LMV791/LMV792 17 MHz, Low Noise, CMOS Input, 1.8V Operational Amplifiers General Description Features The LMV791 (Single) and the LMV792 (Dual) low noise, CMOS input operational amplifiers offer a low input voltage while consuming only 1.15 mA noise density of 5.8 nV/ (LMV791) of quiescent current. The LMV791 and LMV792 are unity gain stable op amps and have gain bandwidth of 17 MHz. The LMV791/ LMV792 have a supply voltage range of 1.8V to 5.5V and can operate from a single supply. The LMV791/LMV792 each feature a rail-to-rail output stage capable of driving a 600Ω load and sourcing as much as 60 mA of current. The LMV791 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 femtoAmperes, and an input common mode voltage range which includes ground make the LMV791 and the LMV792 ideal for low power sensor applications. The LMV791 family has a built-in enable feature which can be used to optimize power dissipation in low power applications. The LMV791/LMV792 are manufactured using National’s advanced VIP50 process and are offered in a 6-pin TSOT23 and a 10-pin MSOP package respectively. (Typical 5V supply, unless otherwise noted) n Input referred voltage noise 5.8 nV/ n Input bias current 100 fA n Unity gain bandwidth 17 MHz n Supply current per channel — LMV791 1.15 mA — LMV792 1.30 mA n Rail-to-rail output swing — @ 10 kΩ load 25 mV from rail 35 mV from rail — @ 2 kΩ load n Guaranteed 2.5V and 5.0V performance n Total harmonic distortion 0.01% @1 kHz, 600Ω n Temperature range −40˚C to 125˚C Applications n n n n n Photodiode amplifiers Active filters and buffers Low noise signal processing Medical Instrumentation Sensor interface applications Typical Application 20116869 Photodiode Transimpedance Amplifier © 2005 National Semiconductor Corporation DS201168 20116839 Input Referred Voltage Noise vs. Frequency www.national.com LMV791/LMV792 17 MHz, Low Noise, CMOS Input, 1.8V Operational Amplifiers November 2005 LMV791/LMV792 Absolute Maximum Ratings (Note 1) Soldering Information If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Infrared or Convection (20 sec) 235˚C Wave Soldering Lead Temp (10 sec) 260˚C ESD Tolerance (Note 2) Human Body Model 2000V Machine Model Operating Ratings (Note 1) 200V Temperature Range (Note 3) ± 0.3V VIN Differential Supply Voltage (V+ – V−) Input/Output Pin Voltage Storage Temperature Range + Supply Voltage (V – V ) −40˚C ≤ TA ≤ 125˚C 6.0V V+ +0.3V, V− −0.3V 2.0V to 5.5V 0˚C ≤ TA ≤ 125˚C −65˚C to 150˚C Junction Temperature (Note 3) −40˚C to 125˚C − 1.8V to 5.5V Package Thermal Resistance (θJA (Note 3)) +150˚C 6-Pin TSOT23 170˚C/W 10-Pin MSOP 236˚C/W 2.5V Electrical Characteristics Unless otherwise specified, all limits are guaranteed for TA = 25˚C, V+ = 2.5V, V− = 0V, VCM = V+/2 = VO, VEN = V+. Boldface limits apply at the temperature extremes. Symbol Parameter VOS Input Offset Voltage TC VOS Input Offset Average Drift IB Input Bias Current Conditions Min (Note 5) Typ (Note 4) Max (Note 5) 0.1 ± 1.35 ± 1.65 LMV791 (Note 6) −1.0 LMV792 (Note 6) −1.8 VCM = 1.0V (Notes 7, 8) 0.05 1 25 −40˚C ≤TA ≤ 125˚C 0.05 1 100 Input Offset Current (Note 8) CMRR Common Mode Rejection Ratio 0V ≤ VCM ≤ 1.4V 80 75 94 PSRR Power Supply Rejection Ratio 2.0V ≤ V+ ≤ 5.5V, VCM = 0V 80 75 100 1.8V ≤ V+ ≤ 5.5V, VCM = 0V 80 98 CMVR Input Common-Mode Voltage Range CMRR ≥ 60 dB CMRR ≥ 55 dB AVOL Large Signal Voltage Gain VOUT = 0.15V to 2.2V, RLOAD = 2 kΩ to V+/2 10 VOUT Output Swing High Output Swing Low www.national.com fA dB 1.5 1.5 LMV791 85 80 98 LMV792 82 78 92 88 84 110 V dB RLOAD = 2 kΩ to V+/2 25 75 82 RLOAD = 10 kΩ to V+/2 20 65 71 RLOAD = 2 kΩ to V+/2 30 75 78 RLOAD = 10 kΩ to V+/2 15 65 67 2 pA dB −0.3 -0.3 VOUT = 0.15V to 2.2V, RLOAD = 10 kΩ to V+/2 mV µV/˚C −40˚C ≤TA ≤ 85˚C IOS Units mV from rail IOUT IS Output Short Circuit Current Supply Current per Amplifier (Continued) Sourcing to V− VIN = 200 mV (Note 9) 35 28 47 Sinking to V+ VIN = –200 mV (Note 9) 7 5 15 Enable Mode VEN ≥ 2.1V LMV791 0.95 1.30 1.65 LMV792 per channel 1.1 1.50 1.85 0.02 1 5 