LMV854

LMV851 Single/ LMV852 Dual/ LMV854 Quad 8 MHz Low Power CMOS, EMI Hardened Operational Amplifiers October 2007 LMV851/LMV852/LMV854 8 MHz Low Power CMOS, EMI Hardened Operational Amplifiers General Description National’s LMV851/LMV852/LMV854 are CMOS input, low power op amp ICs, providing a low input bias current, a wide temperature range of −40°C to +125°C and exceptional performance, making them robust general purpose parts. Additionally, the LMV851/LMV852/LMV854 are EMI hardened to minimize any interference so they are ideal for EMI sensitive applications. The unity gain stable LMV851/LMV852/LMV854 feature 8 MHz of bandwidth while consuming only 0.4 mA of current per channel. These parts also maintain stability for capacitive loads as large as 200 pF. The LMV851/LMV852/ LMV854 provide superior performance and economy in terms of power and space usage. This family of parts has a maximum input offset voltage of 1 mV, a rail-to-rail output stage and an input common-mode voltage range that includes ground. Over an operating supply range from 2.7V to 5.5V the LMV851/LMV852/LMV854 provide a CMRR of 92 dB, and a PSRR of 93 dB. The LMV851/LMV852/LMV854 are offered in the space saving 5-Pin SC70 package, the 8-Pin MSOP and the 14-Pin TSSOP package. Features Unless otherwise noted, typical values at TA = 25°C, VSUPPLY = 3.3V 2.7V to 5.5V ■ Supply voltage 0.4 mA ■ Supply current (per channel) 1 mV max ■ Input offset voltage 0.1 pA ■ Input bias current 8 MHz ■ GBW 87 dB ■ EMIRR at 1.8 GHz 11 nV/√Hz ■ Input noise voltage at 1 kHz 4.5 V/µs ■ Slew rate Rail-to-Rail ■ Output voltage swing 30 mA ■ Output current drive ■ Operating ambient temperature range −40°C to 125°C Applications ■ ■ ■ ■ ■ ■ Photodiode preamp Piezoelectric sensors Portable/battery-powered electronic equipment Filters/buffers PDAs/phone accessories Medical diagnosis equipment Typical Application Sensor Amplifiers Close to RF Sources 20202101 © 2007 National Semiconductor Corporation 202021 www.national.com LMV851 Single/ LMV852 Dual/ LMV854 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 Charge-Device Model Machine Model VINDifferential Supply Voltage (V+ – V−) Voltage at Input/Output Pins 2 kV 1 kV 200V ± Supply Voltage 6V V+ +0.4V V− −0.4V (Note 4) Storage Temperature Range Junction Temperature (Note 3) Soldering Information Infrared or Convection (20 sec) −65°C to +150°C +150°C +260°C Operating Ratings Temperature Range (Note 3) Supply Voltage (V+ – V−) (Note 1) −40°C to +125°C 2.7V to 5.5V 313 °C/W 217 °C/W 135 °C/W Package Thermal Resistance (θJA (Note 3)) 5-Pin SC70 8-Pin MSOP 14-Pin TSSOP 3.3V Electrical Characteristics Symbol VOS TCVOS IB IOS CMRR PSRR EMIRR Parameter Input Offset Voltage Input Offset Voltage Drift (Note 10) Input Bias Current (Note 10) Input Offset Current Common Mode Rejection Ratio Power Supply Rejection Ratio EMI Rejection Ratio, IN+ and IN− (Note 8) Unless otherwise specified, all limits are guaranteed for TA = 25°C, V+ = 3.3V, V− = 0V, VCM = V+/2, and RL = 10 kΩ to V+/2. Boldface limits apply at the temperature extremes. Conditions Min (Note 6) Typ (Note 5) ±0.26 (Note 9) ±0.4 (Note 9) 0.1 1 −0.2V < VCM < V+ - 1.2V 76 75 75 74 92 (Note 9) 93 (Note 9) 64 78 87 90 −0.2 100 97 100 97 114 2.1 V dB Max (Note 6) ±1 ±1.2 ±2 10 500 Units mV μV/°C pA pA dB dB 2.7V ≤ V+ ≤ 5.5V, VOUT = 1V VRFpeak = 100 mVP (−20 dBVP), f = 400 MHz VRFpeak = 100 mVP (−20 dBVP), f = 900 MHz VRFpeak = 100 mVP (−20 dBVP), f = 1800 MHz VRFpeak = 100 mVP (−20 dBVP), f = 2400 MHz CMVR AVOL Input Common-Mode Voltage Range CMRR ≥ 76 dB Large Signal Voltage Gain (Note 11) RL = 2 kΩ, VOUT = 0.15V to 1.65V, VOUT = 3.15V to 1.65V RL = 10 kΩ, VOUT = 0.1V to 1.65V, VOUT = 3.2V to 1.65V 115 dB VO Output