LMP7717/LMP7718 88 MHz, Precision, Low Noise, 1.8V CMOS Input, Decompensated Operational Amplifier General Description Features The LMP7717 (single) and the LMP7718 (dual) low noise, CMOS input operational amplifiers offer a low input voltage while consuming only 1.15 mA noise density of 5.8 nV/ (LMP7717) of quiescent current. The LMP7717/LMP7718 are stable at a gain of 10 and have a gain bandwidth (GBW) product of 88 MHz. The LMP7717/LMP7718 have a supply voltage range of 1.8V to 5.5V and can operate from a single supply. The LMP7717/LMP7718 each feature a rail-to-rail output stage. Both amplifiers are part of the LMP® precision amplifier family and are ideal for a variety of instrumentation applications. The LMP7717 family provides optimal performance in low voltage and low noise systems. A CMOS input stage, with typical input bias currents in the range of a few femto-Amperes, and an input common mode voltage range, which includes ground, make the LMP7717/LMP7718 ideal for low power sensor applications where high speeds are needed. The LMP7717/LMP7718 are manufactured using National’s advanced VIP50 process. The LMP7717 is offered in either a 5-Pin SOT23 or an 8-Pin SOIC package. The LMP7718 is offered in either the 8-Pin SOIC or the 8-Pin MSOP. (Typical 5V supply, unless otherwise noted) ±150 µV (max) ■ Input offset voltage 5.8 nV/√Hz ■ Input referred voltage noise 100 fA ■ Input bias current 88 MHz ■ Gain bandwidth product 1.8V to 5.5V ■ Supply voltage range ■ Supply current per channel 1.15 mA — LMP7717 1.30 mA — LMP7718 ■ Rail-to-Rail output swing 25 mV from rail — @ 10 kΩ load 45 mV from rail — @ 2 kΩ load ■ Guaranteed 2.5V and 5.0V performance 0.04% @1 kHz, 600Ω ■ Total harmonic distortion −40°C to 125°C ■ Temperature range Applications ■ ■ ■ ■ ■ ■ ADC interface Photodiode amplifiers Active filters and buffers Low noise signal processing Medical instrumentation Sensor interface applications Typical Application 30010869 Photodiode Transimpedance Amplifier 30010839 Input Referred Voltage Noise vs. Frequency LMP® is a registered trademark of National Semiconductor Corporation. © 2007 National Semiconductor Corporation 300108 www.national.com LMP7717/LMP7718 88 MHz, Precision, Low Noise, 1.8V CMOS Input, Decompensated Operational Amplifier November 6, 2007 LMP7717/LMP7718 Soldering Information Absolute Maximum Ratings (Note 1) Infrared or Convection (20 sec) Wave Soldering Lead Temp (10 sec) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Operating Ratings ESD Tolerance (Note 2) Human Body Model Machine Model VIN Differential Supply Voltage (V+ – V−) Input/Output Pin Voltage Storage Temperature Range Junction Temperature (Note 3) 260°C (Note 1) Temperature Range (Note 3) Supply Voltage (V+ – V−) −40°C ≤ TA ≤ 125°C 2000V 200V ±0.3V 6.0V V+ +0.3V, V− −0.3V −65°C to 150°C +150°C 2.5V Electrical Characteristics 235°C −40°C to 125°C 2.0V to 5.5V 0°C ≤ TA ≤ 125°C 1.8V to 5.5V Package Thermal Resistance (θJA (Note 3)) 5-Pin SOT23 8-Pin SOIC 8-Pin MSOP 180°C/W 190°C/W 236°C/W (Note 4) Unless otherwise specified, all limits are guaranteed for TA = 25°C, V+ = 2.5V, V− = 0V, VCM = V+/2 = VO. Boldface limits apply at the temperature extremes. Symbol Parameter Conditions Min (Note 6) Typ (Note 5) Max (Note 6) ±20 ±180 ±480 µV ±4 μV/°C VOS Input Offset Voltage TC VOS Input Offset Average Drift (Note 7) LMP7717 −1.0 LMP7718 −1.8 Input Bias Current VCM = 1.0V (Notes 8, 9) IB IOS Input Offset Current CMRR Common Mode Rejection Ratio 0V ≤ VCM ≤ 1.4V PSRR Power Supply Rejection Ratio CMVR AVOL −40°C ≤ TA ≤ 85°C 0.05 1 25 −40°C ≤ TA ≤ 125°C 0.05 1 100 .006 0.5 50 VCM = 1.0V (Note 9) 83 80 94 2.0V ≤ V+ ≤ 5.5V, VCM = 0V 85 80 100 1.8V ≤ V+ ≤ 5.5V, VCM = 0V 85 98 Input Common-Mode Voltage Range CMRR ≥ 60 dB Open Loop Gain VOUT = 0.15V to 2.2V, VOUT = 0.15V to 2.2V, 88 82 98 LMP7718 84 80 92 LMP7717 92 88 110 LMP7718 90 86 95 RL = 10 kΩ to V+/2 VOUT Output Swing High Output Swing Low www.national.com V dB RL = 2 kΩ to V+/2 25 70 77 RL = 10 kΩ to V+/2 20 60 66 RL = 2 kΩ to V+/2 30 70 73 RL = 10 kΩ to V+/2 15 60 62 2 pA dB 1.5 1.5 LMP7717 RL = 2 kΩ to V+/2 pA dB −0.3 −0.3 CMRR ≥ 55 dB Units mV from rail IS Output Short Circuit Current Supply Current per Amplifier SR Slew Rate Sourcing to V− VIN = 200 mV (Note 10) 36 