OPA380 OPA2380 SBOS291E − NOVEMBER 2003 − REVISED NOVEMBER 2004 Precision, High-Speed Transimpedance Amplifier FEATURES D D D D D D D D D D D DESCRIPTION > 1MHz TRANSIMPEDANCE BANDWIDTH EXCELLENT LONG-TERM VOS STABILITY BIAS CURRENT: 50pA (max) OFFSET VOLTAGE: 25µV (max) INPUT CURRENT RANGE: 10nA to 1mA DRIFT: 0.1µV/°C (max) GAIN BANDWIDTH: 90MHz QUIESCENT CURRENT: 6.5mA SUPPLY RANGE: 2.7V to 5.5V SINGLE AND DUAL VERSIONS MicroSize PACKAGE: MSOP-8 APPLICATIONS D D D D PHOTODIODE MONITORING PRECISION I/V CONVERSION OPTICAL AMPLIFIERS CAT-SCANNER FRONT-END +5V 7 OPA380 6 VOUT (0V to 4.4V) RP (Optional Pulldown Resistor) Photodiode 1MΩ The signal bandwidth of a transimpedance amplifier depends largely on the GBW of the amplifier and the parasitic capacitance of the photodiode, as well as the feedback resistor. The 90MHz GBW of the OPA380 enables a transimpedance bandwidth of > 1MHz in most configurations. The OPA380 is ideally suited for fast control loops for power level on an optical fiber. As a result of the high precision and low-noise characteristics of the OPA380, a dynamic range of 5 decades can be achieved. This capability allows the measurement of signal currents in the order of 10nA, and up to 1mA in a single I/V conversion stage. In contrast to logarithmic amplifiers, the OPA380 provides very wide bandwidth throughout the full dynamic range. By using an external pulldown resistor to –5V, the output voltage range can be extended to include 0V. RF 2 The OPA380 family of transimpedance amplifiers provides high-speed (90MHz Gain Bandwidth [GBW]) operation, with extremely high precision, excellent long-term stability, and very low 1/f noise. It is ideally suited for high-speed photodiode applications. The OPA380 features an offset voltage of 25µV, offset drift of 0.1µV/°C, and bias current of 50pA. The OPA380 far exceeds the offset, drift, and noise performance that conventional JFET op amps provide. 67pF −5V 100kΩ 3 75pF 4 The OPA380 (single) is available in MSOP-8 and SO-8 packages. The OPA2380 (dual) is available in the miniature MSOP-8 package. They are specified from –40°C to +125°C. OPA380 RELATED DEVICES PRODUCT FEATURES OPA300 150MHz CMOS, 2.7V to 5.5V Supply OPA350 500µV VOS, 38MHz, 2.5V to 5V Supply OPA335 10µV VOS, Zero-Drift, 2.5V to 5V Supply OPA132 16MHz GBW, Precision FET Op Amp, ±15V OPA656/7 230MHz, Precision FET, ±5V LOG112 LOG amp, 7.5 decades, ±4.5V to ±18V Supply LOG114 LOG amp, 7.5 decades, ±2.25V to ±5.5V Supply IVC102 Precision Switched Integrator DDC112 Dual Current Input, 20-Bit ADC Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet. All trademarks are the property of their respective owners. Copyright 2003-2004, Texas Instruments Incorporated ! ! www.ti.com "#$ %"#$ www.ti.com SBOS291E − NOVEMBER 2003 − REVISED NOVEMBER 2004 ABSOLUTE MAXIMUM RATINGS(1) ELECTROSTATIC DISCHARGE SENSITIVITY Voltage Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +7V Signal Input Terminals(2), Voltage . . . . . . . . . . −0.5V to (V+) + 0.5V Current . . . . . . . . . . . . . . . . . . . . . ±10mA Short-Circuit Current(3) . . . . . . . . . . . . . . . . . . . . . . . . Continuous Operating Temperature Range . . . . . . . . . . . . . . . −40°C to +125°C Storage Temperature Range . . . . . . . . . . . . . . . . . −65°C to +150°C Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +150°C Lead Temperature (soldering, 10s) . . . . . . . . . . . . . . . . . . . . . +300°C ESD Rating (Human Body Model) . . . . . . . . . . . . . . . . . . . . . . . 