LMV844MT

LMV841 Single/ LMV842 Dual/ LMV844 Quad CMOS Input, RRIO, Wide Supply Range Operational Amplifiers October 2007 LMV841/LMV842/LMV844 CMOS Input, RRIO, Wide Supply Range Operational Amplifiers General Description The LMV841/LMV842/LMV844 are low-voltage and low-power operational amplifiers that operate with supply voltages ranging from 2.7V to 12V and have rail-to-rail input and output capability. Their low offset voltage, low supply current, and MOS inputs make them ideal for sensor interface and batterypowered applications. The single LMV841 is offered in the space-saving 5-Pin SC70 package, the dual LMV842 in the 8-Pin MSOP and 8-Pin SOIC packages, and the quad LMV844 in the 14-Pin TSSOP and 14-Pin SOIC packages. These small packages are ideal solutions for area-constrained PC boards and portable electronics. Features Unless otherwise noted, typical values at TA = 25°C, V+ = 5V. ■ Space saving 5-Pin SC70 package ■ Supply voltage range 2.7V to 12V ■ Guaranteed at 3.3V, 5V and ±5V 1 mA per channel ■ Low supply current 4.5 MHz ■ Unity gain bandwidth 133 dB ■ Open loop gain 500 μV max ■ Input offset voltage 0.3 pA ■ Input bias current 112 dB ■ CMRR 20 nV/√Hz ■ Input voltage noise −40°C to 125°C ■ Temperature range ■ Rail-to-Rail input ■ Rail-to-Rail output Applications ■ ■ ■ ■ ■ ■ High impedance sensor interface Battery powered instrumentation High gain amplifiers DAC buffer Instrumentation amplifiers Active filters Typical Applications 20168301 © 2007 National Semiconductor Corporation 201683 www.national.com LMV841 Single/ LMV842 Dual/ LMV844 Quad Absolute Maximum Ratings (Note 1) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. ESD Tolerance (Note 2) Human Body Model Machine Model VIN Differential Supply Voltage (V+ – V−) Voltage at Input/Output Pins Input Current Storage Temperature Range Junction Temperature (Note 3) 2 kV 200V ±300 mV 13.2V V++0.3V, V− −0.3V 10 mA −65°C to +150°C +150°C (Note 4) Soldering Information Infrared or Convection (20 sec) Wave Soldering Lead Temp. (10 sec) 235°C 260°C Operating Ratings Temperature Range (Note 3) Supply Voltage (V+ – V−) (Note 1) −40°C to +125°C 2.7V to 12V 334 °C/W 205 °C/W 126 °C/W 110 °C/W 93 °C/W Package Thermal Resistance (θJA (Note 3)) 5-Pin SC70 8-Pin MSOP 8-Pin SOIC 14-Pin TSSOP 14-Pin SOIC 3.3V Electrical Characteristics Symbol VOS TCVOS IB IOS CMRR Parameter Input Offset Voltage Input Offset Voltage Drift (Note 7) Input Bias Current (Notes 7, 8) Input Offset Current Common Mode Rejection Ratio LMV841 Common Mode Rejection Ratio LMV842 and LMV844 PSRR CMVR AVOL Power Supply Rejection Ratio Input Common-Mode Voltage Range Large Signal Voltage Gain Unless otherwise specified, all limits are guaranteed for TA = 25°C, V+ = 3.3V, V− = 0V, VCM = V+/2, and RL > 10 MΩ to V+/2. Boldface limits apply at the temperature extremes. Conditions Min (Note 6) Typ (Note 5) ±50 0.5 0.3 40 0V ≤ VCM ≤ 3.3V 84 80 77 75 2.7V ≤ V+ ≤ 12V, VO = V+/2 CMRR ≥ 50 dB RL = 2 kΩ VO = 0.3V to 3.0V RL = 10 kΩ VO = 0.2V to 3.1V VO Output Swing High, (measured from V+) RL = 2 kΩ to V+/2 RL = 10 kΩ to V+/2 Output Swing Low, (measured from V−) RL = 2 kΩ to V+/2 RL = 10 kΩ to V+/2 IO Output Short Circuit Current (Notes 3, 9) Sourcing VO = V+/2 VIN = 100 mV Sinking VO = V+/2 VIN = −100 mV IS SR GBW Supply Current Slew Rate (Note 10) Gain Bandwidth Product Per Channel AV = +1, VO = 2.3 VPP 10% to 90% 20 15 20 15 86 82 –0.1 100 96 100 96 123 131 52 28 65 33 32 27 0.93 2.5 4.5 1.5 2 mA 80 120 50 70 100 120 65 75 dB 112 106 108 3.4 dB Max (Note 6) ±500 ±800 ±5 10 300 Units μV μV/°C pA fA dB V mV mV mA V/μs MHz www.national.com 2 LMV841 Single/ LMV842 Dual/ LMV844 Quad Symbol Φm en ROUT THD+N CIN Parameter Phase Margin Input-Referred Voltage Noise Open Loop Output Impedance Total Harmonic Distortion + Noise Input Capacitance (Note 4) f = 1 kHz f = 3 MHz Conditions Min (Note 6) Typ (Note 5) 67 20 70 0.005 7 Max (Note 6) Units Deg nV/ Ω % pF f = 1 kHz , AV = 1 RL = 10 kΩ 5V Electrical Characteristics Symbol VOS TCVOS IB IOS CMRR Parameter Input Offset Voltage