Shutdown Mode, VEN < 0.4 per channel SR Slew Rate mA AV = +1, Rising (10% to 90%) 8.5 AV = +1, Falling (90% to 10%) 10.5 mA µA V/µs GBWP Gain Bandwidth Product en Input-Referred Voltage Noise f = 1 kHz 6.2 14 nV/ in Input-Referred Current Noise f = 1 kHz 0.01 pA/ ton Turn-on Time toff Turn-off Time VEN Enable Pin Voltage Range Enable Mode 2.1 Shutdown Mode IEN THD+N Enable Pin Input Current Total Harmonic Distortion + Noise MHz 140 ns 1000 ns 2 to 2.5 0 to 0.5 V 0.4 Enable Mode VEN = 2.5V (Note 7) 1.5 3 Shutdown Mode VEN = 0V (Note 7) 0.003 0.1 f = 1 kHz, AV = 1, RLOAD = 600Ω 0.01 µA % 5V Electrical Characteristics Unless otherwise specified, all limits are guaranteed for TA = 25˚C, V+ = 5V, V− = 0V, VCM = V+/2 = VO, VEN = V+. Boldface limits apply at the temperature extremes. Symbol Parameter VOS Input Offset Voltage TC VOS Input Offset Average Drift IB Input Bias Current Conditions Min (Note 5) Typ (Note 4) Max (Note 5) 0.1 ± 1.35 ± 1.65 LMV791 (Note 6) −1.0 LMV792 (Note 6) −1.8 VCM = 2.0V (Notes 7, 8) 0.1 1 25 −40˚C ≤TA ≤ 125˚C 0.1 1 100 Input Offset Current (Note 8) CMRR Common Mode Rejection Ratio 0V ≤ VCM ≤ 3.7V 80 75 100 PSRR Power Supply Rejection Ratio 2.0V ≤ V+ ≤ 5.5V, VCM = 0V 80 75 100 10 80 98 1.8V ≤ V ≤ 5.5V, VCM = 0V + CMVR Input Common-Mode Voltage Range CMRR ≥ 60 dB CMRR ≥ 55 dB AVOL Large Signal Voltage Gain VOUT = 0.3V to 4.7V, RLOAD = 2 kΩ to V+/2 3 pA fA dB dB −0.3 -0.3 VOUT = 0.3V to 4.7V, RLOAD = 10 kΩ to V+/2 mV µV/˚C −40˚C ≤TA ≤ 85˚C IOS Units 4 4 LMV791 85 80 97 LMV792 82 78 89 88 84 110 V dB www.national.com LMV791/LMV792 2.5V Electrical Characteristics LMV791/LMV792 5V Electrical Characteristics VOUT Output Swing High Output Swing Low (Continued) RLOAD = 2 kΩ to V+/2 35 75 82 RLOAD = 10 kΩ to V+/2 25 65 71 LMV791 42 75 78 LMV792 50 80 83 20 65 67 RLOAD = 2 kΩ to V+/2 RLOAD = 10 kΩ to V+/2 IOUT IS Output Short Circuit Current Supply Current per Amplifier Sourcing to V− VIN = 200 mV (Note 9) 45 37 60 Sinking to V+ VIN = –200 mV (Note 9) 10 6 21 Enable Mode VEN ≥ 4.6V SR Slew Rate mA LMV791 1.15 1.40 1.75 LMV792 per channel 1.30 1.70 2.05 0.14 1 5 Shutdown Mode (VEN ≤ 0.4V) AV = +1, Rising (10% to 90%) 6.0 9.5 AV = +1, Falling (90% to 10%) 7.5 11.5 mV from rail mA µA V/µs GBWP Gain Bandwidth Product en Input-Referred Voltage Noise f = 1 kHz 5.8 17 nV/ in Input-Referred Current Noise f = 1 kHz 0.01 pA/ ton Turn-on Time 110 ns toff Turn-off Time 800 ns VEN Enable Pin Voltage Range Enable Mode 4.6 Shutdown Mode IEN THD+N Enable Pin Input Current Total Harmonic Distortion + Noise MHz 4.5 to 5 0 to 0.5 0.4 Enable Mode VEN = 5.0V (Note 7) 5.6 10 Shutdown Mode VEN = 0V (Note 7) 0.005 0.2 f = 1 kHz, AV = 1, RLOAD = 600Ω 0.01 V µA % 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 is 1.5 kΩ in series with 100 pF. Machine Model is 0Ω in series with 200 pF 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: Typical values represent the parametric norm at the time of characterization. Note 5: 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 6: Offset voltage average drift is determined by dividing the change in VOS by temperature change. Note 7: Positive current corresponds to current flowing into the device. Note 8: Input bias current and input offset current are guaranteed by design Note 9: The short circuit test is a momentary test, the short circuit duration is 1.5 ms. www.national.com 4 LMV791/LMV792 Connection Diagrams 6-Pin TSOT23 10-Pin MSOP 20116801 Top View 20116802 Top View Ordering Information Package 6-Pin TSOT23 10-Pin MSOP Part Number LMV791MK LMV791MKX LMV792MM LMV792MMX Package Marking Transport Media 1k Units Tape and Reel AS1A 3k Units Tape and Reel 1k Units Tape and Reel AX2A 3.5k Units Tape and Reel 5 NSC Drawing MK06A MUB10A www.national.com LMV791/LMV792 Typical Performance Characteristics + Unless otherwise specified, TA = 25˚C, V– = 0, V+ = Supply + Voltage = 5V, VCM = V /2, VEN = V . Supply Current vs. Supply Voltage (LMV791) Supply Current vs. Supply Voltage (LMV792) 20116805 20116881 Supply Current vs. Supply