LMV881 23 MHz Low Power CMOS EMI Hardened Operational Amplifier with 1.8V Logic Shutdown General Description Features The LMV881 is a low power CMOS input operational amplifier that provides low input bias currents, a rail to rail output with high output drive capability and a wide temperature range of −40°C to +125°C. Additionally, the LMV881 is EMI hardened to minimize sensitivity to external interference. The LMV881 has a maximum input offset voltage of 1 mV with an input common-mode voltage range that includes ground. Over an operating supply range from 2.7V to 5.5V, the LMV881 provides a typical PSRR of 110dB and a CMRR of 110dB. This makes the LMV881 ideal for EMI sensitive applications as well as exceptional performance as a robust general purpose part. The unity gain stable LMV881 features 23 MHz of bandwidth while consuming only 1.65 mA of current. This device also maintains stability for capacitive loads as large as 200 pF. LMV881 offers a shutdown pin that can be used to disable the device and reduce the supply current to sub-nanoamp levels. During shutdown, the output is hard-clamped to V- to provide a known output state. The shutdown input thresholds are set for 1.8V logic, regardless of the amplifiers supply voltage. This eliminates the need for additional logic level shifting circuitry or translators. The LMV881 is offered in the space saving 6-Pin micro LLP package and provides excellent performance and economy in terms of power and space usage. Unless otherwise noted, typical values at TA = 25°C, V+ = 3.3V 2.7V to 5.5V ■ Supply voltage Supply current 1.65 mA ■ 200 pA ■ Shutdown current 1 mV max ■ Input offset voltage 0.1 pA ■ Input bias current GBW 23 MHz ■ 105 dB ■ EMIRR at 1.8 GHz 9 nV/√Hz ■ Input noise voltage at 1 kHz 12 V/µs ■ Slew rate Rail-to-Rail ■ Output voltage swing 70 mA ■ Output current drive ■ Operating ambient temperature range −40°C to 125°C 1.5 x 1.0 x 0.5 mm ■ Space saving micro-LLP package Applications ■ Weight scale systems ■ Filters/buffers ■ Medical diagnosis equipment Typical Application EMI Hardened Sensor Application 30151801 © 2012 Texas Instruments Incorporated 301518 SNOSC62A www.ti.com LMV881 Single 23 MHz Low Power CMOS, EMI Hardened Operational Amplifier with 1.8V Logic Shutdown February 3, 2012 LMV881 Connection Diagram 6-Pad LLP 30151872 Ordering Information Package Part Number LMV881LE LLP LMV881LEE LMV881LEX Package Marking* “XTT” (see below) Transport Media NSC Drawing 1K Units Tape and Reel 250 Units Tape and Reel LEH06A 3K Units Tape and Reel * Note: Because of the lack of space on the package surface, the package marking is a changing datecode and is not a fixed device ID. The package marking should not be relied upon for field identification. The marking will use the following convention, where: ”X” = Alphanumeric date code ”TT” =Alphanumeric die traceability code www.ti.com 2 If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for availability and specifications. ESD Tolerance (Note 2) Human Body Model Charge-Device Model Machine Model VIN Differential Supply Voltage (VS = V+ – V−) Voltage at Input/Output Pins 2 kV 1 kV 200V ± Supply Voltage 6V V+ +0.4V V− −0.4V 3.3V Electrical Characteristics Operating Ratings (Note 1) Temperature Range (Note 3) Supply Voltage (VS = V+ – V−) −40°C to +125°C 2.7V to 5.5V Package Thermal Resistance (θJA (Note 3)) 6-Pad µLLP 335°C/W (Note 4) 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. Symbol Parameter Conditions Min Typ Max (Note 6) (Note 5) (Note 6) Units VOS Input Offset Voltage (Note 9) ±273 ±1000 ±1260 μV TCVOS Input Offset Voltage Temperature Drift (Note 10) ±0.7 ±2.6 μV/°C IB Input Bias Current (Note 10) 0.1 30 500 IOS Input Offset Current CMRR Common-Mode Rejection Ratio (Note 9) 0.2V ≤ VCM ≤ PSRR Power Supply Rejection Ratio (Note 9) 2.7V ≤ V+ ≤ 5.5V, VOUT = 1V EMIRR EMI Rejection Ratio, IN+ and IN− (Note 8) VRF_PEAK = 100 mVP (−20 dBVP), f = 400 MHz 70 VRF_PEAK = 100 mVP (−20 dBVP), f = 900 MHz 80 VRF_PEAK = 100 mVP (−20 dBVP), f = 1800 MHz 105 VRF_PEAK = 100 mVP (−20 dBVP), f = 2400 MHz 110 1 V+ - 1.2V 77 76 93 79 78 95 CMVR Input Common-Mode Voltage Range CMRR ≥ 65 dB AVOL Large Signal Voltage Gain (Note 11) RL = 2 kΩ VOUT = 0.15V to 1.65V, VOUT = 3.15V to 1.65V 99 98 110 RL = 10 kΩ VOUT = 0.1V to 1.65V, VOUT = 3.2V to 1.65V 102 101 112 VOUT Output Voltage Swing High Output Voltage Swing Low −0.2 –0.1 pA dB dB dB 2.2 2.1 V dB RL = 2 kΩ to V+/2 12 14 18 RL = 10 kΩ to V+/2 3 4 5 RL = 2 kΩ to V+/2 8 12 16 RL = 10 kΩ to V+/2 2 4 5 3 pA mV from either rail www.ti.com LMV881 Storage Temperature Range −65°C to +150°C Junction Temperature (Note 3) +150°C Soldering Information For soldering specifications: See product folder at www.national.com and www.national.com/ms/MS/MS-SOLDERING.pdf Absolute Maximum Ratings (Note 1) LMV881 Symbol IOUT Parameter Output Short Circuit Current Conditions Min Typ Max (Note 6) (Note 5) (Note 6) Sourcing, VOUT = VCM, VIN = 100 mV 61 52 70 Sinking, VOUT = VCM, VIN = −100 mV 72 58 86 mA ROUT Shutdown Output Resistance VSDN = 0V 8.5 VOSD Shutdown Output Voltage VSDN = 0V, 134 230 ≥1 1.5 Ohms 200Ω pullup from OUT to V+ VEN Turn-on Voltage (Note 12) Turn-off Voltage (Note 12) IS Supply Current 0.3 1.65 In Shutdown, VSD < 0.676 V 200 AV = +1, VOUT = 1 VPP, 10% to 90% 12 mV V ≤ 0.7 Active, VSD > 0.972 V Units 2.17 2.75 mA pA SR Slew Rate (Note 7) GBW Gain Bandwidth Product 23 MHz Φm Phase Margin 60 deg en Input Referred Voltage Noise Density f = 1 kHz 9 f = 100 kHz Input Referred Current Noise Density CIN Common-Mode Input Capacitance 5 Differential-Mode Input Capacitance 15 Total Harmonic Distortion + Noise 5V Electrical Characteristics nV/ 5.3 in THD+N V/μs f = 1 kHz 0.015 f = 1 kHz, AV = 1, BW ≥ 500 kHz pA/ pF 0.02 % (Note 4) 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. Symbol Parameter Conditions Min Typ Max (Note 6) (Note 5) (Note 6) Units VOS Input Offset Voltage (Note 9) ±273 ±1000 ±1260 μV TCVOS Input Offset Voltage Temperature Drift (Note 10) ±0.7 ±2.6 μV/°C IB Input Bias Current (Note 10) 0.1 30 500 pA IOS Input Offset Current CMRR Common-Mode Rejection Ratio (Note 9) 0V ≤ VCM ≤ V+ –1.2V 79 78 94 PSRR Power Supply Rejection Ratio (Note 9) 2.7V ≤ V+ ≤ 5.5V, VOUT = 1V 79 78 95 EMIRR EMI Rejection Ratio, IN+ and IN− (Note 8) VRF_PEAK = 100 mVP (−20 dBVP), f = 400 MHz 70 VRF_PEAK = 100 mVP (−20 dBVP), f = 900 MHz 80 VRF_PEAK = 100 mVP (−20 dBVP), f = 1800 MHz 105 VRF_PEAK = 100 mVP (−20 dBVP), f = 2400 MHz 110 www.ti.com 1 4 pA dB dB dB Parameter Conditions Min Typ Max (Note 6) (Note 5) (Note 6) CMVR Input Common-Mode Voltage Range CMRR ≥ 65 dB −0.2 –0.1 AVOL Large Signal Voltage Gain (Note 11) RL = 2 kΩ VOUT = 0.15V to 2.5V, VOUT = 4.85V to 2.5V 102 101 110 RL = 10 kΩ VOUT = 0.1V to 2.5V, VOUT = 4.9V to 2.5V 102 101 113 VOUT Output Voltage Swing High, Output Voltage Swing Low, IOUT Output Short Circuit Current 3.9 3.8 13 15 19 RL = 10 kΩ to V+/2 3 4 5 RL = 2 kΩ to V+/2 10 14 18 RL = 10 kΩ to V+/2 3 4 5 90 86 110 Sinking, VOUT = VCM, VIN = −100 mV 90 86 110 Shutdown Output Resistance VSDN = 0V 7 VOSD Shutdown Output Voltage VSDN = 0V, 169 260 ≥1 1.5 Ohms 200Ω pullup from OUT to V+ Turn-on Voltage (Note 12) Turn-off Voltage (Note 12) IS Supply Current 0.3 1.9 In Shutdown, VSD< 0.676 V 200 AV = +1, VOUT = 2VPP, 10% to 90% 12 mV V ≤ 0.7 VSD> 0.972 V mV from