LMH7324_0710

LMH7324 Quad 700 ps High Speed Comparator with RSPECL Outputs October 2007 LMH7324 Quad 700 ps High Speed Comparator with RSPECL Outputs General 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. 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 LLP package. Features (VCCI = VCCO = +5V, VEE = 0V.) 700 ps ■ Propagation delay 20 ps ■ Overdrive dispersion 150 ps ■ Fast rise and fall times 5V to 12V ■ Supply range ■ Input common mode range extends 200 mV below negative rail ■ RSPECL outputs Applications ■ ■ ■ ■ ■ ■ Digital receivers High speed signal restoration Zero-crossing detectors High speed sampling Window comparators High speed signal triggering Typical Application 30017401 © 2007 National Semiconductor Corporation 300174 www.national.com LMH7324 Absolute Maximum Ratings (Note 1) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. ESD Tolerance (Note 2) Human Body Model Machine Model Output Short Circuit Duration Supply Voltages (V+ –V−) Voltage at Input/Output Pins 2.5 kV 250V (Notes 3, 4) 13.2V ±13V Soldering Information Infrared or Convection (20 sec.) Wave Soldering (10 sec.) Storage Temperature Range Junction Temperature (Note 3) 235°C 260°C −65°C to +150°C +150°C Operating Ratings Supply Voltage (V+ –V−) Temperature Range Package Thermal Resistance 32-Pin LLP (Note 1) 5V to 12V −40°C to +125°C 36°C/W 12V DC Electrical Characteristics Unless otherwise specified, all limits are guaranteed for TJ = 25°C. VCCI = VCCO = 12V, VEE = 0V, RL = 50Ω to VCCO-2V, VCM = 300 mV. (Note 7) Symbol Parameter Conditions Min (Note 6) −5 −250 −9.5 7 VEE −12 83 75 54 Fixed Internal Value VIN Differential = 25 mV VIN Differential = 25 mV VIN Differential = 25 mV VIN Differential = 25 mV Load Current Excluded VIN Differential = 25 mV Load Current Excluded 10.78 10.43 300 20.8 10.85 10.50 345 5.6 11.6 10.93 10.58 400 8 17 mA VCCI−2 +12 Typ (Note 5) −2.5 40 0.15 +9.5 250 Max (Note 6) Units Input Characteristics IB IOS TC IOS VOS TC VOS VRI VRID CMRR PSRR AV Hyst VOH VOL VOD IVCCI IVCCO Input Bias Current (Note 11) Input Offset Current Input Offset Current TC (Note 10) Input Offset Voltage Input Offset Voltage TC (Note 10) Input Voltage Range Input Differential Voltage Range Common Mode Rejection Ratio Power Supply Rejection Ratio Active Gain Hysteresis Output Voltage High Output Voltage Low Output Voltage Differential VCCI Supply Current VCCO Supply Current VIN Differential = 0V VIN Differential = 0V VIN Differential = 0V VCM = 0V VCM = 0V CMRR > 50 dB VEE ≤ INP or INM ≤ VCCI 0V ≤ VCM ≤ VCC−2V VCM = 0V, 5V ≤ VCC ≤ 12V µA nA nA/°C mV μV/°C V V dB dB dB mV V V mV Output Characteristics Power Supplies 12V AC Electrical Characteristics Unless otherwise specified, all limits are guaranteed for TJ = 25°C. VCCI = VCCO = 12V, VEE = 0V, RL = 50Ω to VCCO-2V, VCM = 300 mV. (Note 7) Symbol TR Parameter Maximum Toggle Rate Minimum Pulse Width Jitter Conditions Overdrive = ±50 mV, CL = 2 pF @ 50% Output Swing Overdrive = ±50 mV, CL = 2 pF @ 50% Output Swing Overdrive = ±50 mV, CL = 2 pF @ freq = 140 MHz Min (Note 6) Typ (Note 5) 3.84 280 50 dB VEE ≤ INP or INM ≤ VCCI 0V ≤ VCM ≤ VCC−2V VCM = 0V, 5V ≤ VCC ≤ 12V VEE −5 80 75 54 22.5 3.87 3.52 345 5.4 11 3.95 3.60 400 7.5 15 −9.5 7 VCCI−2 +5 Min (Note 6) −5 −250 Typ (Note 5) −2.2 30 0.1 +9.5 +250 Max (Note 6) Units µA nA nA/°C mV μV/°C V V dB dB dB mV V V mV mA mA Output Characteristics Power Supplies 3 www.national.com LMH7324 5V AC Electrical Characteristics Unless otherwise specified, all limits are guaranteed for TJ = 25°C. VCCI = VCCO = 5V, VEE = 0V, RL = 50Ω to VCCO-2V, VCM = 300 mV. (Note 7) Symbol TR Parameter Maximum Toggle Rate Minimum Pulse Width Jitter tPDH Propagation Delay (see Figure 3 application note) Input SR = Constant VIN Startvalue = VREF −100 mV tOD-disp Input Overdrive Dispersion Conditions Overdrive = ±50 mV, CL = 2 pF @ 50% Output Swing Overdrive = ±50 mV, CL = 2 pF @ 50% Output Swing Overdrive = ±50 mV, CL = 2 pF @ freq = 140 MHz Overdrive 20 mV Overdrive 50 mV Overdrive 100 mV Overdrive 1V TPDH @ Overdrive 20 mV ↔ 100 mV TPDH @ Overdrive 100 mV ↔ 1V tSR-disp tCM-disp Input Slew Rate Dispersion Input Common Mode Dispersion 0.1 V/ns to 1 V/ns, Overdrive = 100 mV SR = 1 V/ns, Overdrive 100 mV, 0V ≤ VCM ≤ VCCI – 2V Overdrive = 50 mV, CL = 2 pF Overdrive = 50 mV, CL = 2 pF Overdrive = 50 mV, CL = 2 pF Overdrive = 50 mV, CL = 2 pF 60 40 145 145 ps ps ps ps Min (Note 6) Typ (Note 5) 3.72 290 TA. Note 8: Propagation Delay Skew, ΔtPD, is defined as the average of ΔtPDLH and ΔtPDHL. Note 9: The rise or fall time is the average of the Q and Q rise or fall time. Note 10: Average Temperature Coefficient is determined by dividing the change in a parameter at temperature extremes by the total temperature change. Note 11: Positive current