2000V (1) Stresses above these ratings may cause permanent damage. Exposure to absolute maximum conditions for extended periods may degrade device reliability. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those specified is not implied. (2) Input terminals are diode clamped to the power-supply rails. Input signals that can swing more than 0.5V beyond the supply rails should be current limited to 10mA or less. (3) Short-circuit to ground; one amplifier per package. This integrated circuit can be damaged by ESD. Texas Instruments recommends that all integrated circuits be handled with appropriate precautions. Failure to observe proper handling and installation procedures can cause damage. ESD damage can range from subtle performance degradation to complete device failure. Precision integrated circuits may be more susceptible to damage because very small parametric changes could cause the device not to meet its published specifications. PACKAGE/ORDERING INFORMATION(1) PACKAGE-LEAD OPA380 MSOP-8 AUN OPA380 SO-8 OPA380A OPA2380 MSOP-8 BBX (1) For the most current package and ordering information, see the Package Option Addendum located at the end of this data sheet. PIN ASSIGNMENTS Top View OPA380 OPA2380 NC (1) 1 8 NC (1) Out A 1 8 V+ −In 2 7 V+ − In A 2 7 Out B +In 3 6 Out +In A 3 6 − In B V− 4 5 NC (1) V− 4 5 +In B MSOP−8, SO−8 NOTES: (1) NC indicates no internal connection. 2 PACKAGE MARKING PRODUCT MSOP−8 "#$ %"#$ www.ti.com SBOS291E − NOVEMBER 2003 − REVISED NOVEMBER 2004 ELECTRICAL CHARACTERISTICS: OPA380 (SINGLE), VS = 2.7V to 5.5V Boldface limits apply over the temperature range, TA = −40°C to +125°C. All specifications at TA = +25°C, RL = 2kΩ connected to VS/2, and VOUT = VS/2, unless otherwise noted. OPA380 PARAMETER OFFSET VOLTAGE Input Offset Voltage Drift vs Power Supply Over Temperature Long-Term Stability(1) CONDITION VOS dVOS/dT PSRR MIN VS = +5V, VCM = 0V VS = +2.7V to +5.5V, VCM = 0V VS = +2.7V to +5.5V, VCM = 0V NOISE Input Voltage Noise, f = 0.1Hz to 10Hz Input Voltage Noise Density, f = 10kHz Input Voltage Noise Density, f > 1MHz Input Current Noise Density, f = 10kHz INPUT VOLTAGE RANGE Common-Mode Voltage Range Common-Mode Rejection Ratio IB VCM = VS/2 FREQUENCY RESPONSE Gain-Bandwidth Product Slew Rate Settling Time, 0.01%(3) Overload Recovery Time(4)(5) OUTPUT Voltage Output Swing from Positive Rail Voltage Output Swing from Negative Rail Voltage Output Swing from Positive Rail Voltage Output Swing from Negative Rail Output Current Short-Circuit Current Capacitive Load Drive Open-Loop Output Impedance POWER SUPPLY Specified Voltage Range Quiescent Current Over Temperature TEMPERATURE RANGE Specified and Operating Range Storage Range Thermal Resistance MSOP-8, SO-8 UNITS 4 0.03 2.4 25 0.1 10 10 µV µV/°C µV/V µV/V IOS VCM = VS/2 en en en in VCM CMRR VS = +5V, VCM = 0V VS = +5V, VCM = 0V VS = +5V, VCM = 0V VS = +5V, VCM = 0V (V−) < VCM < (V+) – 1.8V AOL 0.1V < VO < (V+) − 0.7V, VS = 5V, VCM = VS/2 0.1V < VO < (V+) − 0.6V, VS = 5V, VCM = VS/2, TA = -40°C to +85°C 0V < VO < (V+) − 0.7V, VS = 5V, VCM = 0V, RP = 2kΩ to −5V(2) 0V < VO < (V+) − 0.6V, VS = 5V, VCM = 0V, RP = 2kΩ to −5V(2), TA = -40°C to +85°C µV/V 3 ±50 Typical Characteristics 6 ±100 V− 100 pA pA µVPP nV/√Hz nV/√Hz fA/√Hz 3 67 5.8 10 INPUT IMPEDANCE Differential Capacitance Common-Mode Resistance and Inverting Input Capacitance OPEN-LOOP GAIN Open-Loop Voltage Gain MAX See Note (1) 1 Channel Separation, dc INPUT BIAS CURRENT Input Bias Current Over Temperature Input Offset Current TYP 110 (V+) − 1.8V V dB 1.1 pF 1013 || 3 Ω || pF 110 130 dB 110 130 dB 106 120 dB 106 120 dB 90 80 2 100 MHz V/µs µs ns CL = 50pF GBW SR tS G = +1 VS = +5V, 4V Step, G = +1 VIN • G = > VS RL = 2kΩ RL = 2kΩ RP = 2kΩ to −5V(2) RP = 2kΩ to −5V(2) IOUT ISC CLOAD RO VS IQ f = 1MHz, IO = 0A 400 600 60 100 400 600 −20 0 See Typical Characteristics 150 See Typical Characteristics 40 2.7 IO = 0A 6.5 −40 −65 mV mV mV mV mA Ω 5.5 8.3 8.8 V mA mA +125 +150 °C °C qJA 150 °C/W (1) 300-hour life test at 150°C demonstrated randomly distributed variation approximately equal to measurement repeatability of 1µV. (2) Tested with output connected only to R , a pulldown resistor connected between V P OUT and −5V, as shown in Figure 5. See also applications section, Achieving Output Swing to Ground. (3) Transimpedance frequency of 1MHz. (4) Time required to return to linear operation. (5) From positive rail. 3 "#$ %"#$ www.ti.com SBOS291E − NOVEMBER 2003 − REVISED NOVEMBER 2004 ELECTRICAL CHARACTERISTICS: OPA2380 (DUAL), VS = 2.7V to 5.5V Boldface limits apply over the temperature range, TA = −40°C to +125°C. All specifications at TA = +25°C, RL = 2kΩ connected to VS/2, and VOUT = VS/2, unless otherwise noted. OPA2380 PARAMETER OFFSET VOLTAGE Input Offset Voltage Drift vs Power Supply Over Temperature Long-Term Stability(1) CONDITION VOS dVOS/dT PSRR MIN VS = +5V, VCM = 0V VS = +2.7V to +5.5V, VCM = 0V VS = +2.7V to +5.5V, VCM = 0V NOISE Input Voltage Noise, f = 0.1Hz to 10Hz Input Voltage Noise Density, f = 10kHz Input Voltage Noise Density, f > 1MHz Input Current Noise Density, f = 10kHz INPUT VOLTAGE RANGE Common-Mode Voltage Range Common-Mode Rejection Ratio IB IB VCM = VS/2 VCM = VS/2 FREQUENCY RESPONSE Gain-Bandwidth Product Slew Rate Settling Time, 0.01%(3) Overload Recovery Time(4), (5) OUTPUT Voltage Output Swing from Positive Rail Voltage Output Swing from Negative Rail Voltage Output Swing from Positive Rail Voltage Output Swing from Negative Rail Output Current Short-Circuit Current Capacitive Load Drive Open-Loop Output Impedance POWER SUPPLY Specified Voltage Range Quiescent Current (per amplifier) Over Temperature TEMPERATURE RANGE Specified and Operating Range Storage Range Thermal Resistance MSOP-8 UNITS 4 0.03 2.4 25 0.1 10 10 µV µV/°C µV/V µV/V en en en in VS = +5V, VCM = 0V VS = +5V, VCM = 0V VS = +5V, VCM = 0V VS = +5V, VCM = 0V VCM CMRR (V−) < VCM < (V+) – 1.8V AOL 0.12V < VO < (V+) − 0.7V, VS = 5V, VCM = VS/2 0.12V < VO < (V+) − 