Input Offset Voltage Drift (Note 7) Input Bias Current (Notes 7, 8) Input Offset Current Common Mode Rejection Ratio LMV841 Common Mode Rejection Ratio LMV842 and LMV844 PSRR CMVR AVOL Power Supply Rejection Ratio Input Common-Mode Voltage Range Large Signal Voltage Gain Unless otherwise specified, all limits are guaranteed for TA = 25°C, V+ = 5V, V− = 0V, VCM = V+/2, and RL > 10 MΩ to V+/2. Boldface limits apply at the temperature extremes. Conditions Min (Note 6) Typ (Note 5) ±50 0.35 0.3 40 0V ≤ VCM ≤ 5V 86 80 81 79 2.7V ≤ V+ ≤ 12V, VO = V+/2 CMRR ≥ 50 dB RL = 2 kΩ VO = 0.3V to 4.7V RL = 10 kΩ VO = 0.2V to 4.8V VO Output Swing High, (measured from V+) RL = 2 kΩ to V+/2 RL = 10 kΩ to V+/2 Output Swing Low, (measured from V-) RL = 2 kΩ to V+/2 RL = 10 kΩ to V+/2 IO Output Short Circuit Current (Notes 3, 9) Sourcing VO = V+/2 VIN = 100 mV Sinking VO = V+/2 VIN = −100 mV IS SR GBW Φm en ROUT Supply Current Slew Rate (Note 10) Gain Bandwidth Product Phase Margin Input-Referred Voltage Noise Open Loop Output Impedance f = 1 kHz f = 3 MHz Per Channel AV = +1, VO = 4 VPP 10% to 90% 20 15 20 15 86 82 −0.2 100 96 100 96 125 133 68 32 78 38 33 28 0.96 2.5 4.5 67 20 70 1.5 2 mA 100 120 50 70 120 140 70 80 dB 112 106 108 5.2 dB Max (Note 6) ±500 ±800 ±5 10 300 Units μV μV/°C pA fA dB V mV mV mA V/μs MHz Deg nV/ Ω 3 www.national.com LMV841 Single/ LMV842 Dual/ LMV844 Quad Symbol THD+N CIN Parameter Total Harmonic Distortion + Noise Input Capacitance (Note 4) Conditions f = 1 kHz , AV = 1 RL = 10 kΩ Min (Note 6) Typ (Note 5) 0.003 6 Max (Note 6) Units % pF ±5V Electrical Characteristics Symbol VOS TCVOS IB IOS CMRR Parameter Input Offset Voltage Input Offset Voltage Drift (Note 7) Input Bias Current (Notes 7, 8) Input Offset Current Common Mode Rejection Ratio LMV841 Common Mode Rejection Ratio LMV842 and LMV844 PSRR CMVR AVOL Power Supply Rejection Ratio Input Common-Mode Voltage Range Large Signal Voltage Gain Unless otherwise specified, all limits are guaranteed for TA = 25°C, V+ = 5V, V− = –5V, VCM = 0V, and RL > 10 MΩ to VCM. Boldface limits apply at the temperature extremes. Conditions Min (Note 6) Typ (Note 5) ±50 0.25 0.3 40 –5V ≤ VCM ≤ 5V 86 80 86 80 2.7V ≤ V+ ≤ 12V, VO = 0V CMRR ≥ 50 dB RL = 2 kΩ VO = −4.7V to 4.7V RL = 10 kΩ VO = −4.8V to 4.8V VO Output Swing High, (measured from V+) RL = 2 kΩ to 0V RL = 10 kΩ to 0V Output Swing Low, (measured from V−) RL = 2 kΩ to 0V RL = 10 kΩ to 0V IO Output Short Circuit Current (Notes 3, 9) Sourcing VO = 0V VIN = 100 mV Sinking VO = 0V VIN = −100 mV IS SR GBW Φm en ROUT THD+N CIN Supply Current Slew Rate (Note 10) Gain Bandwidth Product Phase Margin Input-Referred Voltage Noise Open Loop Output Impedance Total Harmonic Distortion + Noise Input Capacitance f = 1 kHz f = 3 MHz f = 1 kHz , AV = 1 RL = 10 kΩ 3 Per Channel AV = +1, VO = 9 VPP 10% to 90% 20 15 20 15 86 82 −5.2 100 96 100 96 126 136 95 44 105 52 37 29 1.03 2.5 4.5 67 20 70 0.006 1.7 2 mA 130 155 75 95 160 200 80 100 dB 112 106 108 5.2 dB Max (Note 6) ±500 ±800 ±5 10 300 Units μV μV/°C pA fA dB V mV mV mA V/μs MHz Deg nV/ Ω % pF www.national.com 4 LMV841 Single/ LMV842 Dual/ LMV844 Quad 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) FieldInduced 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, and TA. 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. 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 statistical quality control (SQC) method. Note 7: This parameter is guaranteed by design and/or characterization and is not tested in production. Note 8: Positive current corresponds to current flowing into the device. Note 9: Short circuit test is a momentary test. Note 10: Number specified is the slower of positive and negative slew rates. Connection Diagrams 5-Pin SC70 8-Pin SOIC and MSOP 14–Pin SOIC and TSSOP 20168302 Top View 20168303 Top View 20168304 Top View Ordering Information Package 5-Pin SC70 8-Pin MSOP 8-Pin SOIC 14-Pin SOIC 14-Pin TSSOP Part Number LMV841MG LMV841MGX LMV842MM LMV842MMX LMV842MA LMV842MAX LMV844MA LMV844MAX LMV844MT LMV844MTX Package Marking