LMP2234 Quad Micropower, Precision, RRO Amplifier with CMOS Input General Description Features The LMP2234 is a quad micropower precision amplifier designed for battery powered applications. The 1.8 to 5.5V operating voltage range and quiescent power consumption of only 62 μW extend the battery life in portable systems. The LMP2234 is part of the LMP® precision amplifier family. The high impedance CMOS input makes it ideal for instrumentation and other sensor interface applications. The LMP2234 has a maximum offset voltage of 150 μV and 0.3 μV/°C offset drift along with low bias current of only ±20 fA. These precise specifications make the LMP2234 a great choice for maintaining system accuracy and long term stability. The LMP2234 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 LMP2234 is ideal for ground sensing in single supply applications. The LMP2234 is offered in 14-Pin SOIC and TSSOP packages. (For VS = 5V, Typical unless otherwise noted) 9 µA ■ Supply current (per channel) ±1.0 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 ■ Rail-to-rail output Applications ■ ■ ■ ■ ■ Precision instrumentation amplifiers Battery powered medical instrumentation High impedance sensors Strain gauge bridge amplifier Thermocouple amplifiers Typical Application Strain Gauge Bridge Amplifier 20203468 LMP® is a registered trademark of National Semiconductor Corporation. © 2007 National Semiconductor Corporation 202034 www.national.com LMP2234 Quad Micropower, Precision, RRO, Operational Amplifier with CMOS Input September 2007 LMP2234 Quad Mounting Temperature Infrared or Convection (20 sec.) Wave Soldering Lead Temperature (10 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 Junction Temperature(Note 3) Operating Ratings 2000V 100V ±300 mV 6V V+ + 0.3V, V– – 0.3V −65°C to 150°C 150°C +235°C +260°C (Note 1) Operating Temperature Range (Note 3) −40°C to 125°C Supply Voltage (VS = V+ - V–) 1.8V to 5.5V Package Thermal Resistance (θJA) (Note 3) 14-Pin SOIC 101.5 °C/W 14-Pin TSSOP 121 °C/W 5V DC Electrical Characteristics (Note 4) Unless otherwise specified, all limits are 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 Conditions Min (Note 6) Typ (Note 5) Max (Note 6) Units ±10 ±150 ±230 μV LMP2234A ±0.3 ±1.0 LMP2234B ±0.3 ±2.5 ±0.02 ±1 ±50 IBias Input Bias Current IOS Input Offset Current CMRR Common Mode Rejection Ratio 0V ≤ VCM ≤ 4V 81 80 97 PSRR Power Supply Rejection Ratio 1.8V ≤ V+ ≤ 5.5V VCM = 0V 84 84 120 CMVR 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 V+/2 VO IO IS Output Swing High pA ±5 CMRR ≥ 79 dB AVOL μV/°C dB dB 4.2 4.2 V 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 fA 36 dB 50 50 50 50 mV from either rail mA 48 50 µA 5V AC Electrical Characteristics (Note 4) Unless otherwise specified, all limits are 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 GBWP Gain Bandwidth Product CL = 20 pF, RL = 10 kΩ SR Slew Rate AV = +1 Min (Note 6) Typ (Note 5) 130 Falling Edge 33 58 Rising Edge 33 48 Max (Note 6) Units kHz V/ms θm Phase Margin CL = 20 pF, RL = 10 kΩ 68 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 Density 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 are 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 LMP2234A ±0.3 ±1.0 LMP2234B ±0.3 ±2.5 ±0.02 ±1 ±50 0V ≤ VCM ≤ 2V 92 Power Supply Rejection Ratio 1.8V ≤ V+ ≤ 5.5V VCM = 0V 84 84 120 Common Mode Voltage Range CMRR ≥ 78 dB −0.2 −0.2 VO = 0.3V to 3V 108 107 Large Signal Voltage Gain RL = 10 kΩ to VO IO IS Output Swing High pA ±5 79 77 CMRR ≥ 77 dB AVOL μV/°C V+/2 dB 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 fA 34 dB 50 50 mV from either rail 50 50 mA 44 46 µA 3.3V AC Electrical Characteristics (Note 4) Unless otherwise is specified, all limits are 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 GBWP 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 66 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 2.4 0.1 Hz to 10 Hz in Input-Referred Current Noise Density 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 LMP2234 Quad Symbol LMP2234 Quad 2.5V DC Electrical Characteristics (Note 4) Unless otherwise specified, all limits are 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 Conditions Min (Note 6) Typ (Note 5) Max (Note 6) Units ±10 ±190 ±275 μV LMP2234A ±0.3 ±1.0 LMP2234B ±0.3 ±2.5 ±0.02 ±1.0 ±50 0V ≤ VCM ≤ 1.5V 91 Power Supply Rejection Ratio 1.8V ≤ V+ ≤ 5.5V VCM = 0V 84 84 120 CMVR Common Mode Voltage Range CMRR ≥ 77 dB −0.2 −0.2 AVOL Large Signal