Voltage in Shutdown Mode VOS vs. VCM 20116806 20116809 VOS vs. VCM VOS vs. VCM 20116811 20116851 www.national.com 6 Voltage = 5V, VCM = V+/2, VEN = V+. (Continued) VOS vs. Supply Voltage Slew Rate vs. Supply Voltage 20116829 20116812 Supply Current vs. Enable Pin Voltage (LMV791) Supply Current vs. Enable Pin Voltage(LMV791) 20116807 20116808 Supply Current vs. Enable Pin Voltage (LMV792) Supply Current vs. Enable Pin Voltage (LMV792) 20116882 20116883 7 www.national.com LMV791/LMV792 Typical Performance Characteristics Unless otherwise specified, TA = 25˚C, V– = 0, V+ = Supply LMV791/LMV792 Typical Performance Characteristics Unless otherwise specified, TA = 25˚C, V– = 0, V+ = Supply Voltage = 5V, VCM = V+/2, VEN = V+. (Continued) Input Bias Current vs. VCM Input Bias Current vs. VCM 20116887 20116862 Sourcing Current vs. Supply Voltage Sinking Current vs. Supply Voltage 20116820 20116819 Sourcing Current vs. Output Voltage Sinking Current vs. Output Voltage 20116850 www.national.com 20116854 8 Voltage = 5V, VCM = V+/2, VEN = V+. (Continued) Positive Output Swing vs. Supply Voltage Negative Output Swing vs. Supply Voltage 20116817 20116815 Positive Output Swing vs. Supply Voltage Negative Output Swing vs. Supply Voltage 20116816 20116814 Positive Output Swing vs. Supply Voltage Negative Output Swing vs. Supply Voltage 20116813 20116818 9 www.national.com LMV791/LMV792 Typical Performance Characteristics Unless otherwise specified, TA = 25˚C, V– = 0, V+ = Supply LMV791/LMV792 Typical Performance Characteristics Unless otherwise specified, TA = 25˚C, V– = 0, V+ = Supply Voltage = 5V, VCM = V+/2, VEN = V+. (Continued) Input Referred Voltage Noise vs. Frequency Overshoot and Undershoot vs. CLOAD 20116839 20116830 THD+N vs. Peak-to-Peak Output Voltage (VOUT) THD+N vs. Peak-to-Peak Output Voltage (VOUT) 20116826 20116804 THD+N vs. Frequency THD+N vs. Frequency 20116875 20116874 www.national.com 10 Voltage = 5V, VCM = V+/2, VEN = V+. (Continued) Open Loop Gain and Phase with Capacitive Load Open Loop Gain and Phase with Resistive Load 20116873 20116841 Closed Loop Output Impedance vs. Frequency Crosstalk Rejection 20116832 20116880 Small Signal Transient Response, AV = +1 Large Signal Transient Response, AV = +1 20116838 20116837 11 www.national.com LMV791/LMV792 Typical Performance Characteristics Unless otherwise specified, TA = 25˚C, V– = 0, V+ = Supply LMV791/LMV792 Typical Performance Characteristics Unless otherwise specified, TA = 25˚C, V– = 0, V+ = Supply Voltage = 5V, VCM = V+/2, VEN = V+. (Continued) Small Signal Transient Response, AV = +1 Large Signal Transient Response, AV = +1 20116834 20116833 Phase Margin vs. Capacitive Load (Stability) Phase Margin vs. Capacitive Load (Stability) 20116845 20116846 Positive PSRR vs. Frequency Negative PSRR vs. Frequency 20116828 20116827 www.national.com 12 Voltage = 5V, VCM = V+/2, VEN = V+. (Continued) CMRR vs. Frequency Input Common Mode Capacitance vs. VCM 20116856 20116876 13 www.national.com LMV791/LMV792 Typical Performance Characteristics Unless otherwise specified, TA = 25˚C, V– = 0, V+ = Supply LMV791/LMV792 Small Size Application Notes The small footprint of the LMV791 and the LMV792 package saves space on printed circuit boards, and enables the design of smaller electronic products, such as cellular phones, pagers, or other portable systems. Long traces between the signal source and the opamp make the signal path susceptible to noise. By using a physically smaller LMV791 and LMV792 package, the opamp can be placed closer to the signal source, reducing noise pickup and increasing signal integrity. ADVANTAGES OF THE LMV791/LMV792 Wide Bandwidth at Low Supply Current The LMV791 and LMV792 are high performance op amps that provide a unity gain bandwidth of 17 MHz while drawing a low supply current of 1.15 mA. This makes them ideal for providing wideband amplification in portable applications. The enable and shutdown feature can also be used to design more power efficient systems that offer wide bandwidth and high