A97 AC4A LMV842MA LMV844MA LMV844MT Transport Media 1k Units Tape and Reel 3k Units Tape and Reel 1k Units Tape and Reel 3.5k Units Tape and reel 95 Units/Rail 2.5k Units Tape and Reel 55 Units/Rail 2.5k Units Tape and Reel 94 Units/Rail 2.5k Units Tape and Reel NSC Drawing MAA05A MUA08A M08A M14A MTC14 5 www.national.com LMV841 Single/ LMV842 Dual/ LMV844 Quad Typical Performance Characteristics VOS vs. VCM Over Temperature at 3.3V At TA = 25°C, RL = 10 kΩ, VS = 5V. Unless otherwise specified. VOS vs. VCM Over Temperature at 5.0V 20168310 20168311 VOS vs. VCM Over Temperature at ±5.0V VOS vs. Supply Voltage 20168312 20168313 VOS vs. Temperature DC Gain vs. VOUT 20168314 20168315 www.national.com 6 LMV841 Single/ LMV842 Dual/ LMV844 Quad Input Bias Current vs. VCM Input Bias Current vs. VCM 20168316 20168317 Input Bias Current vs. VCM Supply Current per Channel vs. Supply Voltage 20168318 20168319 Sinking Current vs. Supply Voltage Sourcing Current vs. Supply Voltage 20168320 20168321 7 www.national.com LMV841 Single/ LMV842 Dual/ LMV844 Quad Output Swing High vs. Supply Voltage RL = 2 kΩ Output Swing High vs. Supply Voltage RL = 10 kΩ 20168322 20168323 Output Swing Low vs. Supply Voltage RL = 2 kΩ Output Swing Low vs. Supply Voltage RL = 10 kΩ 20168324 20168325 Output Voltage Swing vs. Load Current Open Loop Frequency Response Over Temperature 20168326 20168327 www.national.com 8 LMV841 Single/ LMV842 Dual/ LMV844 Quad Open Loop Frequency Response Over Load Conditions Phase Margin vs. CL 20168328 20168329 PSRR vs. Frequency CMRR vs. Frequency 20168330 20168331 Channel Separation vs. Frequency Large Signal Step Response with Gain = 1 20168373 20168332 9 www.national.com LMV841 Single/ LMV842 Dual/ LMV844 Quad Large Signal Step Response with Gain = 10 Small Signal Step Response with Gain = 1 20168374 20168375 Small Signal Step Response with Gain = 10 Slew Rate vs Supply Voltage 20168376 20168337 Overshoot vs. CL Input Voltage Noise vs. Frequency 20168338 20168339 www.national.com 10 LMV841 Single/ LMV842 Dual/ LMV844 Quad THD+N vs. Frequency THD+N vs. VOUT 20168340 20168341 Closed Loop Output Impedance vs. Frequency 20168343 11 www.national.com LMV841 Single/ LMV842 Dual/ LMV844 Quad Application Information INTRODUCTION The LMV841/LMV842/LMV844 are operational amplifiers with near-precision specifications: low noise, low temperature drift, low offset, and rail-to-rail input and output. Possible application areas include instrumentation, medical, test equipment, audio, and automotive applications. Its low supply current of 1mA per amplifier, temperature range of −40°C to 125°C, 12V supply with CMOS input, and the small SC70 package for the LMV841 make the LMV841/ LMV842/LMV844 a unique op amp family and a perfect choice for portable electronics. INPUT PROTECTION The LMV841/LMV842/LMV844 have a set of anti-parallel diodes D1 and D2 between the input pins, as shown in Figure 1. These diodes are present to protect the input stage of the amplifier. At the same time, they limit the amount of differential input voltage that is allowed on the input pins. A differential signal larger than one diode voltage drop can damage the diodes. The differential signal between the inputs needs to be limited to ±300 mV or the input current needs to be limited to ±10 mA. Note that when the op amp is slewing, a differential input voltage exists that forward biases the protection diodes. This may result in current being drawn from the signal source. While this current is already limited by the internal resistors R1 and R2 (both 130Ω), a resistor of 1 kΩ can be placed in the feedback path, or a 500Ω resistor can be placed in series with the input signal for further limitation. region. Note that the CMRR and PSRR limits in the tables are large-signal numbers that express the maximum variation of the amplifier's input offset over the full common-mode voltage and supply voltage range, respectively. When the amplifier's common-mode input voltage is within the transition region, the small signal CMRR and PSRR