LMV796/LMV797 17 MHz, Low Noise, CMOS Input, 1.8V Operational Amplifiers General Description Features The LMV796 (Single) and the LMV797 (Dual) low noise, CMOS input operational amplifiers offer a low input voltage while consuming only 1.15 mA noise density of 5.8 nV/ (LMV796) of quiescent current. The LMV796 and LMV797 are unity gain stable op amps and have gain bandwidth of 17 MHz. The LMV796/ LMV797 have a supply voltage range of 1.8V to 5.5V and can operate from a single supply. The LMV796/LMV797 each feature a rail-to-rail output stage capable of driving a 600Ω load and sourcing as much as 60 mA of current. The LMV796 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 LMV796 and the LMV797 ideal for low power sensor applications. (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 — LMV796 1.15 mA — LMV797 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 The LMV796/LMV797 are manufactured using National’s advanced VIP50 process and are offered in a 5-pin SOT23 and an 8-pin MSOP package respectively. n n n n n Applications Photodiode amplifiers Active filters and buffers Low noise signal processing Medical instrumentation Sensor interface applications Typical Application 20183569 Photodiode Transimpedance Amplifier © 2006 National Semiconductor Corporation DS201835 20183539 Input Referred Voltage Noise vs. Frequency www.national.com LMV796/LMV797 17 MHz, Low Noise, CMOS Input, 1.8V Operational Amplifiers May 2006 LMV796/LMV797 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 5-Pin SOT23 180˚C/W 8-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. 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 LMV796 (Note 6) −1.0 LMV797 (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 LMV796 85 80 98 LMV797 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 SR Slew Rate (Continued) Sourcing to V− VIN = 200 mV (Note 9) 35 28 47 Sinking to V+ VIN = –200 mV (Note 9) 7 5 15 mA LMV796 0.95 1.30 1.65 LMV797 per channel 1.1 1.50 1.85 AV = +1, Rising (10% to 90%) 8.5 AV = +1, Falling (90% to 10%) 10.5 mA V/µs GBWP Gain Bandwidth Product 14 MHz 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, RLOAD = 600Ω 0.01 % 5V Electrical Characteristics 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 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 LMV796 (Note 6) −1.0 LMV797 (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 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 = 0.3V to 4.7V, RLOAD = 2 kΩ to V+/2 10 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 LMV796 85 80 97 LMV797 82 78 89 88 84 110 V dB www.national.com LMV796/LMV797 2.5V Electrical Characteristics LMV796/LMV797 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 LMV796 42 75 78 LMV797 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 SR Slew Rate Sourcing to V− VIN = 200 mV (Note 9) 45 37 60 Sinking to V+ VIN = –200 mV (Note 9) 10 6 21 mA LMV796 1.15 1.40 1.75 LMV797 per channel 1.30 1.70 2.05 AV = +1, Rising (10% to 90%) 6.0 9.5 AV = +1, Falling (90% to 10%) 7.5 11.5 17 mV from rail mA V/µs GBWP Gain Bandwidth Product en Input-Referred Voltage Noise f = 1 kHz 5.8 nV/ MHz in Input-Referred Current Noise f = 1 kHz 0.01 pA/ THD+N Total Harmonic Distortion + Noise f = 1 kHz, AV = 1, RLOAD = 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 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 LMV796/LMV797 Connection Diagrams 5-Pin SOT23 8-Pin MSOP 20183501 Top View 20183502 Top View Ordering Information Package 5-Pin SOT23 8-Pin MSOP Part Number LMV796MF LMV796MFX LMV797MM LMV797MMX Package Marking Transport Media 1k Units Tape and Reel AT3A 3k Units Tape and Reel 1k Units Tape and Reel AU3A 3.5k Units Tape and Reel 5 NSC Drawing MF05A MUA08A www.national.com LMV796/LMV797 Typical Performance Characteristics Unless otherwise specified, TA = 25˚C, V– = 0, V+ = Supply + Voltage = 5V, VCM = V /2. Supply Current vs. Supply Voltage (LMV796) Supply Current vs. Supply Voltage (LMV797) 20183505 20183581 VOS vs. VCM VOS vs. VCM 20183551 20183509 VOS vs. VCM VOS vs. Supply Voltage 20183511 www.national.com 20183512 6 Voltage = 5V, VCM = V+/2. (Continued) Slew Rate vs. Supply Voltage Input Bias Current vs. VCM 20183529 20183562 Input Bias Current vs. VCM Sourcing Current vs. Supply Voltage 20183587 20183520 Sinking Current vs. Supply Voltage Sourcing Current vs. Output Voltage 20183550 20183519 7 www.national.com LMV796/LMV797 Typical Performance Characteristics Unless otherwise specified, TA = 25˚C, V– = 0, V+ = Supply LMV796/LMV797 Typical Performance Characteristics Unless otherwise specified, TA = 25˚C, V– = 0, V+ = Supply Voltage = 5V, VCM = V+/2. (Continued) Sinking Current vs. Output Voltage Positive Output Swing vs. Supply Voltage 20183517 20183554 Negative Output Swing vs. Supply Voltage Positive Output Swing vs. Supply Voltage 20183515 20183516 Negative Output Swing vs. Supply Voltage Positive Output Swing vs. Supply Voltage 20183514 www.national.com 20183518 8 Voltage = 5V, VCM = V+/2. (Continued) Negative Output Swing vs. Supply Voltage Input Referred Voltage Noise vs. Frequency 20183513 20183539 Overshoot and Undershoot vs. CLOAD THD+N vs. Peak-to-Peak Output Voltage (VOUT) 20183526 20183530 THD+N vs. Peak-to-Peak Output Voltage (VOUT) THD+N vs. Frequency 20183574 20183504 9 www.national.com LMV796/LMV797 Typical Performance Characteristics Unless otherwise specified, TA = 25˚C, V– = 0, V+ = Supply LMV796/LMV797 Typical Performance Characteristics Unless otherwise specified, TA = 25˚C, V– = 0, V+ = Supply Voltage = 5V, VCM = V+/2. (Continued) THD+N vs. Frequency Open Loop Gain and Phase with Capacitive Load 20183575 20183541 Open Loop Gain and Phase with Resistive Load Closed Loop Output Impedance vs. Frequency 20183532 20183573 Crosstalk Rejection Small Signal Transient Response, AV = +1 20183538 20183580 www.national.com 10 Voltage = 5V, VCM = V+/2. (Continued) Large Signal Transient Response, AV = +1 Small Signal Transient Response, AV = +1 20183537 20183533 Large Signal