Swing High, (measured from V+) RL = 2 kΩ to V+/2 RL = 10 kΩ to V+/2 31 7 26 6 35 43 10 12 32 43 11 14 mV Output Swing Low, (measured from V−) RL = 2 kΩ to V+/2 RL = 10 kΩ to V+/2 mV www.national.com 2 LMV851 Single/ LMV852 Dual/ LMV854 Quad Symbol IO Parameter Output Short Circuit Current Conditions Sourcing, VOUT = VCM, VIN = 100 mV Sinking, VOUT = VCM, VIN = −100 mV Min (Note 6) 25 20 28 20 Typ (Note 5) 28 31 0.42 0.79 1.54 4.5 8 62 Max (Note 6) Units mA IS Supply Current LMV851 LMV852 LMV854 0.50 0.58 0.90 1.06 1.67 1.99 V/μs MHz deg nV/ pA/ Ω pF % mA SR GBW Φm en in ROUT CIN THD+N Slew Rate (Note 7) Gain Bandwidth Product Phase Margin Input-Referred Voltage Noise Input-Referred Current Noise Closed Loop Output Impedance Common-Mode Input Capacitance Differential-Mode Input Capacitance Total Harmonic Distortion + Noise AV = +1, VOUT = 1 VPP, 10% to 90% f = 1 kHz f = 10 kHz f = 1 kHz f = 6 MHz 11 10 0.005 400 11 6 f = 1 kHz, AV = 1, BW = >500 kHz (Note 4) 0.006 5V Electrical Characteristics Symbol VOS TCVOS IB IOS CMRR PSRR EMIRR Parameter Input Offset Voltage Input Offset Voltage Drift (Note 10) Input Bias Current (Note 10) Input Offset Current Common Mode Rejection Ratio Power Supply Rejection Ratio EMI Rejection Ratio, IN+ and IN− (Note 8) Unless otherwise specified, all limits are guaranteed for TA = 25°C, V+ = 5V, V− = 0V, VCM = V+/2, and RL = 10 kΩ to V+/2. Boldface limits apply at the temperature extremes. Conditions Min (Note 6) Typ (Note 5) ±0.26 (Note 9) ±0.4 (Note 9) 0.1 1 −0.2V ≤ VCM ≤ V+ −1.2V 2.7V ≤ V+ ≤ 5.5V, VOUT = 1V VRFpeak = 100 mVP (−20 dBVP), f = 400 MHz VRFpeak = 100 mVP (−20 dBVP), f = 900 MHz VRFpeak = 100 mVP (−20 dBVP), f = 1800 MHz VRFpeak = 100 mVP (−20 dBVP), f = 2400 MHz CMVR Input Common-Mode Voltage Range CMRR ≥ 77 dB −0.2 77 76 75 74 94 (Note 9) 93 (Note 9) 64 76 84 89 3.8 V dB Max (Note 6) ±1 ±1.2 ±2 10 500 Units mV μV/°C pA pA dB dB 3 www.national.com LMV851 Single/ LMV852 Dual/ LMV854 Quad Symbol AVOL Parameter Large Signal Voltage Gain (Note 11) Conditions RL = 2 kΩ, VOUT = 0.15V to 2.5V, VOUT = 4.85V to 2.5V RL = 10 kΩ, VOUT = 0.1V to 2.5V, VOUT = 4.9V to 2.5V Min (Note 6) 105 102 105 102 Typ (Note 5) 118 Max (Note 6) Units 120 dB VO Output Swing High, (measured from V+) RL = 2 kΩ to V+/2 RL = 10 kΩ to V+/2 34 7 31 7 60 48 58 44 65 62 0.43 0.82 1.59 4.5 8 64 39 47 11 13 38 50 12 15 mV Output Swing Low, (measured from V−) RL = 2 kΩ to V+/2 RL = 10 kΩ to V+/2 mV IO Output Short Circuit Current Sourcing, VOUT = VCM, VIN = 100 mV Sinking, VOUT = VCM, VIN = −100 mV mA IS Supply Current LMV851 LMV852 LMV854 0.52 0.60 0.93 1.09 1.73 2.05 V/μs MHz deg nV/ pA/ Ω pF % mA SR GBW Φm en in ROUT CIN THD+N Slew Rate (Note 7) Gain Bandwidth Product Phase Margin Input-Referred Voltage Noise Input-Referred Current Noise Closed Loop Output Impedance Common-Mode Input Capacitance Differential-Mode Input Capacitance Total Harmonic Distortion + Noise AV = +1, VOUT = 2 VPP, 10% to 90% f = 1 kHz f = 10 kHz f = 1 kHz f = 6 MHz 11 10 0.005 400 11 6 f = 1 kHz, AV = 1, BW = >500 kHz 0.003 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: Number specified is the slower of positive and negative slew rates. Note 8: The EMI Rejection Ratio is defined as EMIRR = 20log ( VRFpeak/ΔVOS). Note 9: The typical value is calculated by applying absolute value transform to the distribution, then taking the statistical average of the resulting distribution Note 10: This parameter is guaranteed by design and/or characterization and is not tested in production. Note 11: The specified limits represent the lower of the measured values for each output range condition. www.national.com 4 LMV851 Single/ LMV852 Dual/ LMV854 Quad Connection Diagrams 5-Pin SC70 