30 47 Sinking to V+ VIN = –200 mV (Note 10) 7.5 5 15 mA LMP7717 0.95 1.30 1.65 LMP7718 per channel 1.1 1.5 1.85 AV = +10, Rising (10% to 90%) 32 AV = +10, Falling (90% to 10%) 24 88 mA V/μs GBWP Gain Bandwidth Product AV = +10, RL = 10 kΩ en Input-Referred Voltage Noise f = 1 kHz 6.2 nV/ in Input-Referred Current Noise f = 1 kHz 0.01 pA/ THD+N Total Harmonic Distortion + Noise f = 1 kHz, AV = 1, RL = 600Ω 0.01 5V Electrical Characteristics MHz % (Note 4) Unless otherwise specified, all limits are guaranteed for TA = 25°C, V+ = 5V, V− = 0V, VCM = V+/2 = VO. Boldface limits apply at the temperature extremes. Symbol Parameter Conditions Min (Note 6) Typ (Note 5) Max (Note 6) ±10 ±150 ±450 µV ±4 μV/°C VOS Input Offset Voltage TC VOS Input Offset Average Drift (Note 7) LMP7717 −1.0 LMP7718 −1.8 Input Bias Current VCM = 2.0V (Notes 8, 9) IB IOS Input Offset Current CMRR Common Mode Rejection Ratio 0V ≤ VCM ≤ 3.7V PSRR Power Supply Rejection Ratio CMVR AVOL −40°C ≤ TA ≤ 85°C 0.1 1 25 −40°C ≤ TA ≤ 125°C 0.1 1 100 .01 0.5 50 VCM = 2.0V (Note 9) 85 80 100 2.0V ≤ V+ ≤ 5.5V, VCM = 0V 85 80 100 1.8V ≤ 85 98 V+ ≤ 5.5V, VCM = 0V Input Common-Mode Voltage Range CMRR ≥ 60 dB Open Loop Gain VOUT = 0.3V to 4.7V, VOUT = 0.3V to 4.7V, 88 82 107 LMP7718 84 80 90 LMP7717 92 88 110 LMP7718 90 86 95 RL = 10 kΩ to V+/2 3 pA dB 4 4 LMP7717 RL = 2 kΩ to V+/2 pA dB −0.3 −0.3 CMRR ≥ 55 dB Units V dB www.national.com LMP7717/LMP7718 IOUT LMP7717/LMP7718 VOUT Output Swing High RL = 2 kΩ to V+/2 LMP7717 35 70 77 LMP7718 45 80 77 25 60 66 LMP7717 42 70 73 LMP7718 50 80 78 25 60 66 RL = 10 kΩ to V+/2 Output Swing Low RL = 2 kΩ to V+/2 RL = 10 kΩ to V+/2 IOUT IS Output Short Circuit Current Supply Current per Amplifier SR Slew Rate Sourcing to V− VIN = 200 mV (Note 10) 46 38 60 Sinking to V+ VIN = –200 mV (Note 10) 10.5 6.5 21 mA LMP7717 1.15 1.40 1.75 LMP7718 per channel 1.30 1.70 2.05 AV = +10, Rising (10% to 90%) 35 AV = +10, Falling (90% to 10%) 28 mV from rail mA V/μs GBWP Gain Bandwidth Product AV = +10, RL = 10 kΩ 88 MHz en Input-Referred Voltage Noise f = 1 kHz 5.8 nV/ in Input-Referred Current Noise f = 1 kHz 0.01 pA/ THD+N Total Harmonic Distortion + Noise f = 1 kHz, AV = 1, RL = 600Ω 0.01 % Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is intended to be functional, but specific performance is not guaranteed. For guaranteed specifications and the test conditions, see the Electrical Characteristics Tables. Note 2: 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). Note 3: 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. Note 4: 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 guarantee of parametric performance is indicated in the electrical tables under conditions of internal self-heating where TJ > TA. Note 5: 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 guaranteed on shipped production material. Note 6: Limits are 100% production tested at 25°C. Limits over the operating temperature range are guaranteed through correlations using the statistical quality control (SQC) method. Note 7: Offset voltage average drift is determined by dividing the change in VOS by temperature change. Note 8: Positive current corresponds to current flowing into the device. Note 9: Input bias current and input offset current are guaranteed by design Note 10: The short circuit test is a momentary test, the short circuit duration is 1.5 ms. www.national.com 4 5-Pin SOT23 (LMP7717) 8-Pin SOIC (LMP7717) 8-Pin SOIC/MSOP (LMP7718) 30010801 Top View 30010802 30010885 Top View Ordering Information Package Part Number Package Marking LMP7717MF 5-Pin SOT23 LMP7717MFE AT4A LMP7717MFX 8-Pin SOIC 250 Units Tape and Reel 2.5k Units Tape and Reel LMP7718MA 95 Units/Rail LMP7718MA M08A 250 Units Tape and Reel 2.5k Units Tape and Reel LMP7718MM LMP7718MME MF05A 95 Units/Rail LMP7717MA LMP7718MAX 8-Pin MSOP 250 Units Tape and Reel LMP7717MAX LMP7718MAE NSC Drawing 3k Units Tape and Reel LMP7717MA LMP7717MAE Transport Media 1k Units Tape and Reel 1k Units Tape and Reel AP4A LMP7718MMX 250 Units Tape and Reel MUA08A 3.5k Units Tape and Reel 5 www.national.com LMP7717/LMP7718 Connection Diagrams LMP7717/LMP7718 Typical Performance Characteristics Unless otherwise specified, TA = 25°C, V– = 0, V+ = 5V, VS = V+ - V−, VCM = VS/2. TCVOS