LMP2232AMM

LMP2232 Dual, Micropower, 1.6V, Precision, Operational Amplifier with CMOS Input December 18, 2008 LMP2232 Dual Micropower, 1.6V, Precision, Operational Amplifier with CMOS Input General Description The LMP2232 is a dual micropower precision amplifier designed for battery powered applications. The 1.6V to 5.5V operating supply voltage range and quiescent power consumption of only 26 μW extend the battery life in portable systems. The LMP2232 is part of the LMP® precision amplifier family. The high impedance CMOS input makes it ideal for instrumentation and other sensor interface applications. The LMP2232 has a maximum offset voltage of 150 μV and maximum offset voltage drift of only 0.5 μV/°C along with low bias current of only ±20 fA. These precise specifications make the LMP2232 a great choice for maintaining system accuracy and long term stability. The LMP2232 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 LMP2232 is ideal for ground sensing in single supply applications. The LMP2232 is offered in 8-pin SOIC and MSOP packages. The LMP2231 is the single version of this product and the LMP2234 is the quad version of this product. Both of these products are available on National Semiconductor's website. Features (For VS = 5V, Typical unless otherwise noted) 16 µA ■ Supply current at 1.8V 1.6V to 5.5V ■ Operating voltage range ±0.5 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 30033974 Strain Gauge Bridge Amplifier LMP® is a registered trademark of National Semiconductor Corporation. © 2008 National Semiconductor Corporation 300339 www.national.com LMP2232 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 Junction Temperature (Note 3) 2000V 100V ±300 mV 6V V+ + 0.3V, V– – 0.3V −65°C to 150°C 150°C Mounting Temperature Infrared or Convection (20 sec.) Wave Soldering Lead Temperature (10 sec.) +235°C +260°C (Note 1) −40°C to 125°C 1.6V to 5.5V 111.2 °C/W 147.4 °C/W Operating Ratings Operating Temperature Range (Note 3) Supply Voltage (VS = V+ - V–) Package Thermal Resistance (θJA)(Note 3) 8-Pin SOIC 8-Pin MSOP 5V DC Electrical Characteristics (Note 4) Symbol VOS TCVOS IBIAS IOS CMRR PSRR CMVR AVOL VO Parameter Input Offset Voltage Input Offset Voltage Drift Input Bias Current Input Offset Current Common Mode Rejection Ratio Power Supply Rejection Ratio Common Mode Voltage Range Large Signal Voltage Gain Output Swing High Output Swing Low IO Output Current (Note 7) 0V ≤ VCM ≤ 4V LMP2232A LMP2232B 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. Conditions Min (Note 6) Typ (Note 5) ±10 ±0.3 ±0.3 0.02 5 81 80 83 83 −0.2 −0.2 110 108 120 17 17 27 19 17 12 30 22 19 27 28 mA 50 50 50 50 97 120 4.2 4.2 Max (Note 6) ±150 ±230 ±0.5 ±2.5 ±3 ±125 Units μV μV/°C pA fA dB dB V dB 1.6V ≤ V+ ≤ 5.5V V− = 0V, VCM = 0V CMRR ≥ 80 dB CMRR ≥ 79 dB VO = 0.3V to 4.7V RL = 10 kΩ to V+/2 RL = 10 kΩ to V+/2 VIN(diff) = 100 mV RL = 10 kΩ to V+/2 VIN(diff) = −100 mV Sourcing, VO to V− VIN(diff) = 100 mV Sinking, VO to V+ VIN(diff) = −100 mV mV from either rail IS Supply Current μA 5V AC Electrical Characteristics (Note 4) Symbol GBW SR Parameter Gain-Bandwidth Product Slew Rate 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. Conditions CL = 20 pF, RL = 10 kΩ AV = +1 Falling Edge Rising Edge 33 32 33 32 Min (Note 6) Typ (Note 5) 130 58 48 68 V/ms Max (Note 6) Units kHz θm Phase Margin CL = 20 pF, RL = 10 kΩ 2 deg www.national.com LMP2232 Symbol Gm en in THD+N