Voltage Gain VO = 0.3V to 2.2V 104 104 CMRR ≥ 76 dB RL = 10 kΩ to IO IS Output Swing High pA ±5 77 76 VO μV/°C V+/2 dB dB 1.7 1.7 120 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 fA V dB 50 50 50 50 mV from either rail mA 32 44 46 µA 2.5V AC Electrical Characteristics (Note 4) Unless otherwise specified, all limits are 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 Typ (Note 6) (Note 5) GBWP 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 64 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 Density f = 1 kHz THD+N Total Harmonic Distortion + Noise www.national.com 10 f = 100 Hz, RL = 10 kΩ 4 0.005 nV/ μVPP fA/ % (Note 4) Unless otherwise specified, all limits are guaranteed for TA = 25°C, V+ = 2.0V, 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 Conditions Typ (Note 5) Max (Note 6) Units ±10 ±210 ±300 μV LMP2234A ±0.3 ±1.0 LMP2234B ±0.3 ±2.5 IBIAS Input Bias Current IOS Input Offset Current CMRR Common Mode Rejection Ratio 0V ≤ VCM ≤ 0.7V PSRR Power Supply Rejection Ratio 1.8V ≤ V+ ≤ 5.5V VCM = 0V CMVR Common Mode Voltage Range CMRR ≥ 77 dB AVOL Large Signal Voltage Gain VO = 0.3V to 1.7V Min (Note 6) μV/°C ±0.02 pA ±5 fA 92 dB 120 dB −0.2 to 1.2 V 84 84 120 dB RL = 10 kΩ to V+/2 VO IO IS Output Swing High 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 Sinking, VO to V+ VIN(diff) = −100 mV 5 Supply Current mV from either rail mA 31 44 46 µA 2.0V AC Electrical Characteristics (Note 4) Unless otherwise is specified, all limits are guaranteed for TA = 25°C, V+ = 2.0V, V− = 0V, VCM = VO = V+/2, and RL > 1 MΩ. Boldface limits apply at the temperature extremes. Symbol Parameter Conditions GBWP 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 2.4 0.1 Hz to 10 Hz in Input-Referred Current Noise Density f = 1 kHz THD+N Total Harmonic Distortion + Noise 10 f = 100 Hz, RL = 10 kΩ 5 0.005 nV/ μVPP fA/ % www.national.com LMP2234 Quad 2.0V DC Electrical Characteristics LMP2234 Quad 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 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: All limits are guaranteed by testing, statistical analysis or design. Note 7: The short circuit test is a momentary open loop test. Ordering Information Package Part Number Temperature Range Package Marking Transport Media LMP2234AMA 250 Units Tape and Reel LMP2234AMA 55 Units/Rail LMP2234AMAE 14-Pin SOIC LMP2234AMAX 2.5k Units Tape and Reel LMP2234BMA 55 Units/Rail LMP2234BMAE LMP2234BMAX LMP2234AMT LMP2234AMTE 14-Pin TSSOP LMP2234BMA 250 Units Tape and Reel -40°C to 125°C 94 Units/Rail LMP2234AMT 250 Units Tape and Reel 2.5k Units Tape and Reel LMP2234BMT 94 Units/Rail LMP2234BMT 250 Units Tape and Reel LMP2234BMTX 2.5k Units Tape and Reel Connection Diagram 14-Pin TSSOP/SOIC 20203404 www.national.com M14A 2.5k Units Tape and Reel LMP2234AMTX LMP2234BMTE NSC Drawing 6 MTC14 Unless otherwise Specified: TA = 25°C, VS = 5V, VCM = VS/2, where Offset Voltage Distribution TCVOS Distribution 20203407 20203411 Offset Voltage Distribution TCVOS Distribution 20203406 20203410 Offset Voltage Distribution TCVOS Distribution 20203405 20203409 7 www.national.com LMP2234 Quad Typical Performance Characteristics VS = V+ - V− LMP2234 Quad Offset Voltage Distribution TCVOS Distribution 20203408 20203403 Offset Voltage vs. VCM Offset Voltage vs. VCM 20203464 20203417 Offset Voltage vs. VCM Offset Voltage vs. VCM 20203418 20203465 www.national.com 8 LMP2234 Quad Offset Voltage vs. Temperature Offset Voltage vs. Supply Voltage 20203427 20203420 0.1 Hz to 10 Hz Voltage Noise 0.1 Hz to 10 Hz Voltage Noise 20203433 20203434 0.1 Hz to 10 Hz Voltage Noise 0.1 Hz to 10 Hz Voltage Noise 20203432 20203431 9 www.national.com LMP2234 Quad Input Bias Current vs. VCM Input Bias Current vs. VCM 20203455 20203456 Input Bias Current vs. VCM Input Bias Current vs. VCM 20203457 20203458 Input Bias Current vs. VCM Input Bias Current vs. VCM 20203459 www.national.com 20203460 10 LMP2234 Quad Input Bias Current vs. VCM Input Bias Current vs. VCM 20203461 20203462 PSRR vs. Frequency Supply Current vs. Supply Voltage (per channel) 20203466 20203412 Sinking Current vs. Supply Voltage Sourcing Current vs. Supply Voltage 20203413 20203414 11 www.national.com LMP2234 Quad Output Swing High vs. Supply Voltage Output Swing Low vs. Supply Voltage 20203415 20203416 Open Loop Frequency Response Open Loop Frequency Response 20203421 20203422 Phase Margin vs. Capacitive Load Slew Rate vs. Supply Voltage 20203463 www.national.com 