDL as shown in Figure 21. Both parameters do not necessarily have the same value. It is
possible that differences will occur due to a different response of the internal circuitry. As a derivative of this
effect another parameter is defined: ΔtPD. This parameter is defined as the absolute value of the difference
between tPDH and tPDL.
PW
80%
80%
50%
50%
VIN
20%
20%
tPDH
tPDL
80%
80%
50%
Output Q
50%
20%
20%
tr
tf
Figure 21. Propagation Delay
Input Signal
If ΔtPD is not 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 will produce a symmetrical square wave at the output
with a duty cycle of 50%. When tPDH and tPDL are different, the duty cycle of the output signal will not remain at
50%, but will be increased or decreased. In addition to the propagation delay parameters for single ended
outputs discussed before, there are other parameters in the 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 22.) 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 15.
time
VREF
Output Q
tPDH
time
VO
Output Q
'tPDLH
time
VO
tPDL
Figure 22. tPD with Complementary Outputs
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Both output circuits should be symmetrical. At the moment one output is switching ‘on’ the other is switching ‘off’
with ideally no skew between both outputs. The design of the LMH7324 is optimized so that this timing difference
is minimized. The propagation delay, tPD, is defined as the average delay of both outputs at both slopes: (tPDLH +
tPDHL)/2. Both overdrive and starting point should be equally divided around the VREF (absolute values).
DISPERSION
There are several circumstances that will produce a variation of the propagation delay time. This effect is called
dispersion.
Amplitude Overdrive Dispersion
One of the parameters that causes dispersion is the amplitude variation of the input signal. Figure 23 shows the
dispersion due to a variation of the input overdrive voltage. The overdrive is defined as the ‘go to’ differential
voltage applied to the inputs. Figure 23 shows the impact it has on the propagation delay time if the overdrive is
varied from 10 mV to 100 mV. This parameter is measured with a constant slew rate of the input signal.
Overdrive 100 mV
Input Differential Signal
+
Overdrive 10 mV
0
time
-100 mV
Overdrive Dispersion
Output Differential Signal
+
Dispersion
0
time
-
Figure 23. Overdrive Dispersion
The overdrive dispersion is caused by the switching currents in the input stage which are dependent on the level
of the differential input signal.
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Slew Rate Dispersion
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. (See Figure 24.)
+
0
time
Output Differential Signal
-
Slew Rate Dispersion
+
Dispersion
0
time
-
Figure 24. Slew Rate Dispersion
A combination of overdrive and slew rate dispersion occurs when applying signals with different amplitudes 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. (See Figure 25.) When
VREF is swept through the CMVR (Common Mode Voltage Range), it results in a variation of the propagation
delay time. This variation is called Common Mode Dispersion.
Input Differential Signal
Vin cm
+
0
Vin cm
time
-
Output Differential Signal
Common Mode Dispersion
+
Dispersion
0
time
-
Figure 25. Common Mode Dispersion
All of the dispersion effects described previously influence the propagation delay. In practice the dispersion is
often caused by a combination of more than one varied parameter.
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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 take a relatively long time. This enables the comparators
output 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 actions.
The next sections explain these phenomena in situations where no hysteresis is applied, and discuss the
possible improvement hysteresis can give.
Using No Hysteresis
Figure 26 shows 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.
VREF
fast output
time
1
slow output
Input Signal
mV
1
time
0
time
0
t=0
t=1
Figure 26. Oscillations on Output Signal
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. With hysteresis the
switching level is forced to a new level at the moment the input signal crosses this level. This can be seen in
Figure 27.
Input Signal
mV
VREF
A
B
Output
1
0
t=0
t=1
Figure 27. Hysteresis
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In this picture there are two dotted lines A and B, both indicating the resulting level at which the comparator
output will switch over. Assume that for this situation the input signal is connected to the negative input and the
switching level (VREF) to the positive input. The LMH7324 has a built-in hysteresis voltage that is fixed at
approximately 20 mVPP. The input level of Figure 27 starts much lower than the reference level and this means
that the state of the input stage is well defined with the inverting input much lower than the non-inverting input.