may be slightly lower than the large signal limits. CAPACITIVE LOAD The LMV841/LMV842/LMV844 can be connected as non-inverting unity gain amplifiers. This configuration is the most sensitive to capacitive loading. The combination of a capacitive load placed on the output of an amplifier along with the amplifier’s output impedance creates a phase lag, which reduces the phase margin of the amplifier. If the phase margin is significantly reduced, the response will be underdamped which causes peaking in the transfer and, when there is too much peaking, the op amp might start oscillating. The LMV841/LMV842/LMV844 can directly drive capacitive loads up to 100 pF without any stability issues. In order to drive heavier capacitive loads, an isolation resistor, RISO, should be used, as shown in Figure 2. By using this isolation resistor, the capacitive load is isolated from the amplifier’s output, and hence, the pole caused by CL is no longer in the feedback loop. The larger the value of RISO, the more stable the output voltage will be. If values of RISO are sufficiently large, the feedback loop will be stable, independent of the value of CL. However, larger values of RISO result in reduced output swing and reduced output current drive. 20168350 FIGURE 2. Isolating Capacitive Load DECOUPLING AND LAYOUT For decoupling the supply lines it is suggested that 10 nF capacitors be placed as close as possible to the op amp. For single supply, place a capacitor between V+ and V−. For dual supplies, place one capacitor between V+ and the board ground, and the second capacitor between ground and V−. 20168351 FIGURE 1. Protection Diodes between the Input Pins INPUT STAGE The input stage of this amplifier consists of both a PMOS and an NMOS input pair to achieve a rail-to-rail input range. For input voltages close to the negative rail, only the PMOS pair is active. Close to the positive rail, only the NMOS pair is active. In a transition region that extends from approximately 2V below V+ to 1V below V+, both pairs are active, and one pair gradually takes over from the other. In this transition region, the input-referred offset voltage changes from the offset voltage associated with the PMOS pair to that of the NMOS pair. The input pairs are trimmed independently to guarantee an input offset voltage of less then 0.5 mV at room temperature over the complete rail-to-rail input range. This also significantly improves the CMRR of the amplifier in the transition www.national.com 12 OP AMP CIRCUIT NOISE The LMV841/LMV842/LMV844 have good noise specifications, and will frequently be used in low-noise applications. Therefore it is important to determine the noise of the total circuit. Besides the input referred noise of the op amp, the feedback resistors may have an important contribution to the total noise. For applications with a voltage input configuration it is, in general, beneficial to keep the resistor values low. In these configurations high resistor values mean high noise levels. However, using low resistor values will increase the power consumption of the application. This is not always acceptable for portable applications, so there is a trade-off between noise level and power consumption. Besides the noise contribution of the signal source, three types of noise need to be taken into account for calculating the noise performance of an op amp circuit: LMV841 Single/ LMV842 Dual/ LMV844 Quad • • • Input referred voltage noise of the op amp Input referred current noise of the op amp Noise sources of the resistors in the feedback network, configuring the op amp To calculate the noise voltage at the output of the op amp, the first step is to determine a total equivalent noise source. This requires the transformation of all noise sources to the same reference node. A convenient choice for this node is the input of the op amp circuit. The next step is to add all the noise sources. The final step is to multiply the total equivalent input voltage noise with the gain of the op amp configuration. The input referred