LMP2231 Single Micropower, 1.8V, Precision Operational Amplifier with CMOS Inputs General Description Features The LMP2231 is a single micropower precision amplifier designed for battery powered applications. The 1.8V to 5.0V guaranteed supply voltage range and quiescent power consumption of only 18 μW extend the battery life in portable battery operated systems. The LMP2231 is part of the LMP® precision amplifier family. The high impedance CMOS input makes it ideal for instrumentation and other sensor interface applications. The LMP2231 has a maximum offset of 150 µV and maximum offset voltage drift of only 0.4 µV/°C along with low bias current of only ±20 fA. These precise specifications make the LMP2231 a great choice for maintaining system accuracy and long term stability. The LMP2231 has a rail-to-rail output that swings 15 mV from the supply voltage, which increases system dynamic range. The common mode input voltage range extends 200 mV below the negative supply, thus the LMP2231 is ideal for use in single supply applications with ground sensing. The LMP2231 is offered in 5-Pin SOT23 and 8-pin SOIC packages. The dual and quad versions of this product are also available. The dual, LMP2232 is offered in 8-pin SOIC and MSOP. The quad, LMP2234 is offered in 14-pin SOIC and TSSOP. (For VS = 5V, Typical unless otherwise noted) 10 µA ■ Supply current 1.6V to 5.5V ■ Operating voltage range ±0.4 µV/°C (max) ■ Low TCVOS ±150 µV (max) ■ VOS 20 fA ■ Input bias current 120 dB ■ PSRR 97 dB ■ CMRR 120 dB ■ Open loop gain 130 kHz ■ Gain bandwidth product 58 V/ms ■ Slew rate 60 nV/√Hz ■ Input voltage noise, f = 1 kHz –40°C to 125°C ■ Temperature range Applications ■ ■ ■ ■ ■ Precision instrumentation amplifiers Battery powered medical instrumentation High Impedance Sensors Strain gauge bridge amplifier Thermocouple amplifiers Typical Application 30033874 Strain Gauge Bridge Amplifier LMP® is a registered trademark of National Semiconductor Corporation. © 2008 National Semiconductor Corporation 300338 www.national.com LMP2231 Single Micropower, 1.8V, Precision, Operational Amplifier with CMOS Inputs February 7, 2008 LMP2231 Single Junction Temperature (Note 3) Mounting Temperature Infrared or Convection (20 sec.) 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 Differential Input Voltage Supply Voltage (VS = V+ - V–) Voltage on Input/Output Pins Storage Temperature Range Operating Ratings 150°C +235°C (Note 1) Operating Temperature Range (Note 3) Supply Voltage (VS = V+ - V−) 2000V 100V ±300 mV 6V V+ + 0.3V, V– – 0.3V −65°C to 150°C −40°C to 125°C 1.6V to 5.5V Package Thermal Resistance (θJA) (Note 3) 5-Pin SOT23 8-Pin SOIC 160.6 °C/W 116.2 °C/W 5V DC Electrical Characteristics (Note 4) Unless otherwise specified, all limits guaranteed for TA = 25°C, V+ = 5V, V− = 0V, VCM = VO = V+/2, and RL > 1 MΩ. Boldface limits apply at the temperature extremes. Symbol Parameter VOS Input Offset Voltage TCVOS Input Offset Voltage Drift IBIAS Input Bias Current IOS Input Offset Current CMRR Common Mode Rejection Ratio PSRR CMVR Conditions Min (Note 6) Typ (Note 5) Max (Note 6) Units ±10 ±150 ±230 μV LMP2231A ±0.3 ±0.4 LMP2231B ±0.3 ±2.5 0.02 ±1 ±50 5 AVOL 0V ≤ VCM ≤ 4V 97 Power Supply Rejection Ratio 1.6V ≤ V+ ≤ 5.5V V− = 0V, VCM = 0V 83 83 120 Common Mode Voltage Range CMRR ≥ 80 dB −0.2 −0.2 VO = 0.3V to 4.7V 110 108 Large Signal Voltage Gain RL = 10 kΩ to VO IO IS Output Swing High V+/2 dB dB 4.2 4.2 120 RL = 10 kΩ to V+/2 VIN(diff) = 100 mV 17 Output Swing Low RL = 10 kΩ to V+/2 VIN(diff) = −100 mV 17 Output Current (Note 7) Sourcing, VO to V– VIN(diff) = 100 mV 27 19 30 Sinking, VO to V+ VIN(diff) = −100 mV 17 12 22 Supply Current pA fA 81 80 CMRR ≥ 79 dB μV/°C V dB 50 50 50 50 mV from either rail mA 10 16 18 µA 5V AC Electrical Characteristics (Note 4) Unless otherwise