either rail mA ROUT VEN V dB RL = 2 kΩ to V+/2 Sourcing, VOUT = VCM, VIN = 100 mV Units 2.45 2.95 mA pA SR Slew Rate (Note 7) GBW Gain Bandwidth Product 23 MHz Φm Phase Margin 61 deg en Input Referred Voltage Noise Density f = 1 kHz 9 f = 100 kHz 5.5 in Input Referred Current Noise Density CIN Common-Mode Input Capacitance 5 Differential-Input Capacitance 15 THD+N Total Harmonic Distortion + Noise f = 1 kHz 0.015 f = 1 kHz, AV= 1, BW ≥ 500 kHz 0.02 V/μs nV/ pA/ pF % 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 such that TJ = TA. No guarantee of parametric performance is indicated in the electrical tables under conditions of internal self-heating where TJ > TA. Note 5: Typical values represent the most likely parametric norm as determined at the time of characterization. Actual typical values may vary over time and will also depend on the application and configuration. The typical values are not tested and are not guaranteed on shipped production material. 5 www.ti.com LMV881 Symbol LMV881 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 ( VRF_PEAK/ΔVOS). Note 9: The typical value is calculated by applying the 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. Note 12: The shutdown logic levels are fixed to match 1.8V logic levels (referenced to V-), and do not change with the total power supply voltage. www.ti.com 6 At TA = 25°C, V+ = 5V, V− = 0V, VCM = V+/2, and RL =10 kΩ to V unless otherwise specified. Input Bias Current vs. VCM at 25°C Input Bias Current vs. VCM at 85°C 30151816 30151815 Input Bias Current vs. VCM at 125°C Supply Current vs. Supply Voltage 2.2 +125°C SUPPLY CURRENT (mA) +/2, 2.0 +85°C 1.8 1.6 +25°C 1.4 1.2 -40°C 1.0 2.5 30151817 3.0 3.5 4.0 4.5 5.0 SUPPLY VOLTAGE (V) 5.5 30151883 Output Swing High vs. Supply Voltage RL = 2 kΩ Output Swing High vs. Supply Voltage RL = 10 kΩ 30151826 30151827 7 www.ti.com LMV881 Typical Performance Characteristics LMV881 Output Swing Low vs. Supply Voltage RL = 2 kΩ Output Swing Low vs. Supply Voltage RL = 10 kΩ 30151828 30151829 Output Voltage Swing vs. Load Current at 3.3V Output Voltage Swing vs. Load Current at 5V 30151830 30151831 Open Loop Gain vs. Capacitive Load at 3.3V Open Loop Gain vs. Capacitive Load at 5V 30151832 www.ti.com 30151833 8 CMRR vs. Frequency 120 120 5V 3.3V 3.3V 60 100 -PSRR 5V Vs = 3.3V and 5V 110 CMRR (dB) PSRR (dB) 100 80 LMV881 PSRR vs. Frequency +PSRR 40 90 80 70 60 20 50 0 40 10 100 1k 10k 100k FREQUENCY (Hz) 1M 10M 100 1k 10k 100k 1M FREQUENCY (Hz) 10M 30151890 30151894 Large Signal Step Response at VS = 3.3V 0.8 0.8 RL=10KΩ 0.6 OUTPUT VOLTAGE (V) 0.4 0.2 0.0 -0.2 -0.4 -0.6 CL=20pF AV=+1 0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 0.0 RL=10KΩ 0.6 CL=20pF AV=+1 -0.8 0.2 0.4 0.6 TIME (μs) 0.8 1.0 0.0 0.2 0.4 0.6 TIME (μs) 0.8 1.0 30151887 30151888 Turn-on Time vs Supply Voltage Slew Rate vs. Supply Voltage 20 RL=10KΩ 19 CL=5pF 18 SLEW RATE (V/μs) OUTPUT VOLTAGE (V) Large Signal Step Response at VS = 5V 17 16 15 NEG 14 13 12 11 POS 10 2.5 30151898 3.0 3.5 4.0 4.5 SUPPLY VOLTAGE (V) 5.0 30151889 9 www.ti.com LMV881 Input Voltage Noise vs. Frequency THD+N vs. Frequency 1k 3.3V and 5V NOISE (nV/√) 100 10 1 100m 1 10 100 1k 10k 100k 1M FREQUENCY (Hz) 30151882 30151845 THD+N vs. Amplitude EMIRR IN+ vs. Power at 400 MHz 30151846 30151848 EMIRR IN+ vs. Power at 900 MHz EMIRR IN+ vs. Power at 1800 MHz 30151849 www.ti.com 30151850 10 LMV881 EMIRR IN+ vs. Power at 2400 MHz EMIRR IN+ vs. Frequency 30151852 30151851 11 www.ti.com LMV881 source impedances above 10KΩ. The effect of source resistance on the peaking is shown in Figure 1 below, where the source resistance is effectively the value of RF. Application