performance while consuming less average power. CAPACITIVE LOAD TOLERANCE The LMV791 and LMV792 can directly drive 120 pF in unity-gain without oscillation. The unity-gain follower is the most sensitive configuration to capacitive loading. Direct capacitive loading reduces the phase margin of amplifiers. The combination of the amplifier’s output impedance and the capacitive load induces phase lag. This results in either an underdamped pulse response or oscillation. To drive a heavier capacitive load, the circuit in Figure 1 can be used. In Figure 1, the isolation resistor RISO and the load capacitor CL form a pole to increase stability by adding more phase margin to the overall system. The desired performance depends on the value of RISO. The bigger the RISO resistor value, the more stable VOUT will be. Increased RISO would, however, result in a reduced output swing and short circuit current. Low Input Referred Noise and Low Input Bias Current The LMV791/LMV792 have a very low input referred voltage at 1 kHz). A CMOS input stage noise density (5.8 nV/ ensures a small input bias current (100 fA) and low input ). This is very helpful in referred current noise (0.01 pA/ maintaining signal fidelity, and makes the LMV791 and LMV792 ideal for audio and sensor based applications. Low Supply Voltage The LMV791 and the LMV792 have performance guaranteed at 2.5V and 5V supply. The LMV791 family is 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 LMV791 and LMV792 are also guaranteed to be operational at 1.8V supply voltage, for temperatures between 0˚C and 125˚C. This makes the LMV791 family ideal for usage in low-voltage commercial applications. RRO and Ground Sensing Rail-to-rail output swing provides maximum possible dynamic range at the output. 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 LMV791 and the LMV792 to source more than 40 mA of current at 1.8V supply. This also limits the performance of the LMV791 family as comparators, and hence the usage of the LMV791 and the LMV792 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. 20116861 FIGURE 1. Isolation of CL to Improve Stability INPUT CAPACITANCE AND FEEDBACK CIRCUIT ELEMENTS The LMV791 family has a very low input bias current (100 fA) and a low 1/f noise corner frequency (400 Hz), which makes it ideal for sensor applications. However, to obtain this performance a large CMOS input stage is used, which adds to the input capacitance of the op-amp, CIN. Though this does not affect the DC and low frequency performance, at higher frequencies the input capacitance interacts with the input and the feedback impedances to create a pole, which results in lower phase margin and gain peaking. This can be controlled by being selective in the use of feedback resistors, as well as by using a feedback capacitance, CF. For example, in the inverting amplifier shown in Figure 2, if CIN and CF are ignored and the open loop gain of the op amp is considered infinite then the gain of the circuit is −R2/R1. An op amp, however, usually has a dominant pole, which causes its gain to drop with frequency. Hence, this gain is only valid for DC and low frequency. To understand the effect of the input capacitance coupled with the non-ideal gain of the op amp, the circuit needs to be analyzed in the frequency domain using a Laplace transform. Enable and Shutdown Features The LMV791 family is ideal for battery powered systems. With a low supply current of 1.15 mA and a shutdown current of 140 nA typically, the LMV791 and LMV792 allow the designer to maximize battery life. The enable pin of the LMV791 and the LMV792 allows the op amp to be turned off and reduce its supply current to less than 1 µA. To power on the op amp the enable pin should be higher than V+ - 0.5V, where V+ is the positive supply. To disable the op amp, the enable pin voltage should be less than V− + 0.5V, where V− is the negative supply. www.national.com 14 and R2 to obtain a wider and flatter response. Lower resistances also help in high sensitivity circuits