Gain Margin Parameter Conditions CL = 20 pF, RL = 10 kΩ f = 1 kHz 0.1 Hz to 10 Hz f = 1 kHz f = 100 Hz, RL = 10 kΩ Min (Note 6) Typ (Note 5) 27 60 2.3 10 0.002 Max (Note 6) Units dB nV/ μVPP fA/ % Input-Referred Voltage Noise Density Input Referred Voltage Noise Input-Referred Current Noise Total Harmonic Distortion + Noise 3.3V DC Electrical Characteristics Symbol VOS TCVOS IBIAS IOS CMRR PSRR CMVR AVOL VO Parameter Input Offset Voltage Input Offset Voltage Drift Input Bias Current Input Offset Current Common Mode Rejection Ratio Power Supply Rejection Ratio Common Mode Voltage Range Large Signal Voltage Gain Output Swing High Output Swing Low IO Output Current (Note 7) (Note 4) Unless otherwise specified, all limits guaranteed for T A = 25°C, V+ = 3.3V, V− = 0V, VCM = VO = V+/2, and RL > 1 MΩ. Boldface limits apply at the temperature extremes. Conditions Min (Note 6) Typ (Note 5) ±10 LMP2232A LMP2232B ±0.3 ±0.3 0.02 5 0V ≤ VCM ≤ 2.3V 1.6V ≤ V+ ≤ 5.5V V− = 0V, VCM = 0V CMRR ≥ 78 dB CMRR ≥ 77 dB VO = 0.3V to 3V RL = 10 kΩ to V+/2 RL = 10 kΩ to V+/2 VIN(diff) = 100 mV RL = 10 kΩ to V+/2 VIN(diff) = −100 mV Sourcing, VO to V− VIN(diff) = 100 mV Sinking, VO to V+ VIN(diff) = −100 mV IS Supply Current 11 8 8 5 79 77 83 83 −0.2 −0.2 108 107 120 14 14 14 11 17 25 26 mA 50 50 50 50 92 120 2.5 2.5 Max (Note 6) ±160 ±250 ±0.5 ±2.5 ±3 ±125 Units μV μV/°C pA fA dB dB V dB mV from either rail μA 3.3V AC Electrical Characteristics Symbol GBW SR θm Gm en Parameter Gain-Bandwidth Product Slew Rate Phase Margin Gain Margin Input-Referred Voltage Noise Density Input-Referred Voltage Noise (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. Conditions CL = 20 pF, RL = 10 kΩ AV = +1, CL = 20 pF Falling Edge RL = 10 kΩ Rising Edge CL = 20 pF, RL = 10 kΩ CL = 20 pF, RL = 10 kΩ f = 1 kHz 0.1 Hz to 10 Hz Min (Note 6) Typ (Note 5) 128 58 48 66 26 60 2.4 Max (Note 6) Units kHz V/ms deg dB nV/ μVPP 3 www.national.com LMP2232 Symbol in THD+N Parameter Input-Referred Current Noise Total Harmonic Distortion + Noise f = 1 kHz Conditions Min (Note 6) Typ (Note 5) 10 0.003 Max (Note 6) Units fA/ % f = 100 Hz, RL = 10 kΩ 2.5V DC Electrical Characteristics Symbol VOS TCVOS IBias IOS CMRR PSRR CMVR AVOL VO Parameter Input Offset Voltage Input Offset Voltage Drift Input Bias Current Input Offset Current Common Mode Rejection Ratio Power Supply Rejection Ratio Common Mode Voltage Range Large Signal Voltage Gain Output Swing High Output Swing Low IO Output Current (Note 7) LMP2232A LMP2232B (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. Conditions Min (Note 6) Typ (Note 5) ±10 ±0.3 ±0.3 0.02 5 0V ≤ VCM ≤ 1.5V 1.6V ≤ V+ ≤ 5.5V V– = 0V, VCM = 0V CMRR ≥ 77 dB CMRR ≥ 76 dB VO = 0.3V to 2.2V RL = 10 kΩ to V+/2 RL = 10 kΩ to V+/2 VIN(diff) = 100 mV RL = 10 kΩ to V+/2 VIN(diff) = –100 mV Sourcing, VO to V– VIN(diff) = 100 mV Sinking, VO to V+ VIN(diff) = –100 mV IS Supply Current 5 4 3.5 2.5 77 76 83 83 −0.2 −0.2 104 104 120 12 13 8 7 16 24 25 mA 50 50 50 50 91 120 1.7 1.7 Max (Note 6) ±190 ±275 ±0.5 ±2.5 ±3 ±125 Units μV μV/°C pA fA dB dB V dB mV from either rail µA 2.5V AC Electrical Characteristics Symbol GBW SR θm Gm en in THD+N Parameter Gain-Bandwidth Product Slew Rate Phase Margin Gain Margin (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. Conditions CL = 20 pF, RL = 10 kΩ AV = +1, CL = 20 pF RL = 10 kΩ Falling Edge Rising Edge Min Typ (Note 6) (Note 5) 128 58 48 64 26 60 2.5 10 0.005 Max (Note 6) Units kHz V/ms deg dB nV/ μVPP fA/ % CL = 20 pF, RL = 10 kΩ CL = 20 pF, RL = 10 kΩ 0.1 Hz to 10 Hz f = 1 kHz f = 100 Hz, RL = 10 kΩ Input-Referred Voltage Noise Density f = 1 kHz Input-Referred Voltage Noise Input-Referred Current Noise Total Harmonic Distortion + Noise www.national.com 4 LMP2232 1.8V DC Electrical Characteristics Symbol VOS TCVOS IBIAS IOS CMRR PSRR CMVR AVOL VO Parameter Input Offset Voltage Input Offset Voltage Drift Input Bias Current Input Offset Current Common Mode Rejection Ratio Power Supply Rejection Ratio Common Mode Voltage Range Large Signal Voltage Gain Output Swing High Output Swing Low IO Output Current (Note 7) (Note 4) Unless otherwise specified, all limits guaranteed for T A = 25°C, V+ = 1.8V, V− = 0V, VCM = VO = V+/2, and RL > 1 MΩ. Boldface limits apply at the temperature extremes. Conditions Min (Note 6) Typ (Note 5) ±10 LMP2232A LMP2232B ±0.3 ±0.3 0.02 5 0V ≤ VCM ≤ 0.8V 1.6V ≤ V+ ≤ 5.5V V− = 0V, VCM = 0V CMRR ≥ 76 dB CMRR ≥ 75 dB VO = 0.3V to 1.5V RL = 10 kΩ to V+/2 RL = 10 kΩ to V+/2 VIN(diff) = 100 mV RL = 10 kΩ to V+/2 VIN(diff) = −100 mV Sourcing, VO to V– VIN(diff) = 100 mV Sinking, VO to V+ VIN(diff) = −100 mV IS Supply Current 2.5 2 2 1.5 76 75 83 83 −0.2 0 103 103 120 12 13 5 5 16 24 25 mA 50 50 50 50 92 120 1.0 1.0 Max (Note 6) ±230 ±325 ±0.5 ±2.5 ±3 ±125 Units μV μV/°C pA fA dB dB V dB mV from either rail µA 1.8V AC Electrical Characteristics Symbol GBW SR θm Gm en in THD+N Parameter Gain-Bandwidth Product Slew Rate Phase Margin Gain Margin (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. Conditions CL = 20 pF, RL = 10 kΩ AV = +1, CL = 20 pF Falling Edge RL = 10 kΩ Rising Edge CL = 20 pF, RL = 10 kΩ CL = 20 pF, RL = 10 kΩ 0.1 Hz to 10 Hz f = 1 kHz f = 100 Hz, RL = 10 kΩ Min (Note 6) Typ (Note 5) 127 58 48 60 25 60 2.4 10 0.005 Max (Note 6) Units kHz V/ms deg dB nV/ μVPP fA/ % Input-Referred Voltage Noise Density f = 1 kHz Input-Referred Voltage Noise Input-Referred Current Noise Total Harmonic Distortion + Noise 5 www.national.com LMP2232 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, 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. 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 Diagram 8-Pin MSOP/SOIC 30033938 Top View Ordering Information Package Part Number LMP2232AMA LMP2232AMAE 8-Pin SOIC LMP2232AMAX LMP2232BMA LMP2232BMAE LMP2232BMAX LMP2232AMM LMP2232AMME 8-Pin MSOP LMP2232AMMX LMP2232BMM LMP2232BMME LMP2232BMMX AK5B –40°C to 125°C AK5A LMP2232BMA LMP2232AMA Temperature Range Package Marking Transport Media 95 Units/Rail 250 Units Tape and Reel 2.5k Units Tape and Reel 95 Units/Rail 250 Units Tape and Reel 2.5k Units Tape and Reel 1k Units Tape and Reel 250 Units Tape and Reel 3.5k Units Tape and Reel 1k Units Tape and Reel 250 Units Tape and Reel 3.5k Units Tape and Reel MUA08A M08A NSC Drawing www.national.com 6 LMP2232 Typical Performance Characteristics VS = V+ - V− Offset Voltage Distribution Unless otherwise Specified: TA = 25°C, VS = 5V, VCM = VS/2, where TCVOS Distribution 30033907 30033911 Offset Voltage Distribution TCVOS Distribution 30033906 30033910 Offset Voltage Distribution TCVOS Distribution 30033905 30033909 7 www.national.com LMP2232 Offset Voltage Distribution TCVOS Distribution 30033973 30033969 Offset Voltage vs. VCM Offset Voltage vs. VCM 30033918 30033965 Offset Voltage vs. VCM Offset Voltage vs. VCM 30033964 30033972 www.national.com 8 LMP2232 Offset Voltage