Distribution (LMP7717) Offset Voltage Distribution 30010890 30010891 TCVOS Distribution (LMP7717) Offset Voltage Distribution 30010893 30010892 Supply Current vs. Supply Voltage (LMP7717) VOS vs. VCM 30010805 www.national.com 30010809 6 LMP7717/LMP7718 VOS vs. VCM VOS vs. VCM 30010811 30010851 VOS vs. Supply Voltage Slew Rate vs. Supply Voltage 30010852 30010812 Input Bias Current vs. VCM Input Bias Current vs. VCM 30010887 30010862 7 www.national.com LMP7717/LMP7718 Sourcing Current vs. Supply Voltage Sinking Current vs. Supply Voltage 30010820 30010819 Sourcing Current vs. Output Voltage Sinking Current vs. Output Voltage 30010850 30010854 Positive Output Swing vs. Supply Voltage Negative Output Swing vs. Supply Voltage 30010817 30010815 www.national.com 8 Negative Output Swing vs. Supply Voltage 30010816 30010814 Positive Output Swing vs. Supply Voltage Negative Output Swing vs. Supply Voltage 30010813 30010818 Input Referred Voltage Noise vs. Frequency Overshoot and Undershoot vs. CLOAD 30010839 30010830 9 www.national.com LMP7717/LMP7718 Positive Output Swing vs. Supply Voltage LMP7717/LMP7718 THD+N vs. Frequency THD+N vs. Frequency 30010804 30010826 THD+N vs. Peak-to-Peak Output Voltage (VOUT) THD+N vs. Peak-to-Peak Output Voltage (VOUT) 30010874 30010875 Open Loop Gain and Phase Closed Loop Output Impedance vs. Frequency 30010832 30010806 www.national.com 10 Small Signal Transient Response, AV = +10 30010853 30010880 Large Signal Transient Response, AV = +10 Small Signal Transient Response, AV = +10 30010855 30010857 Large Signal Transient Response, AV = +10 PSRR vs. Frequency 30010863 30010870 11 www.national.com LMP7717/LMP7718 Crosstalk Rejection LMP7717/LMP7718 CMRR vs. Frequency Input Common Mode Capacitance vs. VCM 30010856 www.national.com 30010876 12 ADVANTAGES OF THE LMP7717/LMP7718 Wide Bandwidth at Low Supply Current The LMP7717/LMP7718 are high performance op amps that provide a GBW of 88 MHz with a gain of 10 while drawing a low supply current of 1.15 mA. This makes them ideal for providing wideband amplification in data acquisition applications. With the proper external compensation the LMP7717 can be operated at gains of ±1 and still maintain much faster slew rates than comparable unity gain stable amplifiers. The increase in bandwidth and slew rate is obtained without any additional power consumption over the LMP7715. Low Input Referred Noise and Low Input Bias Current The LMP7717/LMP7718 have a very low input referred voltat 1 kHz). A CMOS input stage age noise density (5.8 nV/ ensures a small input bias current (100 fA) and low input referred current noise (0.01 pA/ ). This is very helpful in maintaining signal integrity, and makes the LMP7717/ LMP7718 ideal for audio and sensor based applications. Low Supply Voltage The LMP7717 and the LMP7718 have performance guaranteed at 2.5V and 5V supply. These parts are guaranteed to be operational at all supply voltages between 2.0V and 5.5V, for ambient temperatures ranging from −40°C to 125°C, thus utilizing the entire battery lifetime. The LMP7717/LMP7718 are also guaranteed to be operational at 1.8V supply voltage, for temperatures between 0°C and 125°C optimizing their usage in low-voltage applications. 30010822 FIGURE 1. LMP7717 AVOL vs. Frequency RRO and Ground Sensing Rail-to-Rail output swing provides the maximum possible dynamic range. This is particularly important when operating at low supply voltages. An innovative positive feedback scheme is used to boost the current drive capability of the output stage. This allows the LMP7717/LMP7718 to source more than 40 mA of current at 1.8V supply. This also limits the performance of the these parts as comparators, and hence the usage of the LMP7717 and the LMP7718 in an open-loop configuration is not recommended. The input common-mode range includes the negative supply rail which allows direct sensing at ground in single supply operation. Small Size The small footprints of the LMP7717 packages and the LMP7718 packages save space on printed circuit boards, and enable the design of smaller electronic products, such as cellular phones, pagers, or other portable systems. Long traces between the signal source and the op amp make the signal path more susceptible to noise pick up. The physically smaller LMP7717 or LMP7718 packages allow the op amp to be placed closer to the signal source, thus reducing noise pickup and maintaining signal integrity. 