since they add less noise. (Continued) 20116864 FIGURE 2. Inverting Amplifier 20116859 For simplicity, the op amp is modelled as an ideal integrator with a unity gain frequency of A0 . Hence, its transfer function (or gain) in the frequency domain is A0/s. Solving the circuit equations in the frequency domain, ignoring CF for the moment, results in an expression for the gain shown in Equation (1). FIGURE 3. Gain Peaking Caused by Large R1, R2 A way of reducing the gain peaking is by adding a feedback capacitance CF in parallel with R2. This introduces another pole in the system and prevents the formation of pairs of complex conjugate poles which cause the gain to peak. Figure 4 shows the effect of CF on the frequency response of the circuit. Adding a capacitance of 2 pF removes the peak, while a capacitance of 5 pF creates a much lower pole and reduces the bandwidth excessively. (1) It can be inferred from the denominator of the transfer function that it has two poles, whose expressions can be obtained by solving for the roots of the denominator and are shown in Equation (2). (2) Equation (2) shows that as the values of R1 and R2 are increased, the magnitude of the poles, and hence the bandwidth of the amplifier, is reduced. This theory is verified by using different values of R1 and R2 in the circuit shown in Figure 1 and by comparing their frequency responses. In Figure 3 the frequency responses for three different values of R1 and R2 are shown. When both R1 and R2 are 1 kΩ, the response is flattest and widest; whereas, it narrows and peaks significantly when both their values are changed to 10 kΩ or 30 kΩ. So it is advisable to use lower values of R1 20116860 FIGURE 4. Gain Peaking Eliminated by CF 15 www.national.com LMV791/LMV792 Application Notes LMV791/LMV792 Application Notes (Continued) AUDIO PREAMPLIFIER WITH BANDPASS FILTERING With low input referred voltage noise, low supply voltage and low supply current, and a low harmonic distortion, the LMV791 family is ideal for audio applications. Its wide unity gain bandwidth allows it to provide large gain for a wide range of frequencies and it can be used to design a preamplifier to drive a load of as low as 600Ω with less than 0.01% distortion. Two amplifier circuits are shown in Figure 5 and Figure 6. Figure 5 is an inverting amplifier, with a 10 kΩ feedback resistor, R2, and a 1kΩ input resistor, R1, and hence provides a gain of −10. Figure 6 is a non-inverting amplifier, using the same values of R1and R2, and provides a gain of 11. In either of these circuits, the coupling capacitor CC1 decides the lower frequency at which the circuit starts providing gain, while the feedback capacitor CF decides the frequency at which the gain starts dropping off. Figure 7 shows the frequency response of the inverting amplifier with different values of CF. 20116858 FIGURE 7. Frequency Response of the Inverting Audio Preamplifier TRANSIMPEDANCE AMPLIFIER CMOS input op amps are often used in transimpedance applications as they have an extremely high input impedance. A transimpedance amplifier converts a small input current into a voltage. This current is usually generated by a photodiode. The transimpedance gain, measured as the ratio of the output voltage to the input current, is expected to be large and wide-band. Since the circuit deals with currents in the range of a few nA, low noise performance is essential. The LMV791/LMV792 are CMOS input op amps providing wide bandwidth and low noise performance, and are hence ideal for transimpedance applications. Usually, a transimpedance amplifier is designed on the basis of the current source driving the input. A photodiode is a very common capacitive current source, which requires transimpedance gain for transforming its miniscule current into easily detectable voltages. The photodiode and amplifier’s gain are selected with respect to the speed and accuracy required of the circuit. A faster circuit would require a photodiode with lesser capacitance and a faster amplifier. A