LPV521MGE

LPV521 Nanopower, 1.8V, RRIO, CMOS Input, Operational Amplifier August 24, 2009 LPV521 Nanopower, 1.8V, RRIO, CMOS Input, Operational Amplifier General Description The LPV521 is a single nanopower 552 nW amplifier designed for ultra long life battery applications. The operating voltage range of 1.6V to 5.5V coupled with typically 351 nA of supply current make it well suited for RFID readers and remote sensor nanopower applications. The device has input common mode voltage 0.1V over the rails, guaranteed TCVOS and voltage swing to the rail output performance. The LPV521 has a carefully designed CMOS input stage that outperforms competitors with typically 40 fA IBIAS currents. This low input current significantly reduces IBIAS and IOS errors introduced in megohm resistance, high impedance photodiode, and charge sense situations. The LPV521 is a member of the PowerWise® family and has an exceptional power-to-performance ratio. The wide input common mode voltage range, guaranteed 1 mV VOS and 3.5 µV/°C TCVOS enables accurate and stable measurement for both high side and low side current sensing. EMI protection was designed into the device to reduce sensitivity to unwanted RF signals from cell phones or other RFID readers. The LPV521 is offered in the 5-pin SC-70 package. Features (For VS = 5V, Typical unless otherwise noted) 400 nA (max) ■ Supply current at VCM = 0.3V 1.6V to 5.5V ■ Operating voltage range 3.5 µV/°C (max) ■ Low TCVOS 1 mV (max) ■ VOS 40 fA ■ Input bias current 109 dB ■ PSRR 102 dB ■ CMRR 132 dB ■ Open loop gain 6.2 kHz ■ Gain bandwidth product 2.4 V/ms ■ Slew rate 255 nV/√Hz ■ Input voltage noise at f = 100 Hz −40°C to 125°C ■ Temperature range Applications ■ ■ ■ ■ ■ ■ ■ ■ Wireless remote sensors Powerline monitoring Power meters Battery powered industrial sensors Micropower oxygen sensor and gas sensor Active RFID readers Zigbee based sensors for HVAC control Sensor network powered by energy scavenging Typical Application 30054578 30054577 © 2009 National Semiconductor Corporation 300545 www.national.com LPV521 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 Charge-Device Model Any pin relative to VIN+, IN-, OUT Pins V+, V-, OUT Pins Differential Input Voltage (VIN+ - VIN-) 2000V 200V 1000V 6V, −0.3V V+ + 0.3V, V– – 0.3V 40mA ±300 mV (Note 4) Storage Temperature Range Junction Temperature (Note 3) Mounting Temperature Infrared or Convection (30 sec.)   Wave Soldering Lead Temp. (4 sec.) −65°C to 150°C 150°C 260°C 260°C Operating Ratings Temperature Range (Note 3) Supply Voltage (VS = V+ - V−) (Note 1) −40°C to 125°C 1.6V to 5.5V 456 °C/W Package Thermal Resistance (θJA) (Note 3) 5-Pin SC-70 1.8V DC Electrical Characteristics Symbol VOS Parameter Input Offset Voltage 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. Conditions VCM = 0.3V VCM = 1.5V TCVOS IBIAS IOS CMRR Input Offset Voltage Drift (Note 9) Input Bias Current Input Offset Current Common Mode Rejection Ratio 0V ≤ VCM ≤ 1.8V 0V ≤ VCM ≤ 0.7V 1.2V ≤ VCM ≤ 1.8V PSRR CMVR AVOL VO Power Supply Rejection Ratio Common Mode Voltage Range Large Signal Voltage Gain Output Swing High Output Swing Low IO Output Current (Note 7) 1.6V ≤ V+ ≤ 5.5V VCM = 0.3V CMRR ≥ 67 dB CMRR ≥ 60 dB VO = 0.5V to 1.3V RL = 100 kΩ to V+/2 RL = 100 kΩ to V+/2 VIN(diff) = 100 mV RL = 100 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 VCM = 0.3V VCM = 1.5V 1 0.5 1 0.5 66 60 75 74 75 53 85 76 0 0 74 73 125 2 2 3 3 345 472 400 580 600 850 mA 50 50 50 50 Min (Note 6) Typ (Note 5) 0.1 0.1 ±0.4 0.01 10 92 101 120 109 1.8 1.8 dB Max (Note 6) ±1.0 ±1.23 ±1.0 ±1.23 ±3 ±1 ±50 Units mV μV/°C pA fA dB V dB mV from either rail nA www.national.com 2 LPV521 1.8V AC Electrical Characteristics Symbol GBW SR θm Gm en in Parameter Gain-Bandwidth Product Slew Rate Phase Margin Gain Margin (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. Conditions CL = 20 pF, RL = 100 kΩ AV = +1, VIN = 0V to 1.8V Falling Edge Rising Edge Min (Note 6) Typ (Note 5) 6.1 2.9 2.3 72 19 265 24 100 Max (Note 6) Units kHz V/ms deg dB nV/ μVPP fA/ CL = 20 pF, RL = 100 kΩ CL = 20 pF, RL = 100 kΩ 0.1 Hz to 10 Hz f = 100 Hz Input-Referred Voltage Noise Density f = 100 Hz Input-Referred Voltage Noise Input-Referred Current Noise 3.3V DC Electrical Characteristics Symbol VOS Parameter Input Offset Voltage (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. Conditions VCM = 0.3V VCM = 3V TCVOS IBIAS IOS CMRR Input Offset Voltage Drift (Note 9) Input Bias Current Input Offset Current Common Mode Rejection Ratio 0V ≤ VCM ≤ 3.3V 0V ≤ VCM ≤ 2.2V 2.7V ≤ VCM ≤ 3.3V PSRR CMVR AVOL VO Power Supply Rejection Ratio Common Mode Voltage Range Large Signal Voltage Gain Output Swing High Output Swing Low IO Output Current (Note 7) 1.6V ≤ V+ ≤ 5.5V VCM = 0.3V CMRR ≥ 72 dB CMRR ≥ 70 dB VO = 0.5V to 2.8V RL = 100 kΩ to V+/2 RL = 100 kΩ to V+/2 VIN(diff) = 100 mV RL = 100 kΩ to V+/2 VIN(diff) = −100 mV Sourcing, VO to V– VIN(diff) = 100 mV Sinking, VO to V+ VIN(diff) = −100 mV 5 4 5 4 72 70 78 75 77 76 85 76 −0.1 0 82 76 120 3 2 11 12 mA 50 50 50 50 Min (Note 6) Typ (Note 5) 0.1 0.1 ±0.4 0.01 20 97 106 121 109 3.4 3.3 Max (Note 6) ±1.0 ±1.23 ±1.0 ±1.23 ±3 ±1 ±50 Units mV μV/°C pA fA dB dB V dB mV from either rail 3 www.national.com LPV521 Symbol IS Parameter Supply Current VCM = 0.3V VCM = 3V Conditions Min (Note 6) Typ (Note 5) 346 471 Max (Note 6) 400 600 600 860 Units nA 3.3V AC Electrical Characteristics Symbol GBW SR θm Gm en in Parameter Gain-Bandwidth Product Slew Rate Phase Margin Gain Margin (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 = 100 kΩ AV = +1, VIN = 0V to 3.3V Falling Edge Rising Edge Min (Note 6) Typ (Note 5) 6.2 2.9 2.5 73 19 259 22 100 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 = 100 Hz Input-Referred Voltage Noise Density f = 100 Hz Input-Referred Voltage Noise Input-Referred Current Noise 5V DC Electrical Characteristics Symbol VOS Parameter Input Offset Voltage (Note 4) Unless otherwise specified, all limits guaranteed for TA = 25°C, V+ = 5V, V− = 0V, VCM = VO = V+/2, and RL > 1MΩ. Boldface limits apply at the temperature extremes. Conditions VCM = 0.3V VCM = 4.7V TCVOS IBIAS IOS CMRR Input Offset Voltage Drift (Note 9) Input Bias Current Input Offset Current Common Mode Rejection Ratio 0V ≤ VCM ≤ 5.0V 0V ≤ VCM ≤ 3.9V 4.4V ≤ VCM ≤ 5.0V PSRR CMVR AVOL VO Power Supply Rejection Ratio Common Mode Voltage Range Large Signal Voltage Gain Output Swing High Output Swing Low 1.6V ≤ V+ ≤ 5.5V VCM = 0.3V CMRR ≥ 75 dB CMRR ≥ 74 dB VO = 0.5V to 4.5V RL = 100 kΩ to V+/2 RL = 100 kΩ to V+/2 VIN(diff) = 100 mV RL = 100 kΩ to VIN (diff) = −100 mV V+/2 4 Min (Note 6) Typ (Note 5) 0.1 0.1 ±0.4 0.04 60 Max (Note 6) ±1.0 ±1.23 ±1.0 ±1.23 ±3.5 ±1 ±50 Units mV μV/°C pA fA 75 74 84 80 77 76 85 76 −0.1 0 84 76 102 108 115 109 5.1 5 132 3 3 50 50 50 50 dB dB V dB mV from either rail www.national.com LPV521 Symbol IO Parameter Output Current (Note 7) Conditions Sourcing, VO to V− VIN(diff) = 100 mV Sinking, VO to V+ VIN(diff) = −100 mV Min (Note 6) 15 8 15 8 Typ (Note 5) 23 22 351 475 Max (Note 6) Units mA IS Supply Current VCM = 0.3V VCM = 4.7V 400 620 600 870 nA 5V AC Electrical Characteristics Symbol GBW SR Parameter Gain-Bandwidth Product Slew Rate (Note 4) Unless otherwise specified, all limits guaranteed for TA = 25°C, V+ = 5V, V− = 0V, VCM = VO = V+/2, and RL > 1MΩ. Boldface limits apply at the temperature extremes. Conditions CL = 20 pF, RL = 100 kΩ AV = +1, VIN = 0V to 5V Falling Edge Rising Edge θm Gm en in EMIRR Phase Margin Gain Margin CL = 20 pF, RL = 100 kΩ CL = 20 pF, RL = 100 kΩ 0.1 Hz to 10 Hz f = 100 Hz VRF_PEAK = 100 mVP (−20 dBP), f = 400 MHz VRF_PEAK = 100 mVP (−20 dBP), f = 900 MHz VRF_PEAK = 100 mVP (−20 dBP), f = 1800 MHz VRF_PEAK = 100 mVP (−20 dBP), f = 2400 MHz 1.1 1.2 1.1 1.2 Min (Note 6) Typ (Note 5) 6.2 2.7 2.4 73 20 255 22 100 121 121 124 142 dB V/ms Max (Note 6) Units kHz deg dB nV/ μVPP fA/ Input-Referred Voltage Noise Density f = 100 Hz Input-Referred Voltage Noise Input-Referred Current Noise EMI Rejection Ratio, IN+ and IN− (Note 8) 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. Note 8: The EMI Rejection Ratio is defined as EMIRR = 20log (VRF_PEAK/ΔVOS). Note 9: The offset voltage average drift is determined by dividing the change in VOS at the temperature extremes by the total temperature change. 