8-Pin MSOP 14-Pin TSSOP 20202102 Top View 20202103 Top View 20202104 Top View Ordering Information Package 5-Pin SC70 8-Pin MSOP 14-Pin TSSOP Part Number LMV851MG LMV851MGX LMV852MM LMV852MMX LMV854MT LMV854MTX Package Marking A98 AB5A LMV854MT Transport Media 1k Units Tape and Reel 3k Units Tape and Reel 1k Units Tape and Reel 3.5k Units Tape and Reel 94 Units/Rail 2.5k Units Tape and Reel NSC Drawing MAA05A MUA08A MTC14 5 www.national.com LMV851 Single/ LMV852 Dual/ LMV854 Quad Typical Performance Characteristics VOS vs. VCM at 3.3V At TA = 25°C, RL = 10 kΩ, VS = 3.3V, unless otherwise specified. VOS vs. VCM at 5.0V 20202110 20202111 VOS vs. Supply Voltage VOS vs. Temperature 20202112 20202113 VOS vs. VOUT Input Bias Current vs. VCM at 25°C 20202114 20202115 www.national.com 6 LMV851 Single/ LMV852 Dual/ LMV854 Quad Input Bias Current vs. VCM at 85°C Input Bias Current vs. VCM at 125°C 20202116 20202117 Supply Current vs. Supply Voltage Single LMV851 Supply Current vs. Supply Voltage Dual LMV852 20202118 20202119 Supply Current vs. Supply Voltage Quad LMV854 Supply Current vs. Temperature Single LMV851 20202120 20202121 7 www.national.com LMV851 Single/ LMV852 Dual/ LMV854 Quad Supply Current vs. Temperature Dual LMV852 Supply Current vs. Temperature Quad LMV854 20202122 20202123 Sinking Current vs. Supply Voltage Sourcing Current vs. Supply Voltage 20202124 20202125 Output Swing High vs. Supply Voltage RL = 2 kΩ Output Swing High vs. Supply Voltage RL = 10 kΩ 20202126 20202127 www.national.com 8 LMV851 Single/ LMV852 Dual/ LMV854 Quad Output Swing Low vs. Supply Voltage RL = 2 kΩ Output Swing Low vs. Supply Voltage RL = 10 kΩ 20202128 20202129 Output Voltage Swing vs. Load Current at 3.3V Output Voltage Swing vs. Load Current at 5.0V 20202155 20202156 Open Loop Frequency Response vs. Temperature Open Loop Frequency Response vs. Load Conditions 20202131 20202132 9 www.national.com LMV851 Single/ LMV852 Dual/ LMV854 Quad Phase Margin vs. Capacitive Load PSRR vs. Frequency 20202133 20202135 CMRR vs. Frequency Channel Separation vs. Frequency 20202136 20202137 Large Signal Step Response with Gain = 1 Large Signal Step Response with Gain = 10 20202138 20202139 www.national.com 10 LMV851 Single/ LMV852 Dual/ LMV854 Quad Small Signal Step Response with Gain = 1 Small Signal Step Response with Gain = 10 20202140 20202141 Slew Rate vs. Supply Voltage Overshoot vs. Capacitive Load 20202142 20202143 Input Voltage Noise vs. Frequency THD+N vs. Frequency 20202144 20202145 11 www.national.com LMV851 Single/ LMV852 Dual/ LMV854 Quad THD+N vs. Amplitude ROUT vs. Frequency 20202146 20202148 EMIRR IN+ vs. Power at 400 MHz EMIRR IN+ vs. Power at 900 MHz 20202149 20202150 EMIRR IN+ vs. Power at 1800 MHz EMIRR IN+ vs. Power at 2400 MHz 20202151 20202152 www.national.com 12 LMV851 Single/ LMV852 Dual/ LMV854 Quad EMIRR IN+ vs. Frequency at 3.3V EMIRR IN+ vs. Frequency at 5.0V 20202153 20202154 13 www.national.com LMV851 Single/ LMV852 Dual/ LMV854 Quad Application Information INTRODUCTION The LMV851/LMV852/LMV854 are operational amplifiers with very good specifications, such as low offset, low noise and a rail-to-rail output. These specifications make the LMV851/LMV852/LMV854 great choices to use in areas such as medical and instrumentation. The low supply current is perfect for battery powered equipment. The small packages, SC-70 package for the LMV851, the MSOP package for the dual LMV852 and the TSSOP package for the quad LMV854, make any of these parts a perfect choice for portable electronics. Additionally, the EMI hardening makes the LMV851/ LMV852 or LMV854 a must for almost all op amp applications. Most applications are exposed to Radio Frequency (RF) signals such as