specified, all limits guaranteed for TA = 25°C, V+ = 5V, V− = 0V, VCM = VO = V+/2, and RL > 1 MΩ. Boldface limits apply at the temperature extremes. Symbol Parameter Conditions GBW Gain-Bandwidth Product CL = 20 pF, RL = 10 kΩ SR Slew Rate AV = +1 Min (Note 6) Typ (Note 5) 130 Falling Edge 33 32 58 Rising Edge 33 32 48 Max (Note 6) Units kHz V/ms θm Phase Margin CL = 20 pF, RL = 10 kΩ 78 deg Gm Gain Margin CL = 20 pF, RL = 10 kΩ 27 dB www.national.com 2 en Parameter Conditions Min (Note 6) Typ (Note 5) Input-Referred Voltage Noise Density f = 1 kHz 60 Input-Referred Voltage Noise 0.1 Hz to 10 Hz 2.3 in Input-Referred Current Noise f = 1 kHz 10 THD+N Total Harmonic Distortion + Noise f = 100 Hz, RL = 10 kΩ Max (Note 6) Units nV/ μVPP fA/ 0.002 % 3.3V DC Electrical Characteristics (Note 4) Unless otherwise specified, all limits guaranteed for TA = 25°C, V+ = 3.3V, V− = 0V, VCM = VO = V+/2, and RL > 1 MΩ. Boldface limits apply at the temperature extremes. Symbol Parameter VOS Input Offset Voltage TCVOS Input Offset Voltage Drift IBIAS Input Bias Current IOS Input Offset Current CMRR Common Mode Rejection Ratio PSRR CMVR Conditions Min (Note 6) Typ (Note 5) Max (Note 6) Units ±10 ±160 ±250 μV LMP2231A ±0.3 ±0.4 LMP2231B ±0.3 ±2.5 0.02 ±1 ±50 0V ≤ VCM ≤ 2.3V Power Supply Rejection Ratio 1.6V ≤ V+ ≤ 5.5V V− = 0V, VCM = 0V 83 83 120 Common Mode Voltage Range CMRR ≥ 78 dB −0.2 −0.2 VO = 0.3V to 3V 108 107 RL = 10 kΩ to VO IO IS fA 92 Large Signal Voltage Gain Output Swing High pA 5 79 77 CMRR ≥ 77 dB AVOL μV/°C V+/2 dB 2.5 2.5 V 120 RL = 10 kΩ to V+/2 VIN(diff) = 100 mV 14 Output Swing Low RL = 10 kΩ to V+/2 VIN(diff) = −100 mV 14 Output Current (Note 7) Sourcing, VO to V– VIN(diff) = 100 mV 11 8 14 Sinking, VO to V+ VIN(diff) = −100 mV 8 5 11 Supply Current dB 10 dB 50 50 mV from either rail 50 50 mA 15 16 µA 3.3V AC Electrical Characteristics (Note 4) Unless otherwise is specified, all limits guaranteed for TA = 25°C, V+ = 3.3V, V− = 0V, VCM = VO = V+/2, and RL > 1 MΩ. Boldface limits apply at the temperature extremes. Symbol Parameter Conditions GBW Gain-Bandwidth Product CL = 20 pF, RL = 10 kΩ 128 SR Slew Rate AV = +1, CL = 20 pF Falling Edge 58 RL = 10 kΩ 48 Rising Edge Min (Note 6) Typ (Note 5) Max (Note 6) Units kHz V/ms θm Phase Margin CL = 20 pF, RL = 10 kΩ 76 deg Gm Gain Margin CL = 20 pF, RL = 10 kΩ 26 dB en Input-Referred Voltage Noise Density f = 1 kHz 60 Input-Referred Voltage Noise 0.1 Hz to 10 Hz 2.4 in Input-Referred Current Noise f = 1 kHz THD+N Total Harmonic Distortion + Noise 10 f = 100 Hz, RL = 10 kΩ 3 0.003 nV/ μVPP fA/ % www.national.com LMP2231 Single Symbol LMP2231 Single 2.5V DC Electrical Characteristics (Note 4) Unless otherwise specified, all limits guaranteed for TA = 25°C, V+ = 2.5V, V− = 0V, VCM = VO = V+/2, and RL > 1MΩ. Boldface limits apply at the temperature extremes. Symbol Parameter VOS Input Offset Voltage TCVOS Input Offset Voltage Drift IBIAS Input Bias Current IOS Input Offset Current CMRR Common Mode Rejection Ratio PSRR CMVR Conditions Min (Note 6) Typ (Note 5) Max (Note 6) Units ±10 ±190 ±275 μV LMP2231A ±0.3 ±0.4 LMP2231B ±0.3 ±2.5 0.02 ±1.0 ±50 0V ≤ VCM ≤ 1.5V Power Supply Rejection Ratio 1.6V ≤ V+ ≤ 5.5V V− = 0V, VCM = 0V 83 83 120 Common Mode Voltage Range CMRR ≥ 77 dB −0.2 −0.2 VO = 0.3V to 2.2V 104 104 RL = 10 kΩ to VO IO IS fA 91 Large Signal Voltage Gain Output Swing High pA 5 77 76 CMRR ≥ 76 dB AVOL μV/°C V+/2 dB 1.7 1.7 V 120 RL = 10 kΩ to V+/2 VIN(diff) = 100 mV 12 Output Swing Low RL = 10 kΩ to V+/2 VIN (diff) = −100 mV 13 Output Current (Note 7) Sourcing, VO to V− VIN(diff) = 100 mV 5 4 8 Sinking, VO to V+ VIN(diff) = −100 mV 3.5 2.5 7 Supply Current dB dB 50 50 50 50 mV from either rail mA 10 14 15 µA 2.5V AC Electrical Characteristics (Note 4) Unless otherwise specified, all limits guaranteed for TA = 25°C, V+ = 