5 www.national.com LPV521 Connection Diagram 5-Pin SC-70 30054581 Top View Ordering Information Package 5-Pin SC-70 Part Number LPV521MG LPV521MGE LPV521MGX AHA Package Marking Transport Media 1k Units Tape and Reel 250 Units Tape and Reel 3k Units Tape and Reel MAA05A NSC Drawing www.national.com 6 LPV521 Typical Performance Characteristics Supply Current vs. Supply Voltage At TJ = 25°C, unless otherwise specified. Supply Current vs. Supply Voltage 30054504 30054535 Offset Voltage Distribution TCVOS Distribution 30054573 30054574 Offset Voltage Distribution TCVOS Distribution 30054575 30054567 7 www.national.com LPV521 Offset Voltage Distribution TCVOS Distribution 30054568 30054569 Input Offset Voltage vs. Input Common Mode Input Offset Voltage vs. Input Common Mode 30054506 30054505 Input Offset Voltage vs. Input Common Mode Input Offset Voltage vs. Supply Voltage 30054507 30054537 www.national.com 8 LPV521 Input Offset Voltage vs. Supply Voltage Input Offset Voltage vs. Output Voltage 30054538 30054539 Input Offset Voltage vs. Output Voltage Input Offset Voltage vs. Output Voltage 30054540 30054541 Input Offset Voltage vs. Sourcing Current Input Offset Voltage vs. Sourcing Current 30054542 30054543 9 www.national.com LPV521 Input Offset Voltage vs. Sourcing Current Input Offset Voltage vs. Sinking Current 30054544 30054545 Input Offset Voltage vs. Sinking Current Input Offset Voltage vs. Sinking Current 30054546 30054547 Sourcing Current vs. Output Voltage Sinking Current vs. Output Voltage 30054508 30054509 www.national.com 10 LPV521 Sourcing Current vs. Output Voltage Sinking Current vs. Output Voltage 30054510 30054511 Sourcing Current vs. Output Voltage Sinking Current vs. Output Voltage 30054552 30054553 Sourcing Current vs. Supply Voltage Sinking Current vs. Supply Voltage 30054548 30054549 11 www.national.com LPV521 Output Swing High vs. Supply Voltage Output Swing Low vs. Supply Voltage 30054550 30054551 Input Bias Current vs. Common Mode Voltage Input Bias Current vs. Common Mode Voltage 30054514 30054515 Input Bias Current vs. Common Mode Voltage Input Bias Current vs. Common Mode Voltage 30054516 30054564 www.national.com 12 LPV521 Input Bias Current vs. Common Mode Voltage Input Bias Current vs. Common Mode Voltage 30054565 30054566 PSRR vs. Frequency CMRR vs. Frequency 30054527 30054560 Frequency Response vs. Temperature Frequency Response vs. Temperature 30054523 30054524 13 www.national.com LPV521 Frequency Response vs. Temperature Frequency Response vs. RL 30054522 30054520 Frequency Response vs. RL Frequency Response vs. RL 30054521 30054519 Frequency Response vs. CL Frequency Response vs. CL 30054517 30054518 www.national.com 14 LPV521 Frequency Response vs. CL Slew Rate vs. Supply Voltage 30054513 30054536 Voltage Noise vs. Frequency 0.1 to 10 Hz Time Domain Voltage Noise 30054526 30054561 0.1 to 10 Hz Time Domain Voltage Noise 0.1 to 10 Hz Time Domain Voltage Noise 30054562 30054563 15 www.national.com LPV521 Small Signal Pulse Response Small Signal Pulse Response 30054531 30054530 Large Signal Pulse Response Large Signal Pulse Response 30054532 30054533 Overload Recovery Waveform EMIRR vs. Frequency 30054534 30054572 www.national.com 16 LPV521 Application Information The LPV521 is fabricated with National Semiconductor's state-of-the-art VIP50 process. This proprietary process dramatically improves the performance of National Semiconductor's low-power and low-voltage operational amplifiers. The following sections showcase the advantages of the VIP50 process and highlight circuits which enable ultralow power consumption. 