the signals transmitted by mobile phones or wireless computer peripherals. The LMV851/LMV852/ LMV854 will effectively reduce disturbances caused by RF signals to a level that will be hardly noticeable. This again reduces the need for additional filtering and shielding. Using this EMI resistant series of op amps will thus reduce the number of components and space needed for applications that are affected by EMI, and will help applications, not yet identified as possible EMI sensitive, to be more robust for EMI. INPUT CHARACTERISTICS The input common mode voltage range of the LMV851/ LMV852/LMV854 includes ground, and can even sense well below ground. The CMRR level does not degrade for input levels up to 1.2V below the supply voltage. For a supply voltage of 5V, the maximum voltage that should be applied to the input for best CMRR performance is thus 3.8V. When not configured as unity gain, this input limitation will usually not degrade the effective signal range. The output is rail-to-rail and therefore will introduce no limitations to the signal range. The typical offset is only 0.26 mV, and the TCVOS is 0.4 μV/°C, specifications close to precision op amps. CMRR MEASUREMENT The CMRR measurement results may need some clarification. This is because different setups are used to measure the AC CMRR and the DC CMRR. The DC CMRR is derived from ΔVOS versus ΔVCM. This value is stated in the tables, and is tested during production testing. The AC CMRR is measured with the test circuit shown in Figure 1. 20202164 FIGURE 1. AC CMRR Measurement Setup The configuration is largely the usually applied balanced configuration. With potentiometer P1, the balance can be tuned to compensate for the DC offset in the DUT. The main difference is the addition of the buffer. This buffer prevents the open-loop output impedance of the DUT from affecting the balance of the feedback network. Now the closed-loop output impedance of the buffer is a part of the balance. But as the closed-loop output impedance is much lower, and by careful selection of the buffer also has a larger bandwidth, the total effect is that the CMRR of the DUT can be measured much more accurately. The differences are apparent in the larger measured bandwidth of the AC CMRR. One artifact from this test circuit is that the low frequency CMRR results appear higher than expected. This is because in the AC CMRR test circuit the potentiometer is used to compensate for the DC mismatches. So, mainly AC mismatch is all that remains. Therefore, the obtained DC CMRR from this AC CMRR test circuit tends to be higher than the actual DC CMRR based on DC measurements. The CMRR curve in Figure 2 shows a combination of the AC CMRR and the DC CMRR. 20202136 FIGURE 2. CMRR Curve www.national.com 14 LMV851 Single/ LMV852 Dual/ LMV854 Quad OUTPUT CHARACTERISTICS As already mentioned the output is rail to rail. When loading the output with a 10 kΩ resistor the maximum swing of the output is typically 7 mV from the positive and negative rail The LMV851/LMV852/LMV854 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 at 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 under damped which causes peaking in the transfer and, when there is too much peaking, the op amp might start oscillating. The LMV851/LMV852/LMV854 can directly drive capacitive loads up to 200 pF without any stability issues. In order to drive heavier capacitive loads, an isolation resistor, RISO, should be used, as shown in Figure 3. 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 amplifier will be. If the value of RISO is 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. Clearly the output voltage varies in the rhythm of the on-off keying of the RF carrier. 