more sensitive circuit would require a sensitive photodiode and a high gain. A typical transimpedance amplifier is shown in Figure 8. The output voltage of the amplifier is given by the equation VOUT = −IINRF. Since the output swing of the amplifier is limited, RF should be selected such that all possible values of IIN can be detected. The LMV791/LMV792 have a large gain-bandwidth product (17 MHz), which enables high gains at wide bandwidths. A rail-to-rail output swing at 5.5V supply allows detection and amplification of a wide range of input currents. A CMOS input stage with negligible input current noise and low input voltage noise allows the LMV791/LMV792 to provide high fidelity amplification for wide bandwidths. These properties make the LMV791/LMV792 ideal for systems requiring wide-band transimpedance amplification. 20116865 FIGURE 5. Inverting Audio Preamplifier 20116866 FIGURE 6. Non-inverting Audio Preamplifier www.national.com 16 LMV791/LMV792 Application Notes (Continued) 20116884 FIGURE 9. CF Selection for Stability 20116869 Calculating CF from Equation (3) can sometimes return unreasonably small values ( < 1 pF), especially for high speed applications. In these cases, its often more practical to use the circuit shown in Figure 10 in order to allow more reasonable values. In this circuit, the capacitance CF' is (1+ RB/RA) time the effective feedback capacitance, CF. A larger capacitor can now be used in this circuit to obtain a smaller effective capacitance. For example, if a CF of 0.5 pF is needed, while only a 5 pF capacitor is available, RB and RA can be selected such that RB/RA = 9. This would convert a CF' of 5 pF into a CF of 0.5 pF. This relationship holds as long as RA << RF. FIGURE 8. Photodiode Transimpedance Amplifier As mentioned earlier, the following parameters are used to design a transimpedance amplifier: the amplifier gainbandwidth product, A0; the amplifier input capacitance, CCM; the photodiode capacitance, CD; the transimpedance gain required, RF; and the amplifier output swing. Once a feasible RF is selected using the amplifier output swing, these numbers can be used to design an amplifier with the desired transimpedance gain and a maximally flat frequency response. An essential component for obtaining a maximally flat response is the feedback capacitor, CF. The capacitance seen at the input of the amplifier, CIN, combined with the feedback capacitor, RF, generate a phase lag which causes gainpeaking and can destabilize the circuit. CIN is usually just the sum of CD and CCM. The feedback capacitor CF creates a pole, fP in the noise gain of the circuit, which neutralizes the zero in the noise gain, fZ, created by the combination of RF and CIN. If properly positioned, the noise gain pole created by CF can ensure that the slope of the gain remains at 20 dB/decade till the unity gain frequency of the amplifier is reached, thus ensuring stability. As shown in Figure 9, fP is positioned such that it coincides with the point where the noise gain intersects the op amp’s open loop gain. In this case, fP is also the overall 3 dB frequency of the transimpedance amplifier. The value of CF needed to make it so is given by Equation (3). A larger value of CF causes excessive reduction of bandwidth, while a smaller value fails to prevent gain peaking and instability. 20116871 FIGURE 10. Obtaining Small CF from large CF' LMV791 AS A TRANSIMPEDANCE AMPLIFIER The LMV791 was used to design a number of amplifiers with varying transimpedance gains and source capacitances. The gains, bandwidths and feedback capacitances of the circuits created are summarized in Table 1. The frequency responses are presented in Figure 11 and Figure 12. The feedback capacitances are slightly different from the formula in Equation (3), since the parasitic capacitance of the board and the feedback resistor RF had to be accounted for. (3) 17 www.national.com LMV791/LMV792 Application Notes HIGH GAIN WIDEBAND TRANSIMPEDANCE AMPLIFIER USING THE LMV792 The LMV792, dual, low