20203430 12 LMP2234 Quad THD+N vs. Amplitude THD+N vs. Frequency 20203429 20203428 Large Signal Step Response Small Signal Step Response 20203424 20203423 Large Signal Step Response Small Signal Step Response 20203426 20203425 13 www.national.com LMP2234 Quad CMRR vs. Frequency Input Voltage Noise vs. Frequency 20203419 20203467 www.national.com 14 LMP2234 Quad Application Information Where: LMP2234 The LMP2234 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 LMP2234 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 operating voltage range of 1.8V to 5.5V along with the ultra-low supply current extend the battery run time in two ways. The extended power supply voltage range of 1.8V to 5.5V 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 LMP2234 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 LMP2234 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 LMP2234 to interface with high impedance sensors while generating negligible voltage noise. Thus the LMP2234 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 1.8V to 5.5V over the extensive temperature range of −40°C to 125°C makes the LMP2234 an excellent choice for low voltage precision applications with extensive temperature requirements. The LMP2234 is offered in the 14-pin TSSOP and 14-pin SOIC package. These small packages are ideal solutions for area constrained PC boards and portable electronics. The input current noise of the LMP2234 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 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. 20203448 FIGURE 1. Total Input Noise VOLTAGE NOISE REDUCTION . While The LMP2234 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 multiple 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 LMP2234 has very low input bias current, very low input current noise, and low input voltage noise for micropower amplifiers. As a result, this amplifier makes a great choice for circuits with high impedance sensor applications. shows the typical input noise of the LMP2234 as a function of source resistance at f = 1 kHz 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 LMP2234 Quad Figure 2 shows a schematic of this input voltage noise reduction circuit using the LMP2234. Typical resistor values are: RG = 10Ω, RF = 1 kΩ, and RO = 1 kΩ. 20203436 FIGURE 3. Instrumentation Amplifier There are two stages in this amplifier. The last stage, the output stage, is a differential amplifier. In an ideal case the two amplifiers of the first stage, the input stage, would be configured as buffers to isolate the inputs. However they cannot be connected as followers because of mismatch in 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 LMP2234. 20203446 (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, the 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 interest and the common signal is considered noise. A classic circuit implementation that is used 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 20203451 20203450 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 LMP2234 amplifiers is used to buffer the LM4140A's precision output voltage. The LM4140A is a precision voltage reference. The other three amplifiers in the LMP2234 are used to form an instrumentation amplifier. This instrumentation amplifier uses the LMP2234'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 LMP2234 Quad 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 GAUGE 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 LMP2234 Quad 20203468 FIGURE 5. Strain Gauge Bridge Amplifier 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 LMP2234 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 LMP2234, such as its very low offset voltage, low TCVOS, low input bias current, low CMRR, and low PSRR are other factors which make the LMP2234 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 very small traces of oxygen in ppm. Figure 6 shows a typical circuit used to amplify the output signal of an oxygen detector. The LMP2234 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 www.national.com 20203449 FIGURE 6. Precision Oxygen Sensor 18 LMP2234 Quad Physical Dimensions inches (millimeters) unless otherwise noted 14-Pin SOIC NS Package Number M14A 14-Pin TSSOP NS Package Number MTC14 19 www.national.com LMP2234 Quad Micropower, Precision, RRO, Operational Amplifier with CMOS Input Notes THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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