2.5V, V− = 0V, VCM = VO = V+/2, and RL > 1MΩ. Boldface limits apply at the temperature extremes. Symbol Parameter Conditions Min (Note 6) Typ (Note 5) GBW Gain-Bandwidth Product CL = 20 pF, RL = 10 kΩ 128 SR Slew Rate AV = +1, CL = 20 pF Falling Edge 58 RL = 10 kΩ Rising Edge 48 Max (Note 6) Units kHz V/ms θm Phase Margin CL = 20 pF, RL = 10 kΩ 74 deg Gm Gain Margin CL = 20 pF, RL = 10 kΩ 26 dB en Input-Referred Voltage Noise Density f = 1 kHz 60 Input-Referred Voltage Noise 0.1 Hz to 10 Hz 2.5 in Input-Referred Current Noise f = 1 kHz 10 THD+N Total Harmonic Distortion + Noise f = 100 Hz, RL = 10 kΩ www.national.com 4 0.005 nV/ μVPP fA/ % (Note 4) Unless otherwise specified, all limits guaranteed for TA = 25°C, V+ = 1.8V, V− = 0V, VCM = VO = V+/2, and RL > 1 MΩ. Boldface limits apply at the temperature extremes. Symbol Parameter VOS Input Offset Voltage TCVOS Input Offset Voltage Drift IBIAS Input Bias Current IOS Input Offset Current CMRR Common Mode Rejection Ratio PSRR CMVR Conditions Typ (Note 5) Max (Note 6) Units ±10 ±230 ±325 μV LMP2231A ±0.3 ±0.4 LMP2231B ±0.3 ±2.5 0.02 ±1.0 ±50 0V ≤ VCM ≤ 0.8V Power Supply Rejection Ratio 1.6V ≤ V+ ≤ 5.5V V− = 0V, VCM = 0V 83 83 120 Common Mode Voltage Rang CMRR ≥ 76 dB −0.2 0 VO = 0.3V to 1.5V 103 103 RL = 10 kΩ to V+/2 VO IO IS Output Swing High pA fA 92 Large Signal Voltage Gain μV/°C 5 76 75 CMRR ≥ 75 dB AVOL Min (Note 6) dB 1.0 1.0 V 120 RL = 10 kΩ to V+/2 VIN(diff) = 100 mV 12 Output Swing Low RL = 10 kΩ to V+/2 VIN(diff) = −100 mV 13 Output Current (Note 7) Sourcing, VO to V− VIN(diff) = 100 mV 2.5 2 5 Sinking, VO to V+ VIN(diff) = −100 mV 2 1.5 5 Supply Current dB dB 50 50 mV from either rail 50 50 mA 10 14 15 µA 1.8V AC Electrical Characteristics (Note 4) Unless otherwise is specified, all limits guaranteed for TA = 25°C, V+ = 1.8V, V− = 0V, VCM = VO = V+/2, and RL > 1 MΩ. Boldface limits apply at the temperature extremes. Symbol Parameter Conditions GBW Gain-Bandwidth Product CL = 20 pF, RL = 10 kΩ 127 SR Slew Rate AV = +1, CL = 20 pF Falling Edge 58 RL = 10 kΩ 48 Rising Edge Min (Note 6) Typ (Note 5) Max (Note 6) Units kHz V/ms θm Phase Margin CL = 20 pF, RL = 10 kΩ 70 deg Gm Gain Margin CL = 20 pF, RL = 10 kΩ 25 dB en Input-Referred Voltage Noise Density f = 1 kHz 60 Input-Referred Voltage Noise 0.1 Hz to 10 Hz 2.4 in Input-Referred Current Noise f = 1 kHz 10 THD+N Total Harmonic Distortion + Noise f = 100 Hz, RL = 10 kΩ 5 0.005 nV/ μVPP fA/ % www.national.com LMP2231 Single 1.8V DC Electrical Characteristics LMP2231 Single Note 1: Absolute Maximum Ratings indicate limits beyond which damage 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 test conditions, see the Electrical Characteristics. 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) Field-Induced 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. 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. Absolute Maximum Ratings indicate junction temperature limits beyond which the device may be permanently degraded, either mechanically or electrically. Note 5: Typical values represent the most likely parametric norm 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: All limits are guaranteed by testing, statistical analysis or design. Note 7: The short circuit test is a momentary open loop test. Connection Diagrams 5-Pin SOT23 8-Pin SOIC 30033802 30033842 Top View Top View Ordering Information Package Part Number Temperature Range Package Marking LMP2231AMF AL5A 3k Units Tape and Reel LMP2231BMF 1k Units Tape and Reel LMP2231BMFE LMP2231AMA LMP2231AMAE 8-Pin SOIC AL5B MF05A 250 Units Tape and Reel 3k Units Tape and Reel −40°C to 125°C 95 Units/Rail LMP2231AMA 250 Units Tape and Reel LMP2231AMAX 2.5k Units Tape and Reel LMP2231BMA 95 Units/Rail LMP2231BMAE LMP2231BMA LMP2231BMAX www.national.com 250 Units Tape and Reel LMP2231AMFX LMP2231BMFX