20202165 FIGURE 4. Offset Voltage Variation Due to an Interfering RF Signal EMIRR Definition To identify EMI hardened op amps, a parameter is needed that quantitatively describes the EMI performance of op amps. A quantitative measure enables the comparison and the ranking of op amps on their EMI robustness. Therefore the EMI Rejection Ratio (EMIRR) is introduced. This parameter describes the resulting input-referred offset voltage shift of an op amp as a result of an applied RF carrier (interference) with a certain frequency and level. The definition of EMIRR is given by: 20202163 FIGURE 3. Isolating Capacitive Load EMIRR With the increase of RF transmitting devices in the world, the electromagnetic interference (EMI) between those devices and other equipment becomes a bigger challenge. The LMV851/LMV852/LMV854 are EMI hardened op amps which are specifically designed to overcome electromagnetic interference. Along with EMI hardened op amps, the EMIRR parameter is introduced to unambiguously specify the EMI performance of an op amp. This section presents an overview of EMIRR. A detailed description on this specification for EMI hardened op amps can be found in Application Note AN-1698. The dimensions of an op amp IC are relatively small compared to the wavelength of the disturbing RF signals. As a result the op amp itself will hardly receive any disturbances. The RF signals interfering with the op amp are dominantly received by the PCB and wiring connected to the op amp. As a result the RF signals on the pins of the op amp can be represented by voltages and currents. This representation significantly simplifies the unambiguous measurement and specification of the EMI performance of an op amp. RF signals interfere with op amps via the non-linearity of the op amp circuitry. This non-linearity results in the detection of the so called out-of-band signals. The obtained effect is that the amplitude modulation of the out-of-band signal is downconverted into the base band. This base band can easily overlap with the band of the op amp circuit. As an example Figure 4 depicts a typical output signal of a unity-gain connected op amp in the presence of an interfering RF signal. In which VRF_PEAK is the amplitude of the applied un-modulated RF signal (V) and ΔVOS is the resulting input-referred offset voltage shift (V). The offset voltage depends quadratically on the applied RF level, and therefore, the RF level at which the EMIRR is determined should be specified. The standard level for the RF signal is 100 mVP. Application Note AN-1698 addresses the conversion of an EMIRR measured for an other signal level than 100 mVP. The interpretation of the EMIRR parameter is straightforward. When two op amps have an EMIRR which differ by 20 dB, the resulting error signals when used in identical configurations, differs by 20 dB as well. So, the higher the EMIRR, the more robust the op amp. Coupling an RF Signal to the IN+ Pin Each of the op amp pins can be tested separately on EMIRR. In this section the measurements on the IN+ pin (which, based on symmetry considerations, also apply to the IN− pin) are discussed. In Application Note AN-1698 the other pins of the op amp are treated as well. For testing the IN+ pin the op amp is connected in the unity gain configuration. Applying the RF signal is straightforward as it can be connected directly to the IN+ pin. As a result the RF signal path has a minimum of components that might affect the RF signal level at the pin. The circuit diagram is shown in Figure 5. The PCB trace from RFIN to the IN+ pin should be a 50Ω stripline in order to match the RF impedance of the cabling and the RF generator. On the PCB a 50Ω termination is