Transient Response, AV = +1 Phase Margin vs. Capacitive Load (Stability) 20183534 20183545 Phase Margin vs. Capacitive Load (Stability) Positive PSRR vs. Frequency 20183546 20183527 11 www.national.com LMV796/LMV797 Typical Performance Characteristics Unless otherwise specified, TA = 25˚C, V– = 0, V+ = Supply LMV796/LMV797 Typical Performance Characteristics Unless otherwise specified, TA = 25˚C, V– = 0, V+ = Supply Voltage = 5V, VCM = V+/2. (Continued) Negative PSRR vs. Frequency CMRR vs. Frequency 20183556 20183528 Input Common Mode Capacitance vs. VCM 20183576 www.national.com 12 ADVANTAGES OF THE LMV796/LMV797 Wide Bandwidth at Low Supply Current The LMV796 and LMV797 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. Low Input Referred Noise and Low Input Bias Current The LMV796/LMV797 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 LMV796 and LMV797 ideal for audio and sensor based applications. 20183561 Low Supply Voltage The LMV796 and the LMV797 have performance guaranteed at 2.5V and 5V supply. The LMV796 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 LMV796 and LMV797 are also guaranteed to be operational at 1.8V supply voltage, for temperatures between 0˚C and 125˚C. This makes the LMV796 family ideal for usage in low-voltage commercial applications. FIGURE 1. Isolation of CL to Improve Stability INPUT CAPACITANCE AND FEEDBACK CIRCUIT ELEMENTS The LMV796 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. 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 LMV796 and the LMV797 to source more than 40 mA of current at 1.8V supply. This also limits the performance of the LMV796 family as comparators, and hence the usage of the LMV796 and the LMV797 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 footprint of the LMV796 and the LMV797 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 op amp make the signal path susceptible to noise. By using the physically smaller LMV796 or LMV797 package, the op amp can be placed closer to the signal source, reducing noise pickup and increasing signal integrity. CAPACITIVE LOAD TOLERANCE The LMV796 and LMV797 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. 20183564 FIGURE 2. Inverting Amplifier 13 www.national.com LMV796/LMV797 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. Application Information LMV796/LMV797 Application Information 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. (Continued) For simplicity, the op amp is modeled 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). (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). 20183560 FIGURE 4. Gain Peaking Eliminated by CF (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 and R2 to obtain a wider and flatter response. Lower resistances also help in high sensitivity circuits since they add less noise. AUDIO PREAMPLIFIER WITH BAND PASS FILTERING With low input referred voltage noise, low supply voltage and current, and a low harmonic distortion, the LMV796 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. 20183559 FIGURE 3. Gain Peaking Caused by Large R1, R2 20183565 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 www.national.com FIGURE 5. Inverting Audio Preamplifier 14 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. (Continued) The LMV796/LMV797 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 LMV796/LMV797 to provide high fidelity amplification for wide bandwidths. These properties make the LMV796/LMV797 ideal for systems requiring wide-band transimpedance amplification. 20183566 FIGURE 6. Non-inverting Audio Preamplifier 20183569 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. 20183558 FIGURE 7. Frequency Response of the Inverting Audio Preamplifier 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. 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 LMV796/LMV797 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 the 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 15 www.national.com LMV796/LMV797 Application Information LMV796/LMV797 Application Information formula in Equation (3), since the parasitic capacitance of the board and the feedback resistor RF had to be accounted for. (Continued) TABLE 1. (3) 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 20183584 FIGURE 9. CF Selection for Stability Calculating CF from Equation (3) can sometimes return unreasonably small values ( < 1 pF), especially for high speed applications. In these cases, it is 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) times 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. 20183577 FIGURE 11. Frequency Response for ATI = 470000 20183571 20183578 FIGURE 10. Obtaining Small CF from Large CF' FIGURE 12. Frequency Response for ATI = 47000 LMV796 AS A TRANSIMPEDANCE AMPLIFIER The LMV796 was used in the designs for 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 www.national.com HIGH GAIN WIDEBAND TRANSIMPEDANCE AMPLIFIER USING THE LMV797 The LMV797 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 16 (Continued) SENSOR INTERFACES 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 8-pin MSOP 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 The low input bias current and low input referred noise of the LMV796 and LMV797 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 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. 20183586 FIGURE 13. 1.5 MHz Transimpedance Amplifier, with ATI = 470000 20183572 FIGURE 15. IR Radiation Sensor 20183579 FIGURE 14. 1.5 MHz Transimpedance Amplifier Frequency Response 17 www.national.com LMV796/LMV797 Application Information LMV796/LMV797 Physical Dimensions inches (millimeters) unless otherwise noted 5-Pin SOT23 NS Package Number MF05A 8-Pin MSOP NS package Number MUA08A www.national.com 18 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. LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. 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 LMV796/LMV797 17 MHz, Low Noise, CMOS Input, 1.8V Operational Amplifiers Notes