NSC Drawing 1k Units Tape and Reel LMP2231AMFE 5-Pin SOT23 Transport Media 250 Units Tape and Reel 2.5k Units Tape and Reel 6 M08A Unless otherwise Specified: TA = 25°C, VS = 5V, VCM = VS/2, where Offset Voltage Distribution TCVOS Distribution 30033807 30033811 Offset Voltage Distribution TCVOS Distribution 30033806 30033810 Offset Voltage Distribution TCVOS Distribution 30033805 30033809 7 www.national.com LMP2231 Single Typical Performance Characteristics VS = V+ - V− LMP2231 Single Offset Voltage Distribution TCVOS Distribution 30033869 30033873 Offset Voltage vs. VCM Offset Voltage vs. VCM 30033818 30033865 Offset Voltage vs. VCM Offset Voltage vs. VCM 30033864 www.national.com 30033872 8 LMP2231 Single Offset Voltage vs. Temperature Offset Voltage vs. Supply Voltage 30033871 30033870 Time Domain Voltage Noise Time Domain Voltage Noise 30033833 30033834 Time Domain Voltage Noise Time Domain Voltage Noise 30033832 30033831 9 www.national.com LMP2231 Single Input Bias Current vs. VCM Input Bias Current vs. VCM 30033855 30033856 Input Bias Current vs. VCM Input Bias Current vs. VCM 30033857 30033858 Input Bias Current vs. VCM Input Bias Current vs. VCM 30033859 www.national.com 30033860 10 LMP2231 Single Input Bias Current vs. VCM Input Bias Current vs. VCM 30033861 30033862 PSRR vs. Frequency Supply Current vs. Supply Voltage 30033866 30033812 Sinking Current vs. Supply Voltage Sourcing Current vs. Supply Voltage 30033813 30033814 11 www.national.com LMP2231 Single Output Swing High vs. Supply Voltage Output Swing Low vs. Supply Voltage 30033815 30033816 Open Loop Frequency Response Open Loop Frequency Response 30033821 30033822 Phase Margin vs. Capacitive Load Slew Rate vs. Supply Voltage 30033863 www.national.com 30033830 12 LMP2231 Single THD+N vs. Amplitude THD+N vs. Frequency 30033829 30033828 Large Signal Step Response Small Signal Step Response 30033824 30033823 Large Signal Step Response Small Signal Step Response 30033826 30033825 13 www.national.com LMP2231 Single CMRR vs. Frequency Input Voltage Noise vs. Frequency 30033819 30033867 www.national.com 14 Where: LMP2231 The LMP2231 is a single CMOS precision amplifier that offer low offset voltage and low offset voltage drift, and high gain while only consuming 10 μA of current per channel. The LMP2231 is a micropower op amp, consuming only 10 μA of current. Micropower op amps extend the run time of battery powered systems and reduce energy consumption in energy limited systems. The guaranteed supply voltage range of 1.8V to 5.0V along with the ultra-low supply current extend the battery run time in two ways. The extended guaranteed power supply voltage range of 1.8V to 5.0V enables the op amp to function when the battery voltage has depleted from its nominal value down to 1.8V. In addition, the lower power consumption increases the life of the battery. The LMP2231 has an input referred offset voltage of only ±150 μV maximum at room temperature. This offset is guaranteed to be less than ±230 μV over temperature. This minimal offset voltage along with very low TCVOS of only 0.3 µV/°C typical allows more accurate signal detection and amplification in precision applications. The low input bias current of only ±20 fA gives the LMP2231 superiority for use in high impedance sensor applications. Bias Current of an amplifier flows through source resistance of the sensor and the voltage resulting from this current flow appears as a noise voltage on the input of the amplifier. The low input bias current enables the LMP2231 to interface with high impedance sensors while generating negligible voltage noise. Thus the LMP2231 provides better signal fidelity and a higher signal-to-noise ration when interfacing with high impedance sensors. National Semiconductor is heavily committed to precision amplifiers and