Information INTRODUCTION The LMV881 is an operational amplifier with low offset, low noise and a high current rail-to-rail output. These specifications make the LMV881 great choices for medical and instrumentation applications such as diagnosis equipment and power line monitors. The low supply current and 1.8V shutdown logic is perfect for battery powered equipment. The small package make this device a perfect choice for portable electronics. Additionally, the EMI hardening makes the LMV881 a must for applications that are exposed to Radio Frequency (RF) signals such as the signals transmitted by mobile phones or wireless computer peripherals. The LMV881 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 op amp 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. 30151897 FIGURE 1. Effect of Source Resistance on Peaking SHUTDOWN MODE To conserve battery life in portable applications, the LMV881 can be disabled when the shutdown pin voltage is pulled low. The shutdown pin is designed for 1.8V logic levels, with thresholds independent of total supply voltage. In shutdown mode, the amplifier is disabled and the output is hard-clamped by an internal MOSFET to V− to provide a known output state. Care must be taken not to exceed the maximum output sinking current (specified in the electrical table) during shutdown. The shutdown pin input thresholds are referenced to the Vpin, and may need to be level shifted in split supply applications. Continuous voltages between 0.9V and 1.1V on the shutdown pins should be avoided to prevent excessive supply current draw due to internal shoot-through currents. The shutdown pin cannot be left unconnected. In case shut down operation is not needed, the shutdown pin should be connected to V+ for normal operation. Leaving the shutdown pin floating will result in an undefined operation modes, either shutdown or active, or even oscillating between the two modes. The 15pF differential mode capacitance mostly cancels due to the feedback bootstrapping effect at lower frequencies, but there still remains about 8pF of equivalent capacitance on each pin as seen by the circuit. The designer should be aware of this capacitance and make the appropriate adjustments to their circuit. OUTPUT CHARACTERISTICS During shutdown, the output is hard-clamped to V- with a resistance of just a few ohms. In normal operation, the output is rail-to-rail. When loading the output with a 10 kΩ resistor the maximum swing of the output is typically 3 mV from the positive and negative rail. The output of the LMV881 can typically drive peak currents up to 70 mA at 3.3V, and even up to 110 mA at 5V. However, power dissipation in small packages can become an issue at high drive currents. The LMV881 can be connected as a non-inverting unity gain amplifier (“buffer”). 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 LMV881 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 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 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. INPUT CHARACTERISTICS The input common mode voltage range of the LMV881 includes ground, and can even sense well below ground. The CMRR level does not degrade for input levels up to 1.2V below the positive 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 70 µV and the TCVOS is 0.7 μV/°C, placing the specifications close to that of precision op amps. INPUT CAPACITANCE The LMV881's input capacitance is larger than most typical op-amps due to the internal EMIRR circuitry. The differential mode capacitance (capacitance between the two input pins) is about 15pF. The common mode capacitance (“stray” input capacitance) is