30010823 FIGURE 2. LMP7715 AVOL vs. Frequency Figure 1 shows the much larger 88 MHz bandwidth of the LMP7717 as compared to the 17 MHz bandwidth of the LMP7715 shown in Figure 2. The decompensated LMP7717 has five times the bandwidth of the LMP7715. What is a Decompensated Op Amp? The differences between the unity gain stable op amp and the decompensated op amp are shown in Figure 3. This Bode plot assumes an ideal two pole system. The dominant pole of the decompensated op amp is at a higher frequency, f1, as compared to the unity gain stable op amp which is at fd as shown in Figure 3. This is done in order to increase the speed capability of the op amp while maintaining the same power dissipation of the unity gain stable op amp. The LMP7717/ LMP7718 have a dominant pole at 8.6 Hz. The unity gain stable LMP7715/LMP7716 have their dominant pole at 1.6 Hz. USING THE DECOMPENSATED LMP7717 Advantages of Decompensated Op Amp A unity gain stable op amp, which is fully compensated, is designed to operate with good stability down to gains of ±1. The large amount of compensation does provide an op amp that is relatively easy to use; however, a decompensated op amp is designed to maximize the bandwidth and slew rate without any additional power consumption. This can be very advantageous. 13 www.national.com LMP7717/LMP7718 The LMP7717/LMP7718 require a gain of ±10 to be stable. However, with an external compensation network (a simple RC network) these parts can be stable with gains of ±1 and still maintain the higher slew rate. Looking at the Bode plots for the LMP7717 and its closest equivalent unity gain stable op amp, the LMP7715, one can clearly see the increased bandwidth of the LMP7717. Both plots are taken with a parallel combination of 20 pF and 10 kΩ for the output load. Application Information LMP7717/LMP7718 30010825 FIGURE 4. LMP7717 with Lead-Lag Compensation for Inverting Configuration 30010824 To cover how to calculate the compensation network values it is necessary to introduce the term called the feedback factor or F. The feedback factor F is the feedback voltage VA-VB across the op amp input terminals relative to the op amp output voltage VOUT. FIGURE 3. Open Loop Gain for Unity Gain Stable Op Amp and Decompensated Op Amp Having a higher frequency for the dominate pole will result in: 1. The DC open loop gain (AVOL) extending to a higher frequency. 2. A wider closed loop bandwidth. 3. Better slew rate due to reduced compensation capacitance within the op amp. The second open loop pole (f2) for the LMP7717/LMP7718 occurs at 45 MHz. The unity gain (fu’) occurs after the second pole at 51 MHz. An ideal two pole system would give a phase margin of 45° at the location of the second pole. The LMP7717/LMP7718 have parasitic poles close to the second pole, giving a phase margin closer to 0°. Therefore it is necessary to operate the LMP7717/LMP7718 at a closed loop gain of 10 or higher, or to add external compensation in order to assure stability. For the LMP7715, the gain bandwidth product occurs at 17 MHz. The curve is constant from fd to fu which occurs before the second pole. For the LMP7717/LMP7718 the GBW = 88 MHz and is constant between f1 and f2. The second pole at f2 occurs before AVOL =1. Therefore fu’ occurs at 51 MHz, well before the GBW frequency of 88 MHz. For decompensated op amps the unity gain frequency and the GBW are no longer equal. Gmin is the minimum gain for stability and for the LMP7717/LMP7718 this is a gain of 10 or 20 dB. From feedback theory the classic form of the feedback equation for op amps is: A is the open loop gain of the amplifier and AF is the loop gain. Both are highly important in analyzing op amps. Normally AF >>1 and so the above equation reduces to: Deriving the equations for the lead-lag compensation is beyond the scope of this datasheet. The derivation is based on the feedback equations that have just been covered. The inverse of feedback factor for the circuit in Figure 4 is: Input Lead-Lag Compensation The recommended technique which allows the user to compensate the LMP7717/LMP7718 for stable