voltage noise of the op amp is already located at the input, we can use the input referred voltage noise without further transferring. The input referred current noise needs to be converted to an input referred voltage noise. The current noise is negligibly small, as long as the equivalent resistance is not unrealistically large, so we can leave the current noise out for these examples. That leaves us with the noise sources of the resistors, being the thermal noise voltage. The influence of the resistors on the total noise can be seen in the following examples, one with high resistor values and one with low resistor values. Both examples describe an op amp configuration with a gain of 101 which will give the circuit a bandwidth of 44.5 kHz. The op amp noise is the same for both cases, i.e. an input referred noise voltage of 20 nV/ and a negligibly small input referred noise current. where: enr = thermal noise voltage of the equivalent resistor Req (V/ ) k = Boltzmann constant (1.38 x 10–23 J/K) T = absolute temperature (K) Req = resistance (Ω) The total equivalent input voltage noise is given by the equation: where: en in = total input equivalent voltage noise of the circuit env = input voltage noise of the op amp The final step is multiplying the total input voltage noise by the noise gain, which is in this case the gain of the op amp configuration: The equivalent resistance for the first example with a resistor RF of 10 MΩ and a resistor RG of 100 kΩ at 25°C (298 K) equals: Now the noise of the resistors can be calculated, yielding: 20168377 The total noise at the input of the op amp is: FIGURE 3. Noise Circuit To calculate the noise of the resistors in the feedback network, the equivalent input referred noise resistance is needed. For the example in Figure 3, this equivalent resistance Req can be calculated using the following equation: For the first example, this input noise will, multiplied with the noise gain, give a total output noise of: The voltage noise of the equivalent resistance can be calculated using the following equation: In the second example, with a resistor RF of 10 kΩ and a resistor RG of 100Ω at 25°C (298 K), the equivalent resistance equals: 13 www.national.com LMV841 Single/ LMV842 Dual/ LMV844 Quad The resistor noise for the second example is: a center frequency of approximately 10% from the frequency of the total filter: C = 33 nF R1 = 2 kΩ R2 = 6.2 kΩ R3 = 45 Ω The total noise at the input of the op amp is: This will give for filter A: and for filter B with C = 27 nF: For the second example the input noise will, multiplied with the noise gain, give an output noise of Bandwidth can be calculated by: In the first example the noise is dominated by the resistor noise due to the very high resistor values, in the second example the very low resistor values add only a negligible contribution to the noise and now the dominating factor is the op amp itself. When selecting the resistor values, it is important to choose values that don't add extra noise to the application. Choosing values above 100 kΩ may increase the noise too much. Low values will keep the noise within acceptable levels; choosing very low values however, will not make the noise even lower, but will increase the current of the circuit. ACTIVE FILTER The rail-to-rail input and output of the LMV841/LMV842/ LMV844 and the wide supply voltage range make these amplifiers ideal to use in numerous applications. One of the typical applications is an active filter as shown in Figure 4. This example is a band-pass filter, for which the pass band is widened. This is achieved by cascading two band-pass filters, with slightly different center frequencies. For filter A this will give: and for filter B: The response of the two filters and the combined filter is shown in Figure 5. 