about 5pF on each input pin to the supplies. This extra input capacitance will cause peaking to occur with www.ti.com 12 LMV881 30151863 FIGURE 2. Isolating Capacitive Load 30151865 FIGURE 3. Offset voltage variation due to an interfering RF signal A resistor value of around 50Ω would be sufficient. As an example some values are given in the following table, for 5V and an open loop gain of 111 dB. CLOAD RISO 300 pF 62Ω 400 pF 55Ω 500 pF 50Ω 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: When increasing the closed-loop gain the capacitive load can be increased even further. With a closed loop gain of 2 and a 27Ω isolation resistor, the load can be 1 nF 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 LMV881 is a EMI hardened op amp which is 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. The received 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 3 depicts a typical output signal of a unity-gain connected op amp in the presence of an interfering RF signal. Clearly the output voltage varies in the rhythm of the on-off keying of the RF carrier. 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 EMIRRs 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 4. 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 voltage shift corresponds one-to-one to the measured output voltage shift. 13 www.ti.com LMV881 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 5. 30151867 FIGURE 4. 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. The application is show in Figure 6. This application needs two op amps. 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. The op amps are placed in a single supply configuration. The experiment is performed on two different op amps: a typical standard op amp and the LMV881 EMI hardened op amp. A cell phone is placed on a fixed position a couple of centimeters from the op amps in the sensor circuit. 30151862 FIGURE 5. Comparing EMI Robustness The difference between the two types of op amps is clearly visible. The typical standard op amp has an output shift (disturbed signal) larger than 1V as a result of the RF signal transmitted by the cell phone. The LMV881 EMI hardened op amp does not show any significant disturbances. This means that the RF signal will not disturb the signal entering the ADC when using the LMV881. 30151861 FIGURE 6. Pressure Sensor Application features of the LMV881 . Most of the EMI will be shunted out to the supply pins, so the supply pins should have bypassing and grounding suitable well up into the gigahertz range. A small 100pF RF grade capacitor directly from the supply pin to the nearest suitable RF ground is recommended. 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 LMV881 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 www.ti.com LOAD CELL SENSOR APPLICATION The LMV881 can be used for weight measuring system applications which use a load cell sensor. Examples of such systems are: bathroom weight scales, industrial weight scales and weight measurement devices on moving equipment such as forklift trucks. 14 tors of the sensor from affecting the gain of the following gain stage. The third buffer (A3) is used to create a reference voltage, to correct for the offset in the system. Given the differential output voltage VSENSE of the load cell the output signal of this op amp configuration, VOUT, equals: Load Cell Characteristics The load cell used in this example is a Wheatstone bridge. The value of the resistors in the bridge changes 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, the used supply voltage 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 load cell. A typical value for the