vs. Temperature Offset Voltage vs. Supply Voltage 30033971 30033970 0.1 Hz to 10 Hz Voltage Noise 0.1 Hz to 10 Hz Voltage Noise 30033933 30033934 0.1 Hz to 10 Hz Voltage Noise 0.1 Hz to 10 Hz Voltage Noise 30033932 30033931 9 www.national.com LMP2232 Input Bias Current vs. VCM Input Bias Current vs. VCM 30033955 30033956 Input Bias Current vs. VCM Input Bias Current vs. VCM 30033957 30033958 Input Bias Current vs. VCM Input Bias Current vs. VCM 30033959 30033960 www.national.com 10 LMP2232 Input Bias Current vs. VCM Input Bias Current vs. VCM 30033961 30033962 PSRR vs. Frequency Supply Current vs. Supply Voltage (per channel) 30033966 30033912 Sinking Current vs. Supply Voltage Sourcing Current vs. Supply Voltage 30033913 30033914 11 www.national.com LMP2232 Output Swing High vs. Supply Voltage Output Swing Low vs. Supply Voltage 30033915 30033916 Open Loop Frequency Response Open Loop Frequency Response 30033921 30033922 Phase Margin vs. Capacitive Load Slew Rate vs. Supply Voltage 30033963 30033930 www.national.com 12 LMP2232 THD+N vs. Amplitude THD+N vs. Frequency 30033928 30033929 Large Signal Step Response Small Signal Step Response 30033924 30033923 Large Signal Step Response Small Signal Step Response 30033926 30033925 13 www.national.com LMP2232 CMRR vs. Frequency Input Voltage Noise vs. Frequency 30033967 30033919 www.national.com 14 LMP2232 Application Information LMP2232 The LMP2232 is a quad CMOS precision amplifier that offers low offset voltage, low offset voltage drift, and high gain while consuming less than 10 μA of supply current per channel. The LMP2232 is a micropower op amp, consuming only 36 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 LMP2232 has 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 LMP2232 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 LMP2232 to interface with high impedance sensors while generating negligible voltage noise. Thus the LMP2232 provides better signal fidelity and a higher signal-to-noise ratio when interfacing with high impedance sensors. National Semiconductor is heavily committed to precision amplifiers and the market segments they serve. Technical support and extensive characterization data is available for sensitive applications or applications with a constrained error budget. The operating voltage range of 1.6V to 5.5V over the extensive temperature range of −40°C to 125°C makes the LMP2232 an excellent choice for low voltage precision applications with extensive temperature requirements. The LMP2232 is offered in the 8-pin MSOP and 8-pin SOIC packages. These small packages are ideal solutions for area constrained PC boards and portable electronics. TOTAL NOISE CONTRIBUTION The LMP2232 has very low input bias current, very low input current noise, and low input voltage noise for micropower amplifiers. As a result, these amplifiers make great choices for circuits with high impedance sensor applications. Figure 1 shows the typical input noise of the LMP2232 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. Where: The input current noise of the LMP2232 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. 30033948 FIGURE 1. Total Input Noise VOLTAGE NOISE REDUCTION . While The LMP2232 has an input voltage noise of 60nV/ 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: 15 www.national.com LMP2232 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. 