noise, wide bandwidth, CMOS input op amp IC can be used for compact, robust and integrated solutions for sensing and amplifying wide-band signals obtained from sensitive photodiodes. One of the two op amps available can be used to obtain transimpedance gain while the other can be used for amplifying the output voltage to further enhance the transimpedance gain. The wide bandwidth of the op amps (17 MHz) ensures that they are capable of providing high gain for a wide range of frequencies. The ) allows the amplifier to low input referred noise (5.8 nV/ deliver an output with a high SNR (signal to noise ratio). The small MSOP-10 footprint saves space on printed circuit boards and allows ease of design in portable products. The circuit shown in Figure 13, has the first op amp acting as a transimpedance amplifier with a gain of 47000, while the second stage provides a voltage gain of 10. This provides a total transimpedance gain of 470000 with a −3 dB bandwidth of about 1.5 MHz, for a total input capacitance of 50 pF. The frequency response for the circuit is shown in Figure 14 (Continued) TABLE 1. Transimpedance, ATI CIN CF 3 dB Frequency 470000 50 pF 1.5 pF 350 kHz 470000 100 pF 2.0 pF 250 kHz 470000 200 pF 3.0 pF 150 kHz 47000 50 pF 4.5 pF 1.5 MHz 47000 100 pF 6.0 pF 1 MHz 47000 200 pF 9.0 pF 700 kHz 20116877 FIGURE 11. Frequency Response for ATI = 470000 20116886 FIGURE 13. 1.5 MHz Transimpedance Amplifier, with ATI = 470000 20116878 FIGURE 12. Frequency Response for ATI = 47000 20116879 FIGURE 14. 1.5 MHz Transimpedance Amplifier Frequency Response www.national.com 18 LMV791/LMV792 Application Notes (Continued) SENSOR INTERFACES The low input bias current and low input referred noise of the LMV791 and LMV792 make them ideal for sensor interfaces. These circuits are required to sense voltages of the order of a few µV, and currents amounting to less than a nA, and hence the op amp needs to have low voltage noise and low input bias current. Typical applications include infra-red (IR) thermometry, thermocouple amplifiers and pH electrode buffers. Figure 15 is an example of a typical circuit used for measuring IR radiation intensity, often used for estimating the temperature of an object from a distance. The IR sensor generates a voltage proportional to I, which is the intensity of the IR radiation falling on it. As shown in Figure 15, K is the constant of proportionality relating the voltage across the IR sensor (VIN) to the radiation intensity, I. The resistances RA and RB are selected to provide a high gain to amplify this voltage, while CF is added to filter out the high frequency noise. 20116872 FIGURE 15. IR Radiation Sensor 19 www.national.com LMV791/LMV792 Physical Dimensions inches (millimeters) unless otherwise noted 6-Pin TSOT23 NS Package Number MK06A 10-Pin MSOP NS package Number MUB10A www.national.com 20 National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications. For the most current product information visit us at www.national.com. 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BANNED SUBSTANCE COMPLIANCE National Semiconductor manufactures products and uses packing materials that meet the provisions of the Customer Products Stewardship Specification (CSP-9-111C2) and the Banned Substances and Materials of Interest Specification (CSP-9-111S2) and contain no ‘‘Banned Substances’’ as defined in CSP-9-111S2. Leadfree products are RoHS compliant. National Semiconductor Americas Customer Support Center Email: [email protected] Tel: 1-800-272-9959 www.national.com National Semiconductor Europe Customer Support Center Fax: +49 (0) 180-530 85 86 Email: [email protected] Deutsch Tel: +49 (0) 69 9508 6208 English Tel: +44 (0) 870 24 0 2171 Français Tel: +33 (0) 1 41 91 8790 National Semiconductor Asia Pacific Customer Support Center Email: [email protected] National Semiconductor Japan Customer Support Center Fax: 81-3-5639-7507 Email: [email protected] Tel: 81-3-5639-7560 LMV791/LMV792 17 MHz, Low Noise, CMOS Input, 1.8V Operational Amplifiers Notes