corresponds to current flowing into the device. www.national.com 4 LMH7324 Connection Diagram 32-Pin LLP 30017402 30017403 Top View Ordering Information Package 32-Pin LLP Part Number LMH7324SQ LMH7324SQX Package Marking L7324SQ Transport Media 1k Units Tape and Reel 4.5k Units Tape and Reel NSC Drawing SQA32A 5 www.national.com LMH7324 Typical Performance Characteristics Propagation Delay vs. Supply Voltage At TJ = 25°C, V+ = +5V, V− = 0V, unless otherwise specified. Propagation Delay vs. Temperature 30017424 30017425 Propagation Delay vs. Overdrive Voltage Propagation Delay vs. Supply Voltage for Different Overdrive 30017426 30017427 Propagation Delay vs. Common Mode Voltage Propagation Delay vs. Slew Rate 30017428 30017429 www.national.com 6 LMH7324 Pulse Response and Maximum Toggle Rate Bias Current vs. Temperature 30017430 30017431 Input Current vs. Differential Input Voltage Output Voltage vs. Input Voltage 30017432 30017433 Hysteresis Voltage vs. Temperature 30017434 7 www.national.com LMH7324 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 LLP 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 1, 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. The output stage of the LMH7324 is built using two emitter followers, which are referenced to the VCCO. (See Figure 2.) 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. 30017405 FIGURE 2. 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. 30017404 FIGURE 1. Equivalent Input Circuitry www.national.com 8 LMH7324 DEFINITIONS This table provides a short description of the parameters used in the datasheet and in the timing diagram of Figure 3. Symbol IB IOS TC IOS VOS TC VOS VRI VRID Text Input Bias Current Input Offset Current Description Current flowing in or out of the input pins, when both are biased at the VCM voltage as specified in the tables. Difference between the input bias current of the inverting and non-inverting inputs. Voltage difference needed between IN+ and IN− to make the outputs change state, averaged for H to L and L to H transitions. Voltage which can be applied to the input pin maintaining normal operation. 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. Ratio of input offset voltage change and input common mode voltage change. Ratio of input offset voltage change and supply voltage change from VSMIN to VS-MAX. Overall gain of the circuit. Difference between the switching point ‘0’ to ‘1’ and vice versa. High state single ended output voltage (Q or Q). (See Figure 16) Low state single ended output voltage (Q or Q). (See Figure 16) (VODH + VODL)/2 Supply current into the input stage. Supply current into the output stage while current through the load resistors is excluded. Current flowing out of the negative supply pin. Maximum frequency at which the outputs can toggle at 50% of the nominal VOH and VOL. Time from 50% of the rising edge of a signal to 50% of the falling edge. 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). 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). Average of tPDH and tPDL Average of tPDL and tPDH Average of tPDLH and tPDHL 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). Input Overdrive Dispersion Input Slew Rate Dispersion Change in tPD for different overdrive voltages at the input pins. Change in tPD for different slew rates at the input pins. 9 www.national.com Average Input Offset Current Drift Temperature coefficient of IOS. Input Offset Voltage Average Input Offset Voltage Drift Temperature coefficient of VOS. Input Voltage Range Input Differential Voltage Range CMRR PSRR AV Hyst VOH VOL VOD IVCCI IVCCO IVEE TR PW tPDH resp tPDL Common Mode Rejection Ratio Power Supply Rejection Ratio Active Gain Hysteresis Output Voltage High Output Voltage Low Average of VODH and VODL Supply Current Input Stage Supply Current Output Stage Supply Current VEE Pin Maximum Toggle Rate Pulse Width Propagation Delay tPDL resp tPDH tPDLH tPDHL tPD tPDHd resp tPDLd tOD-disp tSR-disp LMH7324 Symbol tCM-disp ΔtPDLH resp ΔtPDHL ΔtPD ΔtPDd tr/trd tf/tfd Text Input Common Mode Dispersion Q to Q Time Skew Description Change in tPD for different common mode voltages at the input pins. 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). Average of tPDLH and tPDHL for L to H and H to L transients. Average of tPDHd and tPDLd for L to H and H to L transients. Time needed for the (single ended or differential) output voltage to change from 20% of its nominal value to 80%. Time needed for the (single ended or differential) output voltage to change from 80% of its nominal value to 20%. Average Q to Q Time Skew Average Diff. Time Skew Output Rise Time (20% - 80%) Output Fall Time (20% - 80%) 30017406 FIGURE 3. Timing Definitions PIN DESCRIPTIONS Pin 1. 2. 3. 4. 5. 6. 7. 8. 