operation at any gain is lead-lag compensation. The compensation components added to the circuit allow the user to shape the feedback function to make sure there is sufficient phase margin when the loop gain is as low as 0 dB and still maintain the advantages over the unity gain op amp. Figure 4 shows the leadlag configuration. Only RC and C are added for the necessary compensation. www.national.com (1) where 1/F's pole is located at (2) 1/F's zero is located at (3) 14 LMP7717/LMP7718 (4) The circuit gain for Figure 4 at low frequencies is −RF/RIN, but F, the feedback factor is not equal to the circuit gain. The feedback factor is derived from feedback theory and is the same for both inverting and non-inverting configurations. Yes, the feedback factor at low frequencies is equal to the gain for the non-inverting configuration. (5) From this formula, we can see that • 1/F's zero is located at a lower frequency compared with 1/F's pole. • 1/F's value at low frequency is 1 + RF/RIN. • This method creates one additional pole and one additional zero. • This pole-zero pair will serve two purposes: — To raise the 1/F value at higher frequencies prior to its intercept with A, the open loop gain curve, in order to meet the Gmin = 10 requirement. For the LMP7717 some overcompensation will be necessary for good stability. — To achieve the previous purpose above with no additional loop phase delay. Please note the constraint 1/F ≥ Gmin needs to be satisfied only in the vicinity where the open loop gain A and 1/F intersect; 1/F can be shaped elsewhere as needed. The 1/F pole must occur before the intersection with the open loop gain A. In order to have adequate phase margin, it is desirable to follow these two rules: Rule 1 1/F and the open loop gain A should intersect at the frequency where there is a minimum of 45° of phase margin. When over-compensation is required the intersection point of A and 1/F is set at a frequency where the phase margin is above 45°, therefore increasing the stability of the circuit. Rule 2 1/F’s pole should be set at least one decade below the intersection with the open loop gain A in order to take advantage of the full 90° of phase lead brought by 1/F’s pole which is F’s zero. This ensures that the effect of the zero is fully neutralized when the 1/F and A plots intersect each other. 30010848 FIGURE 5. LMP7717/LMP7718 Simplified Bode Plot To obtain stable operation with gains under 10 V/V the open loop gain margin must be reduced at high frequencies to where there is a 45° phase margin when the gain margin of the circuit with the external compensation is 0 dB. The pole and zero in F, the feedback factor, control the gain margin at the higher frequencies. The distance between F and AVOL is the gain margin; therefore, the unity gain point (0 dB) is where F crosses the AVOL curve. For the example being used RIN = RF for a gain of −1. Therefore F = 6 dB at low frequencies. At the higher frequencies the minimum value for F is 18 dB for 45° phase margin. From Equation 5 we have the following relationship: Now set RF = RIN = R. With these values and solving for RC we have RC = R/5.9. Note that the value of C does not affect the ratio between the resistors. Once the value of the resistors is set, then the position of the pole in F must be set. A 2 kΩ resistor is used for RF and RIN in this design. Therefore the value for RC is set at 330Ω, the closest standard value for 2 kΩ/5.9. Rewriting Equation 2 to solve for the minimum capacitor value gives the following equation: Calculating Lead-Lag Compensation for LMP7717 Figure 5 is the same plot as Figure 1, but the AVOL and phase curves have been redrawn as smooth lines to more readily show the concepts covered, and to clearly show the key parameters used in the calculations for lead-lag compensation. C = 1/(2πfpRC) The feedback factor curve, F, intersects the AVOL curve at about 12 MHz. Therefore the pole of F should not be any larger than 1.2 MHz. Using this value and RC = 330Ω the minimum value for C is 390 pF. Figure 6 shows that there is too much overshoot, but the part is stable. Increasing C to 2.2 nF did not improve the ringing, as shown in Figure 7. 