30033936 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, the 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 LMP2232. 30033946 (1) FIGURE 2. Noise Reduction Circuit PRECISION INSTRUMENTATION AMPLIFIER Measurement of very small signals with an amplifier requires close attention to the input impedance of the amplifier, gain of the 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. By Ohm’s Law: (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 LMP2232 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 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 differential voltages. Bridges will have no output voltage if the 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. 30033951 30033950 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 one of the LMP2232 amplifiers is used to buffer the LM4140A's precision output voltage. The LM4140A is a precision voltage reference. The other three amplifiers in the LMP2232 are used to form an instrumentation amplifier. This instrumentation amplifier uses the LMP2232'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 LMP2232 30033974 FIGURE 5. Strain Gage Bridge Amplifier 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 very small traces of oxygen in ppm. Figure 6 shows a typical circuit used to amplify the output signal of an oxygen detector. The LMP2232 makes an excellent choice for this application as it draws only 36 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 sensors typically have a life of one to two years. The use of the micropower LMP2232 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 LMP2232, such as its very low offset voltage, low TCVOS, low input bias current, low CMRR, and low PSRR are other factors which make the LMP2232 a great choice for this application.. 30033949 FIGURE 6. Precision Oxygen Sensor www.national.com 18 LMP2232 Physical Dimensions inches (millimeters) unless otherwise noted 8-Pin MSOP NS Package Number MUA08A 8-Pin SOIC NS Package Number M08A 19 www.national.com LMP2232 Dual, Micropower, 1.6V, Precision, Operational Amplifier with CMOS Input Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: Products Amplifiers Audio Clock and Timing Data Converters Interface LVDS Power Management Switching Regulators LDOs LED Lighting Voltage Reference PowerWise® Solutions Serial Digital Interface (SDI) Temperature Sensors Wireless (PLL/VCO) www.national.com/amplifiers www.national.com/audio www.national.com/timing www.national.com/adc www.national.com/interface www.national.com/lvds www.national.com/power www.national.com/switchers www.national.com/ldo www.national.com/led www.national.com/vref www.national.com/powerwise www.national.com/sdi www.national.com/tempsensors www.national.com/wireless WEBENCH® Tools App Notes Reference Designs Samples Eval Boards Packaging Green Compliance Distributors Design Support www.national.com/webench www.national.com/appnotes www.national.com/refdesigns www.national.com/samples www.national.com/evalboards www.national.com/packaging www.national.com/quality/green www.national.com/contacts www.national.com/quality www.national.com/feedback www.national.com/easy www.national.com/solutions www.national.com/milaero www.national.com/solarmagic www.national.com/AU Quality and Reliability Feedback/Support Design Made Easy Solutions Mil/Aero Solar Magic® Analog University® THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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