20168358 FIGURE 4. Active Filter The center frequency of the separate band-pass filters can be calculated by: FIGURE 5. Active Filter Curve 20168359 In this example a filter was designed with its pass band at 10 kHz. The two separate band-pass filters are designed to have The responses of filter A and filter B are shown as the thin lines in Figure 5; the response of the combined filter is shown as the thick line. Shifting the center frequencies of the separate filters farther apart, will result in a wider band; however, positioning the center frequencies too far apart will result in a less flat gain within the band. For wider bands more bandpass filters can be cascaded. 14 www.national.com LMV841 Single/ LMV842 Dual/ LMV844 Quad Tip: Use the WEBENCH internet tools at www.national.com for your filter application. HIGH-SIDE CURRENT SENSING The rail-to-rail input and the low VOS features make the LMV841/LMV842/LMV844 ideal op amps for high-side current sensing applications. To measure a current, a sense resistor is placed in series with the load, as shown in Figure 6. The current flowing through this sense resistor will result in a voltage drop, that is amplified by the op amp. Suppose it is necessary to measure a current between 0A and 2A using a sense resistor of 100 mΩ, and convert it to an output voltage of 0 to 5V. A current of 2A flowing through the load and the sense resistor will result in a voltage of 200 mV across the sense resistor. The op amp will amplify this 200 mV to fit the current range to the output voltage range. Use the formula: VOUT = RF/RG * VSENSE to calculate the gain needed. For a load current of 2A and an output voltage of 5V the gain would be VOUT/VSENSE = 25. If the feedback resistor, RF, is 100 kΩ, then the value for RG will be 4 kΩ. The tolerance of the resistors has to be low to obtain a good common-mode rejection. impedance is 10 MΩ (RS), the signal at the op amp input will be VIN = 1V - 10 nA * 10 MΩ = 1V - 0.1V = 0.9V For the CMOS input of the LMV841/LMV842/LMV844, which has an input bias current of only 0.3 pA, this would give VIN = 1V – 0.3 pA * 10 MΩ = 1V - 3 μV = 0.999997V The conclusion is that a standard op amp, with its high input bias current input, is not a good choice for use in impedance sensor applications. The LMV841/LMV842/LMV844, in contrast, are much more suitable due to the low input bias current. The error is negligibly small; therefore, the LMV841/LMV842/ LMV844 are a must for use with high impedance sensors. 20168352 FIGURE 7. High Impedance Sensor Interface THERMOCOUPLE AMPLIFIER The following is a typical example for a thermocouple amplifier application using an LMV841, LMV842, or LMV844. A thermocouple senses a temperature and converts it into a voltage. This signal is then amplified by the LMV841, LMV842, or LMV844. An ADC can then convert the amplified signal to a digital signal. For further processing the digital signal can be processed by a microprocessor, and can be used to display or log the temperature, or the temperature data can be used in a fabrication process. Characteristics of a Thermocouple A thermocouple is a junction of two different metals. These metals produce a small voltage that increases with temperature. The thermocouple used in this application is a K-type thermocouple. A K-type thermocouple is a junction between Nickel-Chromium and Nickel-Aluminum. This is one of the most commonly used thermocouples. There are several reasons for using the K-type thermocouple. These include temperature range, the linearity, the sensitivity, and the cost. A K-type thermocouple has a wide temperature range. The range of this thermocouple is from approximately −200°C to approximately 1200°C, as can be seen in Figure 8. This covers the generally used temperature ranges. Over the main part of the range the behavior is linear. This is important for converting the analog signal to a digital signal. The K-type thermocouple has good sensitivity when compared to many other types; the sensitivity is 41 uV/°C. Lower sensitivity requires more gain and makes the application more sensitive to noise. In addition, a K-type thermocouple is not expensive, many other thermocouples consist of more expensive materials or are more difficult to produce. 