the market segment they serve. Technical support and extensive characterization data is available for sensitive applications or applications with a constrained error budget. The operating supply voltage range of 1.8V to 5.5V over the extensive temperature range of −40°C to 125°C makes the LMP2231 an excellent choice for low voltage precision applications with extensive temperature requirements. The LMP2231 is offered in the space saving 5-Pin SOT23 and 8-pin SOIC package. These small packages are ideal solutions for area constrained PC boards and portable electronics. The input current noise of the LMP2231 is so low that it will not become the dominant factor in the total noise unless source resistance exceeds 300 MΩ, which is an unrealistically high value. As is evident in Figure 1, at lower RS values, total noise is dominated by the amplifier’s input voltage noise. Once RS is larger than a 100 kΩ, then the dominant noise factor becomes the thermal noise of RS. As mentioned before, the current noise will not be the dominant noise factor for any practical application. 30033848 FIGURE 1. Total Input Noise VOLTAGE NOISE REDUCTION . While The LMP2231 has an input voltage noise of 60 nV/ this value is very low for micropower amplifiers, this input voltage noise can be further reduced by placing N amplifiers in parallel as shown in Figure 2. The total voltage noise on the output of this circuit is divided by the square root of the number of amplifiers used in this parallel combination. This is because each individual amplifier acts as an independent noise source, and the average noise of independent sources is the quadrature sum of the independent sources divided by the number of sources. For N identical amplifiers, this means: TOTAL NOISE CONTRIBUTION The LMP2231 has a very low input bias current, very low input current noise, and low input voltage noise for micropower amplifier. As a result, this amplifier makes a great choice for circuits with high impedance sensor applications. Figure 1 shows the typical input noise of the LMP2231 as a function of source resistance where: en denotes the input referred voltage noise ei is the voltage drop across source resistance due to input referred current noise or ei = RS * in et shows the thermal noise of the source resistance eni shows the total noise on the input. 15 www.national.com LMP2231 Single Application Information LMP2231 Single Figure 2 shows a schematic of this input voltage noise reduction circuit. Typical resistor values are: RG = 10Ω, RF = 1 kΩ, and RO = 1 kΩ. 30033836 FIGURE 3. Instrumentation Amplifier There are two stages in this amplifier. The last stage, output stage, is a differential amplifier. In an ideal case the two amplifiers of the first stage, input stage, would be set up as buffers to isolate the inputs. However they cannot be connected as followers because of mismatch of amplifiers. That is why there is a balancing resistor between the two. The product of the two stages of gain will give the gain of the instrumentation amplifier. Ideally, the CMRR should be infinite. However the output stage has a small non-zero common mode gain which results from resistor mismatch. In the input stage of the circuit, current is the same across all resistors. This is due to the high input impedance and low input bias current of the LMP2231. 