9. Name VCCO Q Q VEE VEE Q Q VCCO VCCI Description Positive Supply Output Stage Inverted Output Output Negative Supply Negative Supply Output Inverted Output Positive Supply Output Stage Positive Supply for Input Stage Negative Input Part A A A A B B B B B B Comment This supply pin is independent of the supply for the input stage. This allows output levels of different logic families. Output levels are determined by the choice of VCCOA. Output levels are determined by the choice of VCCOA. All four VEE pins are circular connected together via two antiparallel diodes. (See Figure 4) All four VEE pins are circular connected together via two antiparallel diodes. (See Figure 4) Output levels are determined by the choice of VCCOB. Output levels are determined by the choice of VCCOB. This supply pin is independent of the supply for the input stage. This allows output levels of different logic families. This supply pin is independent of the supply for the output stage. VCCIand VCCO share the same ground pin VEE. Input for analog voltages between 200 mV below VEE and 2V below VCCI. 10. IN− www.national.com 10 LMH7324 Pin Name Description Positive Input Negative Supply Negative Supply Positive Input Negative Input Positive Supply for Input Stage Positive Supply Output Stage Inverted Output Output Negative Supply Negative Supply Output Inverted Output Positive Supply Output Stage Positive Supply for Input Stage Negative Input Positive Input Negative Supply Negative Supply Positive Input Negative Input Positive Supply for Input Stage Central Pad at the Bottom of the Package Part B B C C C C C C C C D D D D D D D D A A A A All Comment Input for analog voltages between 200 mV below VEE and 2V below VCCI. All four VEE pins are circular connected together via two antiparallel diodes. (See Figure 4) All four VEE pins are circular connected together via two antiparallel diodes. (See Figure 4) Input for analog voltages between 200 mV below VEE and 2V below VCCI. Input for analog voltages between 200 mV below VEE and 2V below VCCI. This supply pin is independent of the supply for the output stage. VCCI and VCCO share the same ground pin VEE. This supply pin is independent of the supply for the input stage. This allows output levels of different logic families. Output levels are determined by the choice of VCCOC. Output levels are determined by the choice of VCCOC. All four VEE pins are circular connected together via two antiparallel diodes. (See Figure 4) All four VEE pins are circular connected together via two antiparallel diodes. (See Figure 4) Output levels are determined by the choice of VCCOD. Output levels are determined by the choice of VCCOD. This supply pin is independent of the supply for the input stage. This allows output levels of different logic families. This supply pin is independent of the supply for the output stage. VCCI and VCCO share the same ground pin VEE. Input for analog voltages between 200 mV below VEE and 2V below VCCI. Input for analog voltages between 200 mV below VEE and 2V below VCCI. All four VEE pins are circular connected together via two antiparallel diodes. (See Figure 4) All four VEE pins are circular connected together via two antiparallel diodes. (See Figure 4) Input for analog voltages between 200 mV below VEE and 2V below VCCI. Input for analog voltages between 200 mV below VEE and 2V below VCCI. This supply pin is independent of the supply for the output stage. VCCI and VCCO share the same ground pin VEE. The purpose of this pad is to transfer heat outside the part. 11. IN+ 12. VEE 13. VEE 14. IN+ 15. IN− 16. VCCI 17. VCCO 18. Q 19. Q 20. VEE 21. VEE 22. Q 23. Q 24. VCCO 25. VCCI 26. IN− 27. IN+ 28. VEE 29. VEE 30. IN+ 31. IN− 32. VCCI 33. DAP 11 www.national.com LMH7324 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 4. 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. 30017408 FIGURE 5. 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 6.) 30017407 FIGURE 4. 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 the minimum supply voltage between VEE and one of the positive supplies must be 5V. The negative supply pins are connected together for all four parts. Using the power pins at different supply voltages makes it possible to create several translations for logic families. 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 5.) 30017409 FIGURE 6. PECL TO RSECL Interface from Analog to LVDS As seen in Figure 7, 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. www.national.com 12 LMH7324 30017410 FIGURE 7. ANALOG TO LVDS STANDARD COMPARATOR SETUP Figure 8 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 k Ω 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.) 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 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 9. 