60 HZ TWIN T NOTCH FILTER Small signals from transducers in remote and distributed sensing applications commonly suffer strong 60 Hz interference from AC power lines. The circuit of Figure 1 notches out the 60 Hz and provides a gain AV = 2 for the sensor signal represented by a 1 kHz sine wave. Similar stages may be cascaded to remove 2nd and 3rd harmonics of 60 Hz. Thanks to the nA power consumption of the LPV521, even 5 such circuits can run for 9.5 years from a small CR2032 lithium cell. These batteries have a nominal voltage of 3V and an end of life voltage of 2V. With an operating voltage from 1.6V to 5.5V the LPV521 can function over this voltage range. The notch frequency is set by F0 = 1/2πRC. To achieve a 60 Hz notch use R = 10 MΩ and C = 270 pF. If eliminating 50 Hz noise, which is common in European systems, use R = 11.8 MΩ and C = 270 pF. The Twin T Notch Filter works by having two separate paths from VIN to the amplifier’s input. A low frequency path through the resistors R - R and another separate high frequency path through the capacitors C - C. However, at frequencies around the notch frequency, the two paths have opposing phase angles and the two signals will tend to cancel at the amplifier’s input. To ensure that the target center frequency is achieved and to maximize the notch depth (Q factor) the filter needs to be as balanced as possible. To obtain circuit balance, while overcoming limitations of available standard resistor and capacitor values, use passives in parallel to achieve the 2C and R/2 circuit requirements for the filter components that connect to ground. To make sure passive component values stay as expected clean board with alcohol, rinse with deionized water, and air dry. Make sure board remains in a relatively low humidity environment to minimize moisture which may increase the conductivity of board components. Also large resistors come with considerable parasitic stray capacitance which effects can be reduced by cutting out the ground plane below components of concern. Large resistors are used in the feedback network to minimize battery drain. When designing with large resistors, resistor thermal noise, op amp current noise, as well as op amp voltage noise, must be considered in the noise analysis of the circuit. The noise analysis for the circuit in Figure 1 can be done over a bandwidth of 5 kHz, which takes the conservative approach of overestimating the bandwidth (LPV521 typical GBW/AV is lower). The total noise at the output is approximately 800 µVpp, which is excellent considering the total consumption of the circuit is only 540 nA. The dominant noise terms are op amp voltage noise (550 µVpp), current noise through the feedback network (430 µVpp), and current noise through the notch filter network (280 µVpp). Thus the total circuit's noise is below 1/2 LSB of a 10 bit system with a 2 V reference, which is 1 mV. 30054576 FIGURE 1. 60 Hz Notch Filter 30054577 FIGURE 2. 60 Hz Notch Filter Waveform BATTERY CURRENT SENSING The rail-to-rail common mode input range and the very low quiescent current make the LPV521 ideal to use in high side and low side battery current sensing applications. The high side current sensing circuit in Figure 3 is commonly used in a battery charger to monitor the charging current in order to prevent over charging. A sense resistor RSENSE is connected in series with the battery. The theoretical output voltage of the circuit is VOUT = [ (RSENSE × R3) / R1 ] × ICHARGE. In reality, however, due to the finite Current Gain, β, of the transistor the current that travels through R3 will not be