span is 10 mV/V. The circuit configuration should be chosen such that loading of the sensor is prevented. Loading of the resistor bridge due to the circuit following the sensor, could result in incorrect output voltages of the sensor. To align the pressure range with the full range of an ADC the correct gain needs to be set. To calculate the correct gain, the power supply voltage and the span of the load cell are needed. For this example a power supply of 5V is used and the span of the sensor, in this case a 125 kg sensor, is 100 mV. With the configuration as shown in Figure 7, this signal is covering almost the full input range of the ADC. With no weight on the load cell, the output of the sensor and the op amp A4 will be close to 0V. With the full weight on the load cell, the output of the sensor is 100 mV, and will be amplified with the gain from the configuration. In the case of the configuration of Figure 7 the gain is R3/R1 = 5 kΩ/100Ω = 50. This will result in a maximum output of 100 mV x 50 = 5V, which covers the full range of the ADC. For further processing the digital signal can be processed by a microprocessor following the ADC, this can be used to display or log the weight on the load cell. To get a resolution of 0.5 kg, the LSB of the ADC should be smaller then 0.5 kg/125 kg = 1/1000. A 12-bit ADC would be sufficient as this gives 4096 steps. A 12-bit ADC such as the two channel 12-bit ADC122S021 can be used for this application. Load Cell Example Figure 7 shows a typical schematic for a load cell application. It uses a single supply and has an adjustment for both positive and negative offset of the load cell. An ADC converts the amplified signal to a digital signal. The op amps A1 and A2 are configured as buffers, and are connected at both the positive and the negative output of the load cell. This is to prevent the loading of the resistor bridge in the sensor by the resistors configuring the differential op amp circuit (op amp A4). The buffers also prevent the resis- 30151869 FIGURE 7. Load Cell Application tions. The boards can be ordered through the web or through your local representative. EVALUATION BOARD The LMV881 has a multi-function evaluation board available for ease of bench testing and prototyping. The board has a separate users guide that describes the various configura- Device LMV881 15 Package µLLP Evaluation Board Part Number LMV881EVAL www.ti.com LMV881 The following example describes a typical load cell sensor application that can be used as a starting point for many different types of sensors and applications. Applications in environments where EMI may appear would especially benefit from the EMIRR performance of the LMV881 . LMV881 Physical Dimensions inches (millimeters) unless otherwise noted 6-Pad 1.5 x 1.0 x 0.55mm, 0.5mm Pitch LLP NS Package Number LEH06A www.ti.com 16 LMV881 Notes 17 www.ti.com LMV881 Single 23 MHz Low Power CMOS, EMI Hardened Operational Amplifier with 1.8V Logic Shutdown Notes www.ti.com IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, modifications, enhancements, improvements, and other changes to its products and services at any time and to discontinue any product or service without notice. Customers should obtain the latest relevant information before placing orders and should verify that such information is current and complete. All products are sold subject to TI’s terms and conditions of sale supplied at the time of order acknowledgment. TI warrants performance of its hardware products to the specifications applicable at the time of sale in accordance with TI’s standard warranty. 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