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. 30017412 FIGURE 9. Propagation Delay 30017411 FIGURE 8. Standard Setup 13 www.national.com LMH7324 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 10.) 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 3. 30017414 FIGURE 11. 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. Slew Rate Dispersion The slew rate is another parameter that affects propagation delay. The higher the input slew rate, the faster the input stage switches. (See Figure 12.) 30017413 FIGURE 10. tPD with Complementary Outputs 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 11 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 11 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. 30017415 FIGURE 12. 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. www.national.com 14 LMH7324 Common Mode Dispersion Dispersion will also occur when changing the common mode level of the input signal. (See Figure 13.) 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. 30017417 FIGURE 14. 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 15. 30017416 FIGURE 13. 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. 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 14 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. 30017418 FIGURE 15. Hysteresis 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 15 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 15 www.national.com LMH7324 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 15. If by design the levels A and B which are due to a change in the built-in hysteresis voltage are changed then the timing of t = 0 and t = 1 will also vary. 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. 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 16.) 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. Another parasitic capacity that can affect the output signal is the capacity directly between both outputs, called CPAR. (See Figure 17.) 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. 30017420 FIGURE 17. 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. 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 18.) 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 16 30017419 FIGURE 16. Output Swing 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 17.) 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. www.national.com LMH7324 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. 30017421 FIGURE 18. Bit Rates Need for Terminated Transmission Lines During the 1980’s and 90’s, National 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 19.) 30017423 FIGURE 20. 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 CMcapacitance 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 National 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 termi17 www.national.com 30017422 FIGURE 19. 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 20.) LMH7324 nating 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. 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 32pin LLP 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 ca- pacitors 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). National suggests the following evaluation board as a guide for high frequency layout and as an aid in device testing: Device LMH7324 Package SQA32A Evaluation Board Ordering ID 551013272 This evaluation board can be shipped when a device sample request is placed with National Semiconductor. www.national.com 18 LMH7324 Physical Dimensions inches (millimeters) unless otherwise noted 32-Pin LLP NS Package Number SQA32A 19 www.national.com LMH7324 Quad 700 ps High Speed Comparator with RSPECL Outputs Notes THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. NATIONAL MAKES NO REPRESENTATIONS OR WARRANTIES WITH RESPECT TO THE ACCURACY OR COMPLETENESS OF THE CONTENTS OF THIS PUBLICATION AND RESERVES THE RIGHT TO MAKE CHANGES TO SPECIFICATIONS AND PRODUCT DESCRIPTIONS AT ANY TIME WITHOUT NOTICE. NO LICENSE, WHETHER EXPRESS, IMPLIED, ARISING BY ESTOPPEL OR OTHERWISE, TO ANY INTELLECTUAL PROPERTY RIGHTS IS GRANTED BY THIS DOCUMENT. TESTING AND OTHER QUALITY CONTROLS ARE USED TO THE EXTENT NATIONAL DEEMS NECESSARY TO SUPPORT NATIONAL’S PRODUCT WARRANTY. EXCEPT WHERE MANDATED BY GOVERNMENT REQUIREMENTS, TESTING OF ALL PARAMETERS OF EACH PRODUCT IS NOT NECESSARILY PERFORMED. NATIONAL ASSUMES NO LIABILITY FOR APPLICATIONS ASSISTANCE OR BUYER PRODUCT DESIGN. BUYERS ARE RESPONSIBLE FOR THEIR PRODUCTS AND APPLICATIONS USING NATIONAL COMPONENTS. PRIOR TO USING OR DISTRIBUTING ANY PRODUCTS THAT INCLUDE NATIONAL COMPONENTS, BUYERS SHOULD PROVIDE ADEQUATE DESIGN, TESTING AND OPERATING SAFEGUARDS. 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