ICHARGE, but instead, will be α × ICHARGE or β/( β+1) × ICHARGE. A Darlington pair can be used to increase the β and performance of the measuring circuit. Using the components shown in Figure 3 will result in VOUT ≈ 4000 Ω × ICHARGE. This is ideal to amplify a 1 mA ICHARGE to near full scale of an ADC with VREF at 4.1V. A resistor, R2 is used at the non-inverting input of the amplifier, with the same value as R1 to minimize offset voltage. Selecting values per Figure 3 will limit the current traveling through the R1 – Q1 – R3 leg of the circuit to under 1 µA which is on the same order as the LPV521 supply current. Increasing resistors R1 , R2 , and R3 will decrease the measuring circuit supply current and extend battery life. Decreasing RSENSE will 17 www.national.com LPV521 minimize error due to resistor tolerance, however, this will also decrease VSENSE = ICHARGE × RSENSE, and in turn the amplifier offset voltage will have a more significant contribution to the total error of the circuit. With the components shown in Figure 3 the measurement circuit supply current can be kept below 1.5 µA and measure 100 µA to 1 mA.. TCVOS , low input bias current, high CMRR, and high PSRR are other factors which make the LPV521 a great choice for this application. 30054559 FIGURE 4. Precision Oxygen Sensor INPUT STAGE The LPV521 has a rail-to-rail input which provides more flexibility for the system designer. Rail-to-rail input is achieved by using in parallel, one PMOS differential pair and one NMOS differential pair. When the common mode input voltage (VCM) is near V+, the NMOS pair is on and the PMOS pair is off. When VCM is near V−, the NMOS pair is off and the PMOS pair is on. When VCM is between V+ and V−, internal logic decides how much current each differential pair will get. This special logic ensures stable and low distortion amplifier operation within the entire common mode voltage range. Because both input stages have their own offset voltage (VOS) characteristic, the offset voltage of the LPV521 becomes a function of VCM. VOS has a crossover point at 1.0V below V+. Refer to the ’VOS vs. VCM’ curve in the Typical Performance Characteristics section. Caution should be taken in situations where the input signal amplitude is comparable to the VOS value and/or the design requires high accuracy. In these situations, it is necessary for the input signal to avoid the crossover point. In addition, parameters such as PSRR and CMRR which involve the input offset voltage will also be affected by changes in VCM across the differential pair transition region. OUTPUT STAGE The LPV521 output voltage swings 3 mV from rails at 3.3V supply, which provides the maximum possible dynamic range at the output. This is particularly important when operating on low supply voltages. The LPV521 Maximum Output Voltage Swing defines the maximum swing possible under a particular output load. The LPV521 output swings 50 mV from the rail at 5V supply with an output load of 100 kΩ. DRIVING CAPACITIVE LOAD The LPV521 is internally compensated for stable unity gain operation, with a 6.2 kHz typical gain bandwidth. However, the unity gain follower is the most sensitive configuration to capacitive load. 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 30054502 FIGURE 3. High Side Current Sensing 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 4 shows a typical circuit used to amplify the output of an oxygen detector. The LPV521 makes an excellent choice for this application as it only draws 345 nA of current and operates on supply voltages down to 1.6V. 