30033846 (1) FIGURE 2. Noise Reduction Circuit By Ohm’s Law: PRECISION INSTRUMENTATION AMPLIFIER Measurement of very small signals with an amplifier requires close attention to the input impedance of the amplifier, gain of the overall signal on the inputs, and the gain on each input of the amplifier. This is because the difference of the input signal on the two inputs is of the interest and the common signal is considered noise. A classic circuit implementation is an instrumentation amplifier. Instrumentation amplifiers have a finite, accurate, and stable gain. They also have extremely high input impedances and very low output impedances. Finally they have an extremely high CMRR so that the amplifier can only respond to the differential signal. A typical instrumentation amplifier is shown in Figure 3. (2) However: (3) So we have: VO1–VO2 = (2a+1)(V1–V2) (4) Now looking at the output of the instrumentation amplifier: (5) Substituting from Equation 4: (6) This shows the gain of the instrumentation amplifier to be: −K(2a+1) Typical values for this circuit can be obtained by setting: a = 12 and K= 4. This results in an overall gain of −100. www.national.com 16 30033851 30033850 FIGURE 4. Bridge Sensor Instrumentation amplifiers are great for interfacing with bridge sensors. Bridge sensors often sense a very small differential signal in the presence of a larger common mode voltage. Instrumentation amplifiers reject this common mode signal. Figure 5 shows a strain gauge bridge amplifier. In this application the LMP2231 is used to buffer the LM4140's precision output voltage. The LM4140A is a precision voltage refer- ence. The other three LMP2231s are used to form an instrumentation amplifier. This instrumentation amplifier uses the LMP2231's high CMRR and low VOS and TCVOS to accurately amplify the small differential signal generated by the output of the bridge sensor. This amplified signal is then fed into the ADC121S021 which is a 12-bit analog to digital converter. This circuit works on a single supply voltage of 5V. 17 www.national.com LMP2231 Single ratios of two adjacent resistor values are equal. This fact is used in null circuit measurements. These are particularly used in feedback systems which involve electrochemical elements or human interfaces. Null systems force an active resistor, such as a strain gauge, to balance the bridge by influencing the measured parameter. Often in sensor applications at lease one of the resistors is a variable resistor, or a sensor. The deviation of this active element from its initial value is measured as an indication of change in the measured quantity. A change in output voltage represents the sensor value change. Since the sensor value change is often very small, the resulting output voltage is very small in magnitude as well. This requires an extensive and very precise amplification circuitry so that signal fidelity does not change after amplification. Sensitivity of a bridge is the ratio of its maximum expected output change to the excitation voltage change. Figure 4 (a) shows a typical bridge sensor and Figure 4(b) shows the bridge with four sensors. R in Figure 4(b) is the nominal value of the sense resistor and the deviations from R are proportional to the quantity being measured. SINGLE SUPPLY STRAIN GAGE BRIDGE AMPLIFIER Strain gauges are popular electrical elements used to measure force or pressure. Strain gauges are subjected to an unknown force which is measured as a the deflection on a previously calibrated scale. Pressure is often measured using the same technique; however this pressure needs to be converted into force using an appropriate transducer. Strain gauges are often resistors which are sensitive to pressure or to flexing. Sense resistor values range from tens of ohms to several hundred kilo ohms. The resistance change which is a result of applied force across the strain gauge might be 1% of its total value. An accurate and reliable system is needed to measure this small resistance change. Bridge configurations offer a reliable method for this measurement. Bridge sensors are formed of four resistors, connected as a quadrilateral. A voltage source or a current source is used across one of the diagonals to excite the bridge while a voltage detector across the other diagonal