used. This 50Ω resistor is also used to set the bias level of the IN+ pin to ground level. For determining the EMIRR, two measurements are needed: one is measuring the DC output level when the RF signal is off; and the other is measuring the DC output level when the RF signal is switched on. The difference of the two DC levels is the output voltage shift as a result of the RF signal. As the op amp is in the unity gain configuration, the input referred offset 15 www.national.com LMV851 Single/ LMV852 Dual/ LMV854 Quad voltage shift corresponds one-to-one to the measured output voltage shift. DECOUPLING AND LAYOUT Care must be given when creating a board layout for the op amp. 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 a second capacitor between ground and V−. Even with the LMV851/LMV852/LMV854 inherent hardening against EMI, it is still recommended to keep the input traces short and as far as possible from RF sources. Then the RF signals entering the chip are as low as possible, and the remaining EMI can be, almost, completely eliminated in the chip by the EMI reducing features of the LMV851/ LMV852/LMV854. PRESSURE SENSOR APPLICATION The LMV851/LMV852/LMV854 can be used for pressure sensor applications. Because of their low power the LMV851/ LMV852/LMV854 are ideal for portable applications, such as blood pressure measurement devices, or portable barometers. This example describes a universal pressure sensor that can be used as a starting point for different types of sensors and applications. Pressure Sensor Characteristics The pressure sensor used in this example functions as a Wheatstone bridge. The value of the resistors in the bridge change when pressure is applied to the sensor. This change of the resistor values will result in a differential output voltage, depending on the sensitivity of the sensor and the applied pressure. The difference between the output at full scale pressure and the output at zero pressure is defined as the span of the pressure sensor. A typical value for the span is 100 mV. A typical value for the resistors in the bridge is 5 kΩ. Loading of the resistor bridge could result in incorrect output voltages of the sensor. Therefore the selection of the circuit configuration, which connects to the sensor, should take into account a minimum loading of the sensor. Pressure Sensor Example The configuration shown in Figure 7 is simple, and is very useful for the read out of pressure sensors. With two op amps in this application, the dual LMV852 fits very well. The op amp configured as a buffer and connected at the negative output of the pressure sensor prevents the loading of the bridge by resistor R2. The buffer also prevents the resistors of the sensor from affecting the gain of the following gain stage. Given the differential output voltage VS of the pressure sensor, the output signal of this op amp configuration, VOUT, equals: 20202167 FIGURE 5. Circuit for Coupling the RF Signal to IN+ Cell Phone Call The effect of electromagnetic interference is demonstrated in a setup where a cell phone interferes with a pressure sensor application (Figure 7). This application needs two op amps and therefore a dual op amp is used. The experiment is performed on two different dual op amps: a typical standard op amp and the LMV852, EMI hardened dual op amp. The op amps are placed in a single supply configuration. The cell phone is placed on a fixed position a couple of centimeters from the op amps. When the cell phone is called, the PCB and wiring connected to the op amps receive the RF signal. Subsequently, the op amps detect the RF voltages and currents that end up at their pins. The resulting effect on the output of the second op amp is shown in Figure 6. 