15 www.national.com LMP7717/LMP7718 30010810 30010803 FIGURE 9. RC = 240Ω and C = 2.2 nF, Gain = −1 FIGURE 6. First Try at Compensation, Gain = −1 To summarize, the following steps were taken to compensate the LMP7717 for a gain of −1: 1. Values for Rc and C were calculated from the Bode plot to give an expected phase margin of 45°. The values were based on RIN = RF = 2 kΩ. These calculations gave Rc = 330Ω and C = 390 pF. 2. To reduce the ringing C was increased to 2.2 nF which moved the pole of F, the feedback factor, farther away from the AVOL curve. 3. There was still too much ringing so 2 dB of overcompensation was added to F. This was done by decreasing RC to 240Ω. The LMP7715 is the fully compensated part which is comparable to the LMP7717. Using the LMP7715 in the same setup, but removing the compensation network, provided the response shown in Figure 10 . 30010807 FIGURE 7. C Increased to 2.2 nF, Gain = −1 Some over-compensation appears to be needed for the desired overshoot characteristics. Instead of intersecting the AVOL curve at 18 dB, 2 dB of over-compensation will be used, and the AVOL curve will be intersected at 20 dB. Using Equation 5 for 20 dB, or 10 V/V, the closest standard value of RC is 240Ω. The following two waveforms show the new resistor value with C = 390 pF and 2.2 nF. Figure 9 shows the final compensation and a very good response for the 1 MHz square wave. 30010821 FIGURE 10. LMP7715 Response For large signal response the rise and fall times are dominated by the slew rate of the op amps. Even though both parts are quite similar the LMP7717 will give rise and fall times about 2.5 times faster than the LMP7715. This is possible because the LMP7717 is a decompensated op amp and even though it is being used at a gain of −1, the speed is preserved by using a good technique for external compensation. 30010808 FIGURE 8. RC = 240Ω and C = 390 pF, Gain = −1 www.national.com 16 30010884 FIGURE 13. LMP7717 with Lead-Lag Compensation for Non-Inverting Configuration Figure 13 is the result of using Equation 5 and additional experimentation in the lab. RP is not part of Equation 5, but it is necessary to introduce another pole at the input stage for good performance at gain = +1. Equation 5 is shown below with RIN = ∞. Using 2 kΩ for RF and solving for RC gives RC = 2000/6.9 = 290Ω. The closest standard value for RC is 300Ω. After some fine tuning in the lab RC = 330Ω and RP = 1.5 kΩ were chosen as the optimum values. RP together with the input capacitance at the non-inverting pin inserts another pole into the compensation for the LMP7717. Adding this pole and slightly reducing the compensation for 1/F (using a slightly higher resistor value for RC) gives the optimum response for a gain of +1. Figure 14 is the response of the circuit shown in Figure 13. Figure 15 shows the response of the LMP7715 in the buffer configuration with no compensation and RP = RF = 0. 30010882 FIGURE 11. RC = 240Ω and C = 2.2 nF, Gain = +2 30010883 FIGURE 12. LMP7715 Response Gain = +2 The response shown in Figure 11 is close to the response shown in Figure 9. The part is actually slightly faster in the non-inverting configuration. Decreasing the value of RC to around 200Ω can decrease the negative overshoot but will have slightly longer rise and fall times. The other option is to add a small resistor in series with the input signal. Figure 12 shows the performance of the LMP7715 with no compensation. Again the decompensated parts are almost 2.5 times faster than the fully compensated op amp. The most difficult op amp configuration to stabilize is the gain of +1. With proper compensation the LMP7717/LMP7718 can be used in this configuration and still maintain higher speeds 30010888 FIGURE 14. RC = 330Ω and C = 10 nF, Gain = +1 17 www.national.com LMP7717/LMP7718 than the fully compensated parts. Figure 13 shows the gain = 1, or the buffer configuration, for these parts. Non-Inverting Compensation For the non-inverting amp the same theory applies for establishing the needed compensation. When setting the inverting configuration for a gain of −1, F has a value of 2. For the noninverting configuration both F and the actual gain are the same, making the non-inverting configuration more difficult to compensate. Using the same circuit as shown in Figure 4, but setting up the circuit for non-inverting