20168371 FIGURE 6. High-Side Current Sensing HIGH IMPEDANCE SENSOR INTERFACE With CMOS inputs, the LMV841/LMV842/LMV844 are particularly suited to be used as high impedance sensor interfaces. Many sensors have high source impedances that may range up to 10 MΩ. The input bias current of an amplifier will load the output of the sensor, and thus cause a voltage drop across the source resistance, as shown in Figure 7. When an op amp is selected with a relatively high input bias current, this error may be unacceptable. The low input current of the LMV841/LMV842/LMV844 significantly reduces such errors. The following examples show the difference between a standard op amp input and the CMOS input of the LMV841/LMV842/LMV844. The voltage at the input of the op amp can be calculated with VIN+ = VS - IB * RS For a standard op amp the input bias Ib can be 10 nA. When the sensor generates a signal of 1V (VS) and the sensors 15 www.national.com LMV841 Single/ LMV842 Dual/ LMV844 Quad If RG is 2 kΩ, then the value for RF can be calculated with this gain of 160. Since AV = RF/RG, RF can be calculated as follow: RF = AV * RG = 160 x 2 kΩ = 320 kΩ. To get a resolution of 0.5°C a step smaller then the minimum resolution is needed. This means that at least 1000 steps are necessary (500°C/0.5°C). A 10-bit ADC would be sufficient as this will give 1024 steps. A 10-bit ADC such as the two channel 10-bit ADC102S021 would be a good choice. Unwanted Thermocouple Effect At the point where the thermocouple wires are connected to the circuit, usually copper wires or traces, an unwanted thermocouple effect will occur. At this connection, this could be the connector on a PCB, the thermocouple wiring forms a second thermocouple with the connector. This second thermocouple disturbs the measurements from the intended thermocouple. Using an isothermal block as a reference will compensate for this additional thermocouple effect . An isothermal block is a good heat conductor. This means that the two thermocouple connections both have the same temperature. The temperature of the isothermal block can be measured, and thereby the temperature of the thermocouple connections. This is usually called the cold junction reference temperature. In the example, an LM35 is used to measure this temperature. This semiconductor temperature sensor can accurately measure temperatures from −55°C to 150°C. The ADC in this example also coverts the signal from the LM35 to a digital signal. Now the microprocessor can compensate the amplified thermocouple signal, for the unwanted thermocouple effect. 20168370 FIGURE 8. K-Type Thermocouple Response Thermocouple Example For this example suppose the range of interest is from 0°C to 500°C, and the resolution needed is 0.5°C. The power supply for both the LMV841, LMV842, or LMV844 and the ADC is 3.3V. The temperature range of 0°C to 500°C results in a voltage range from 0 mV to 20.6 mV produced by the thermocouple. This is shown in Figure 8 To obtain the best accuracy the full ADC range of 0 to 3.3V is used and the gain needed for this full range can be calculated as follows: AV = 3.3V/0.0206V = 160. 20168353 FIGURE 9. Thermocouple Amplifier www.national.com 16 LMV841 Single/ LMV842 Dual/ LMV844 Quad Physical Dimensions inches (millimeters) unless otherwise noted 5-Pin SC70 NS Package Number MAA05A 8-Pin MSOP NS Package Number MUA08A 17 www.national.com LMV841 Single/ LMV842 Dual/ LMV844 Quad 8-Pin SOIC NS Package Number M08A 14-Pin SOIC NS Package Number M14A www.national.com 18 LMV841 Single/ LMV842 Dual/ LMV844 Quad 14-Pin TSSOP NS Package Number MTC14 19 www.national.com LMV841 Single/ LMV842 Dual/ LMV844 Quad CMOS Input, RRIO, Wide Supply Range Operational Amplifiers Notes THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. NATIONAL MAKES NO REPRESENTATIONS OR WARRANTIES WITH RESPECT TO THE ACCURACY OR COMPLETENESS OF THE CONTENTS OF THIS PUBLICATION AND RESERVES THE RIGHT TO MAKE CHANGES TO SPECIFICATIONS AND PRODUCT DESCRIPTIONS AT ANY TIME WITHOUT NOTICE. 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