20202168 FIGURE 6. Comparing EMI Robustness The difference between the two types of dual op amps is clearly visible. The typical standard dual op amp has an output shift (disturbed signal) larger than 1V as a result of the RF signal transmitted by the cell phone. The LMV852, EMI hardened op amp does not show any significant disturbances. To align the pressure range with the full range of an ADC, the power supply voltage and the span of the pressure sensor are needed. For this example a power supply of 5V is used and the span of the sensor is 100 mV. When a 100Ω resistor is used for R2, and a 2.4 kΩ resistor is used for R1, the maximum voltage at the output is 4.95V and the minimum voltage is 0.05V. This signal is covering almost the full input range of the ADC. Further processing can take place in the microprocessor following the ADC. www.national.com 16 LMV851 Single/ LMV852 Dual/ LMV854 Quad 20202160 FIGURE 7. Pressure Sensor Application THERMOCOUPLE AMPLIFIER The following circuit is a typical example for a thermocouple amplifier application using an LMV851/LMV852, or LMV854. A thermocouple converts a temperature into a voltage. This signal is then amplified by the LMV851/LMV852, or LMV854. An ADC can convert the amplified signal to a digital signal. For further processing the digital signal can be processed by a microprocessor and used to display or log the temperature. The temperature data can for instance 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 temperature range the output voltage depends linearly on the temperature. This is important for easily converting the measured signal levels to a temperature reading. The K-type thermocouple has good sensitivity when compared to many other types; the sensitivity is about 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. 20202162 FIGURE 8. K-Type Thermocouple Response Thermocouple Example For this example, suppose the range of interest is 0°C to 500°C, and the resolution needed is 0.5°C. The power supply for both the LMV851/LMV852, or LMV854 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 indicated in Figure 8 by the dotted lines. To obtain the highest resolution, the full ADC range of 0 to 3.3V is used. The gain needed for the full range can be calculated as follows: AV = 3.3V / 0.0206V = 160 If RG is 2 kΩ, then the value for RF can be calculated for a gain of 160. Since AV = RF / RG, RF can be calculated as follows: RF = AV x RG = 160 x 2 kΩ = 320 kΩ To get a resolution of 0.5°C, the LSB of the ADC should be smaller then 0.5°C / 500°C = 1/1000. A 10-bit ADC would be sufficient as this gives 1024 steps. A 10-bit ADC such as the two channel 10-bit ADC102S021 can be used. 17 www.national.com LMV851 Single/ LMV852 Dual/ LMV854 Quad 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 enables correction for this unwanted 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 two channel ADC in this example also converts the signal from the LM35 to a digital signal. Now the microprocessor can compensate the amplified thermocouple signal, for the unwanted thermocouple effect. 20202161 FIGURE 9. Thermocouple Read Out Circuit www.national.com 18 LMV851 Single/ LMV852 Dual/ LMV854 Quad Physical Dimensions inches (millimeters) unless otherwise noted 5-Pin SC70 NS Package Number MAA05A 8-Pin MSOP NS Package Number MUA08A 19 www.national.com LMV851 Single/ LMV852 Dual/ LMV854 Quad 14-Pin TSSOP NS Package Number MTC14 www.national.com 20 LMV851 Single/ LMV852 Dual/ LMV854 Quad Notes 21 www.national.com LMV851 Single/ LMV852 Dual/ LMV854 Quad 8 MHz Low Power CMOS, EMI Hardened 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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