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 nanopower LPV521 means minimal power usage by the op amp and it enhances the battery life. With the components shown in Figure 4 the circuit can consume less than 0.5 µA of current ensuring that even batteries used in compact portable electronics, with low mAh charge ratings, could last beyond the life of the oxygen sensor. The precision specifications of the LPV521, such as its very low offset voltage, low www.national.com 18 LPV521 peaking in the transfer and, when there is too much peaking, the op amp might start oscillating. In order to drive heavy capacitive loads, an isolation resistor, RISO, should be used, as shown in Figure 5. By using this isolation resistor, the capacitive load is isolated from the amplifier’s output. 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. 30054555 FIGURE 5. Resistive Isolation of Capacitive Load Recommended minimum values for RISO are given in the following table, for 5V supply. Figure 6 shows the typical response obtained with the CL = 50 pF and RISO = 154 kΩ. The other values of RISO in the table were chosen to achieve similar dampening at their respective capacitive loads. Notice that for the LPV521 with larger CL a smaller RISO can be used for stability. However, for a given CL a larger RISO will provide a more damped response. For capacitive loads of 20 pF and below no isolation resistor is needed. CL 0 – 20 pF 50 pF 100 pF 500 pF 1 nF 5 nF 10 nF RISO not needed 154 kΩ 118 kΩ 52.3 kΩ 33.2 kΩ 17.4 kΩ 13.3 kΩ EMI SUPPRESSION The near-ubiquity of cellular, bluetooth, and Wi-Fi signals and the rapid rise of sensing systems incorporating wireless radios make electromagnetic interference (EMI) an evermore important design consideration for precision signal paths. Though RF signals lie outside the op amp band, RF carrier switching can modulate the DC offset of the op amp. Also some common RF modulation schemes can induce downconverted components. The added DC offset and the induced signals are amplified with the signal of interest and thus corrupt the measurement. The LPV521 uses on chip filters to reject these unwanted RF signals at the inputs and power supply pins; thereby preserving the integrity of the precision signal path. Twisted pair cabling and the active front-end’s common-mode rejection provide immunity against low frequency noise (i.e. 60 Hz or 50 Hz mains) but are ineffective against RF interference. Even a few centimeters of PCB trace and wiring for sensors located close to the amplifier can pick up significant 1 GHz RF. The integrated EMI filters of the LPV521 reduce or eliminate external shielding and filtering requirements, thereby increasing system robustness. A larger EMIRR means more rejection of the RF interference. For more information on EMIRR, please refer to AN-1698. POWER SUPPLIES AND LAYOUT The LPV521 operates from a single 1.6V to 5.5V power supply. It is recommended to bypass the power supplies with a 0.1 μF ceramic capacitor placed close to the V+ and V− pins. Ground layout improves performance by decreasing the amount of stray capacitance and noise at the op amp's inputs and outputs. To decrease stray capacitance, minimize PC board trace lengths and resistor leads, and place external components close to the op amps' pins. 30054580 FIGURE 6. Step Response 19 www.national.com LPV521 Physical Dimensions inches (millimeters) unless otherwise noted 5-Pin SC-70 NS Package Number MAA05A www.national.com 20 LPV521 Notes 21 www.national.com LPV521 Nanopower, 1.8V, RRIO, CMOS Input, Operational Amplifier 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 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 WEBENCH® Tools App Notes Reference Designs Samples Eval Boards Packaging Green Compliance Distributors Quality and Reliability Feedback/Support Design Made Easy Solutions Mil/Aero PowerWise® Design University 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/training Serial Digital Interface (SDI) www.national.com/sdi www.national.com/wireless www.national.com/tempsensors SolarMagic™ THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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