As a result the output will be in the high state. Internally the switching level is at A, with the input signal sloping
up, this situation remains until VIN crosses level A at t = 1. Now the output toggles, and the internal switching
level is lowered to level B. So before the output has the possibility to toggle again, the difference between the
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 starting situation with the inverting input at a much lower level than the non-inverting input. In the
situation without hysteresis, the output will toggle exactly at VREF. With hysteresis this happens at the internally
introduced levels A and B, as can be seen in Figure 27. If the levels A and B change, due to a change in the
built-in hysteresis voltage depending of e.g. temperature variations, then the timing of t = 0 and t = 1 will also
vary. The variation of the hysteresis voltage over temperature is very low and ranges from 22 mV to 23 mV at 5V
Supply over a temperature variation of -25 °C to 125 °C (see Figure 28). When designing a circuit be aware of
this effect. Introducing hysteresis will cause some time shift between output and input (e.g. duty cycle variations),
but will eliminate undesired switching of the output.
30
HYSTERESIS VOLTAGE (mV)
29
28
VS = 5V
27
VCM = 300 mV
26
25
24
23
22
21
20
-50
-25
0
25
50
75
100
125
TEMPERATURE (°C)
Figure 28. Hysteresis Voltage over Temperature
THE OUTPUT
Output Swing Properties
The LMH7324 has differential outputs, which means that both outputs have the same swing but in opposite
directions. (See Figure 29.) Both outputs swing around the common mode output voltage (VO). This voltage can
be measured at the midpoint between two equal resistors connected to each output. The absolute value of the
difference between both voltages is called VOD. The 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 of whether the output changes from ‘0’ to ‘1’ or vise versa.
VOH
Output Q
VOD
VO
VOL
Output Q
Figure 29. Output Swing
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Loading the Output
Both outputs are activated when current is flowing through a resistor that is externally connected to VT. The
termination voltage should be set 2V below the VCCO. This makes it possible to terminate each of the outputs
directly with 50Ω, and if needed to connect through a transmission line with the same impedance. (See
Figure 30.) Due to the low ohmic nature of the output emitter followers and the 50Ω load resistor, a capacitive
load of several pF does not dramatically affect the speed and shape of the signal. When transmitting the signal
from one output to any input the termination resistor should match the transmission line. The capacitive load (CP)
will distort the received signal. When measuring this input with a probe, a certain amount of capacitance from the
probe is parallel to the termination resistor. The total capacitance can be as large as 10 pF. In this case there is
a pole at:
f = 1/(2*π*C*R)
f = 1e9/ π
f = 318 MHz
For this frequency the current IP has the same value as the current through the termination resistor. This means
that the voltage drops at the input and the rise and fall times are dramatically different from the specified
numbers for this part.
Another parasitic capacity that can affect the output signal is the capacity directly between both outputs, called
CPAR. (See Figure 30.) The LMH7324 has two complementary outputs so there is the possibility that the output
signal will be transported by a symmetrical transmission line. In this case both output tracks form a coupled line
with their own parasitics and both receiver inputs are connected to the transmission line. Actually the line
termination looks like 100Ω and the input capacities, which are in series, are parallel to the 100Ω termination.
The best way to measure the input signal is to use a differential probe directly across both inputs. Such a probe
is very suitable for measuring these fast signals because it has good high frequency characteristics and low
parasitic capacitance.
IP
CP
VCCO
VCCI
RT
IN+
IN-
+
Q
-
Q
VEE
CPAR
RT
CP
IP
VT
Figure 30. Parasitic Capacities
TRANSMISSION LINES & TERMINATION TECHNOLOGIES
The LMH7324 uses complementary RSPECL outputs and emitter followers, which means high output current
capability and low sensitivity to parasitic capacitance. The use of Reduced Swing Positive Emitter Coupled Logic
gives advantages concerning speed and supply. Data rates are growing, which requires increasing speed. Data
is not 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, which consumes low power and is able to handle high data rates. The
complementary outputs of the LMH7324 make it possible to use symmetrical transmission lines. The advantage
over single ended signal transmission is that the LMH7324 has 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 Bit Rates
The maximum toggle rate is defined at an amplitude of 50% of the nominal output signal. This toggle rate is a
number for the maximum transfer rate of the part and can be given in Hz or in Bps. When transmitting signals in
a NRZ (Non Return to Zero) format the bitrate is double this frequency number, because during one period two
bits can be transmitted. (See Figure 31.) The rise and fall times are very important specifications in high speed
circuits. In fact these times determine the maximum toggle rate of the part. Rise and fall times are normally
specified at 20% and 80% of the signal amplitude (60% difference). Assuming that the edges at 50% amplitude
are coming up and down like a sawtooth it is possible to calculate the maximum toggle rate but this number is
too optimistic. In practice the edges are not linear while the pulse shape is more or less a sinewave.