measures the output voltage. Bridges are mainly used as null circuits or to measure a differential voltages. Bridges will have no output voltage if the LMP2231 Single 30033874 FIGURE 5. Strain Gauge Bridge Amplifier sensors typically have a life of one to two years. The use of the micropower LMP2231 means minimal power usage by the op amp and it enhances the battery life. Depending on other components present in the circuit design, the battery could last for the entire life of the oxygen sensor. The precision specifications of the LMP2231, such as its very low offset voltage, low TCVOS , low input bias current, low CMRR, and low PSRR are other factors which make the LMP2231 a great choice for this application. PORTABLE GAS DETECTION SENSOR Gas sensors are used in many different industrial and medical applications. They generate a current which is proportional to the percentage of a particular gas sensed in an air sample. This current goes through a load resistor and the resulting voltage drop is measured. Depending on the sensed gas and sensitivity of the sensor, the output current can be in the order of tens of microamperes to a few milliamperes. Gas sensor datasheets often specify a recommended load resistor value or they suggest a range of load resistors to choose from. Oxygen sensors are used when air quality or oxygen delivered to a patient needs to be monitored. Fresh air contains 20.9% oxygen. Air samples containing less than 18% oxygen are considered dangerous. Oxygen sensors are also used in industrial applications where the environment must lack oxygen. An example is when food is vacuum packed. There are two main categories of oxygen sensors, those which sense oxygen when it is abundantly present (i.e. in air or near an oxygen tank) and those which detect traces of oxygen in ppm. Figure 6 shows a typical circuit used to amplify the output of an oxygen detector. The LMP2231 makes an excellent choice for this application as it only draws 10 µA of current and operates on supply voltages down to 1.8V. This application detects oxygen in air. The oxygen sensor outputs a known current through the load resistor. This value changes with the amount of oxygen present in the air sample. Oxygen sensors usually recommend a particular load resistor value or specify a range of acceptable values for the load resistor. Oxygen www.national.com 30033849 FIGURE 6. Precision Oxygen Sensor 18 LMP2231 Single Physical Dimensions inches (millimeters) unless otherwise noted 5-Pin SOT23 NS Package Number MF0A5 8-Pin SOIC NS Package Number M08A 19 www.national.com LMP2231 Single Micropower, 1.8V, Precision, Operational Amplifier with CMOS Inputs Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: Products Design Support Amplifiers www.national.com/amplifiers WEBENCH www.national.com/webench Audio www.national.com/audio Analog University www.national.com/AU Clock Conditioners www.national.com/timing App Notes www.national.com/appnotes Data Converters www.national.com/adc Distributors www.national.com/contacts Displays www.national.com/displays Green Compliance www.national.com/quality/green Ethernet www.national.com/ethernet Packaging www.national.com/packaging Interface www.national.com/interface Quality and Reliability www.national.com/quality LVDS www.national.com/lvds Reference Designs www.national.com/refdesigns Power Management www.national.com/power Feedback www.national.com/feedback Switching Regulators www.national.com/switchers LDOs www.national.com/ldo LED Lighting www.national.com/led PowerWise www.national.com/powerwise Serial Digital Interface (SDI) www.national.com/sdi Temperature Sensors www.national.com/tempsensors Wireless (PLL/VCO) www.national.com/wireless THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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