operation (gain of +2) results in similar performance as the inverting configuration with the inputs set to half the amplitude to compensate for the additional gain. Figure 11 below shows the results. LMP7717/LMP7718 30010861 FIGURE 16. Transimpedance Amplifier Figure 16 is the complete schematic for a transimpedance amplifier. Only the supply bypass capacitors are not shown. CD represents the photodiode capacitance which is given on its datasheet. CCM is the input common mode capacitance of the op amp and, for the LMP7717 it is shown in the last graph of the Typical Performance Characteristics section of this datasheet. In Figure 16 the inverting input pin of the LMP7717 is kept at virtual ground. Even though the diode is connected to the 2.5V line, a power supply line is AC ground, thus CD is connected to ground. Figure 17 shows the schematic needed to derive F, the feedback factor, for a transimpedance amplifier. In this figure CD + CCM = CIN. Therefore it is critical that the designer knows the diode capacitance and the op amp input capacitance. The photodiode is close to an ideal current source once its capacitance is included in the model. What kind of circuit is this? Without CF there is only an input capacitor and a feedback resistor. This circuit is a differentiator! Remember, differentiator circuits are inherently unstable and must be compensated. In this case CF compensates the circuit. 30010889 FIGURE 15. LMP7715 Response Gain = +1 With no increase in power consumption the decompensated op amp offers faster speed than the compensated equivalent part . These examples used RF = 2 kΩ. This value is high enough to be easily driven by the LMP7717/LMP7718, yet small enough to minimize the effects from the parasitic capacitance of both the PCB and the op amp. Note: When using the LMP7717/LMP7718, proper high frequency PCB layout must be followed. The GBW of these parts is 88 MHz, making the PCB layout significantly more critical than when using the compensated counterparts which have a GBW of 17 MHz. TRANSIMPEDANCE AMPLIFIER An excellent application for either the LMP7717 or the LMP7718 is as a transimpedance amplifier. With a GBW product of 88 MHz these parts are ideal for high speed data transmission by light. The circuit shown on the front page of the datasheet is the circuit used to test the LMP7717/LMP7718 as transimpedance amplifiers. The only change is that VB is tied to the VCC of the part, thus the direction of the diode is reversed from the circuit shown on the front page. Very high speed components were used in testing to check the limits of the LMP7717/LMP7718 in a transimpedance configuration. The photodiode part number is PIN-HR040 from OSI Optoelectronics. The diode capacitance for this part is only about 7 pF for the 2.5V bias used (VCC to virtual ground). The rise time for this diode is 1 nsec. A laser diode was used for the light source. Laser diodes have on and off times under 5 nsec. The speed of the selected optical components allowed an accurate evaluation of the LMP7717 as a transimpedance amplifier. Nationals evaluation board for decompensated op amps, PN 551013271-001 A, was used and only minor modifications were necessary and no traces had to be cut. www.national.com 30010864 FIGURE 17. Transimpedance Feedback Model 18 After a bit of algebraic manipulation the above equation reduces to: The noise gain is 1/F. Because this is a differentiator circuit, a zero must be inserted. The location of the zero is given by: In the above equation the only unknown is CF. In trying to solve this equation the fourth power of CF must be dealt with. An excel spread sheet with this equation can be used and all the known values entered. Then through iteration, the value of CF when both sides are equal will be found. That is the correct value for CF and of course the closest standard value is used for CF. Before moving to the lab, the transfer function of the transimpedance amplifier must be found and the units must be in Ohms. CF has been added for stability. The addition of this part adds a pole to the circuit. The pole is located at: To attain maximum bandwidth and still have good stability the pole is to be located on the open loop gain curve which is A. If additional compensation is