period period
1
2
80%
VOUT
Decision Level
20%
1
bit
0
0
1
0
1
0
1
0
Ideal Pulse Out
Figure 31. Bit Rates
Need for Terminated Transmission Lines
During the 1980’s and 90’s, TI fabricated the 100K ECL logic family. The rise and fall time specifications were
0.75 ns, which were considered very fast. If sufficient care has not been given in designing the transmission lines
and choosing the correct terminations, then errors in digital circuits are introduced. To be helpful to designers
that use ECL with “old” PCB-techniques, the 10K ECL family was introduced with rise and fall time specifications
of 2 ns. This is much slower and easier to use. The RSPECL output signals of the LMH7324 have transition
times that extend 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. (See Figure 32.)
D
2h
d
d
Parallel Wire
Coax Cable
Figure 32. Cable Types
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 microstrip line. 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 33.)
22
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top copper
signal line
PCB
FR4
bottom copper
stripline
signal line
Top Copper
PCB
FR4
bottom copper
Microstrip
signal lines
Top Copper
PCB
FR4
bottom copper
differential microstrip
Figure 33. PCB Lines
Differential Microstrip Line
The transmission line which is ideally suited for complementary signals is the differential microstrip line. This is a
double microstrip line with a narrow space in between. This means both lines have strong coupling and this
determines the characteristic impedance. The fact that they are routed above a copper plane does not affect
differential impedance, only CM-capacitance is added. Each of the structures above has its own geometric
parameters, so for each structure there is a different formula to calculate the right impedance. For calculations on
these transmission lines visit the TI website or order RAPIDESIGNER. At the end of the transmission line there
must be a termination having the same impedance as that of the transmission line itself. It does not 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
microstrip line, line width is fixed for a given impedance and for a specific board material. Other line widths will
result in different impedances.
Advantages of Differential Microstrip Lines
Impedances of transmission lines are always dictated by their geometric parameters. This is also true for
differential microstrip lines. Using this type of transmission line, the distance of the track determines the resulting
impedance. So, if the PCB manufacturer can produce reliable boards with low track spacing the track width for a
given impedance is also small. The wider the spacing, the wider tracks are needed for a specific 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 to create optimal connections to the receiving part or the
terminating resistor, in accordance to 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 the
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 the open lines must be to prevent signal degradation.
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PCB LAYOUT CONSIDERATIONS AND COMPONENT VALUE SELECTION
High frequency designs require that both active and passive components be selected from those that are
specially designed for this purpose. The LMH7324 is fabricated in a 32-pin WQFN package intended for surface
mount design. For reliable high speed design it is highly recommended 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. Good practice dictates that a high frequency design
have at least one ground plane, providing a low impedance path for all decoupling capacitors and other ground
connections. Care should be given 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). 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 should not ‘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, 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, literature number SNOA367).
TI suggests the following evaluation board as a guide for high frequency layout and as an aid in device testing:
Device
Package
Evaluation Board
Ordering ID
LMH7324
RTV0032A
LMH7324EVAL
This evaluation board can be shipped when a device sample request is placed with Texas Instruments.
24
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REVISION HISTORY
Changes from Revision F (March 2013) to Revision G
•
Page
Changed layout of National Data Sheet to TI format .......................................................................................................... 24
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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)
LMH7324SQ/NOPB
ACTIVE
WQFN
RTV
32
1000
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 85
L7324SQ
(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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