required one can always increase the value of CF, but this will also reduce the bandwidth of the circuit. Therefore A = 1/F, or AF = 1. For A the equation is: The LMP7717 was evaluated for RF = 10 kΩ and 100 kΩ, representing a somewhat lower gain configuration and with the 100 kΩ feedback resistor a fairly high gain configuration. The RF = 10 kΩ is covered first. Looking at the Input Common Mode Capacitance vs. VCM chart for CCM for the operating point selected CCM = 15 pF. Note that for split supplies VCM = 2.5V, CIN = 22 pF and fGBW = 88 MHz. Solving for CF the calculated value is 1.75 pF, so 1.8 pF is selected for use. Checking the frequency of the pole finds that it is at 8.8 MHz, which is right at the minimum gain recommended for this part. Some over compensation was necessary for stability and the final selected value for CF is 2.7 pF. This moves the pole to 5.9 MHz. Figure 18 and Figure 19 show the rise and fall times obtained in the lab with a 1V output swing. The laser diode was difficult to drive due to thermal effects making the starting and ending point of the pulse quite different, therefore the two separate scope pictures. The expression fGBW is the gain bandwidth product of the part. For a unity gain stable part this is the frequency where A = 1. For the LMP7717 fGBW = 88 MHz. Multiplying A and F results in the following equation: For the above equation s = jω. To find the actual amplitude of the equation the square root of the square of the real and imaginary parts are calculated. At the intersection of F and A, we have: 30010894 FIGURE 18. Fall Time 19 www.national.com LMP7717/LMP7718 Using feedback theory, F = VA/VOUT, this becomes a voltage divider giving the following equation: LMP7717/LMP7718 pole is at 2.5 MHz. Figure 20 shows the response for a 1V output. 30010895 FIGURE 19. Rise Time 30010896 In Figure 18 the ringing and the hump during the on time is from the laser. The higher drive levels for the laser gave ringing in the light source as well as light changing from the thermal characteristics. The hump is due to the thermal characteristics. Solving for CF using a 100 kΩ feedback resistor, the calculated value is 0.54 pF. One of the problems with more gain is the very small value for CF. A 0.5 pF capacitor was used, its measured value being 0.64 pF. For the 0.64 pF location the www.national.com FIGURE 20. High Gain Response A transimpedance amplifier is an excellent application for the LMP7717. Even with the high gain using a 100 kΩ feedback resistor, the bandwidth is still well over 1 MHz. Other than a little over compensation for the 10 kΩ feedback resistor configuration using the LMP7717 was quite easy. Of course a very good board layout was also used for this test. 20 LMP7717/LMP7718 Physical Dimensions inches (millimeters) unless otherwise noted 5-Pin SOT23 NS Package Number MF05A 8-Pin SOIC NS Package Number M08A 21 www.national.com LMP7717/LMP7718 8-Pin MSOP NS Package Number MUA08A www.national.com 22 LMP7717/LMP7718 23 www.national.com LMP7717/LMP7718 88 MHz, Precision, Low Noise, 1.8V CMOS Input, Decompensated Operational Amplifier Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: Products Design Support Amplifiers www.national.com/amplifiers WEBENCH www.national.com/webench Audio www.national.com/audio Analog University www.national.com/AU Clock Conditioners www.national.com/timing App Notes www.national.com/appnotes Data Converters www.national.com/adc Distributors www.national.com/contacts Displays www.national.com/displays Green Compliance www.national.com/quality/green Ethernet www.national.com/ethernet Packaging www.national.com/packaging Interface www.national.com/interface Quality and Reliability www.national.com/quality LVDS www.national.com/lvds Reference Designs www.national.com/refdesigns Power Management www.national.com/power Feedback www.national.com/feedback Switching Regulators www.national.com/switchers LDOs www.national.com/ldo LED Lighting www.national.com/led PowerWise www.national.com/powerwise Serial Digital Interface (SDI) www.national.com/sdi Temperature Sensors www.national.com/tempsensors Wireless (PLL/VCO) www.national.com/wireless THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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