0.6V, VS = 5V, VCM = VS/2, TA = -40°C to +85°C 0V < VO < (V+) − 0.7V, VS = 5V, VCM = 0V, RP = 2kΩ to −5V(2) 0V < VO < (V+) − 0.6V, VS = 5V, VCM = 0V, RP = 2kΩ to −5V(2), TA = -40°C to +85°C µV/V 3 ±50 3 ±200 Typical Characteristics V− 95 pA pA µVPP nV/√Hz nV/√Hz fA/√Hz 3 67 5.8 10 INPUT IMPEDANCE Differential Capacitance Common-Mode Resistance and Inverting Input Capacitance OPEN-LOOP GAIN Open-Loop Voltage Gain MAX See Note (1) 1 Channel Separation, dc INPUT BIAS CURRENT Input Bias Current, Inverting Input Noninverting Input Over Temperature TYP 105 (V+) − 1.8V V dB 1.1 pF 1013 || 3 Ω || pF 110 130 dB 110 130 dB 106 120 dB 106 120 dB 90 80 2 100 MHz V/µs µs ns CL = 50pF GBW SR tS G = +1 VS = +5V, 4V Step, G = +1 VIN • G = > VS RL = 2kΩ RL = 2kΩ RP = 2kΩ to −5V(2) RP = 2kΩ to −5V(2) IOUT ISC CLOAD RO VS IQ f = 1MHz, IO = 0A 400 600 80 120 400 600 −20 0 See Typical Characteristics 150 See Typical Characteristics 40 2.7 IO = 0A 7.5 −40 −65 mV mV mV mV mA Ω 5.5 9.5 10 V mA mA +125 +150 °C °C qJA 150 °C/W (1) 300-hour life test at 150°C demonstrated randomly distributed variation approximately equal to measurement repeatability of 1µV. (2) Tested with output connected only to R , a pulldown resistor connected between V P OUT and −5V, as shown in Figure 5. See also applications section, Achieving Output Swing to Ground. (3) Transimpedance frequency of 1MHz. (4) Time required to return to linear operation. (5) From positive rail. 4 "#$ %"#$ www.ti.com SBOS291E − NOVEMBER 2003 − REVISED NOVEMBER 2004 TYPICAL CHARACTERISTICS: VS = +2.7V to +5.5V All specifications at TA = +25°C, RL = 2kΩ connected to VS/2, and VOUT = VS/2, unless otherwise noted. POWER−SUPPLY REJECTION RATIO AND COMMON−MODE REJECTION vs FREQUENCY OPEN−LOOP GAIN AND PHASE vs FREQUENCY Gain 100 160 45 140 120 0 80 Phase −45 −90 60 40 −135 20 −180 0 −225 −20 −270 100M 10 100 1k 10k 100k 1M 10M Phase (_) Open−Loop Gain (dB) 120 90 PSRR, CMRR (dB) 140 100 80 PSRR 60 40 20 0 CMRR −20 0.1 1 10 INPUT VOLTAGE NOISE SPECTRAL DENSITY 1k 10k 100k 1M 10M 100M QUIESCENT CURRENT vs TEMPERATURE 1000 8 Quiescent Current (mA) 7 100 10 6 VS = +5.5V 5 4 VS = +2.7V 3 2 1 1 0 10 100 1k 10k 100k 1M −40 −25 10M 0 Frequency (Hz) 25 50 75 100 125 Temperature (_ C) QUIESCENT CURRENT vs SUPPLY VOLTAGE INPUT BIAS CURRENT vs TEMPERATURE 7 1000 Input Bias Current (pA) 6 Quiescent Current (mA) Input Voltage Noise (nV/√(Hz) 100 Frequency (Hz) Frequency (Hz) 5 4 3 2 100 10 1 0 1 2.7 3.0 3.5 4.0 4.5 Supply Voltage (V) 5.0 5.5 −40 −25 0 25 50 75 100 125 Temperature (_C) 5 "#$ %"#$ www.ti.com SBOS291E − NOVEMBER 2003 − REVISED NOVEMBER 2004 TYPICAL CHARACTERISTICS: VS = +2.7V to +5.5V (continued) All specifications at TA = +25°C, RL = 2kΩ connected to VS/2, and VOUT = VS/2, unless otherwise noted. INPUT BIAS CURRENT vs INPUT COMMON−MODE VOLTAGE OUTPUT VOLTAGE SWING vs OUTPUT CURRENT (V+) 25 (V+) −1 15 10 Output Swing (V) Input Bias Current (pA) 20 −IB 5 0 −5 +IB −10 −15 (V+) −2 +125_C +25_ C −40_ C (V−) +2 (V−) +1 −20 −25 (V−) 0 0.5 1.0 1.5 2.0 2.5 3.0 0 3.5 50 100 150 Output Current (mA) Input Common−Mode Voltage (V) SHORT−CIRCUIT CURRENT vs TEMPERATURE OFFSET VOLTAGE PRODUCTION DISTRIBUTION 200 150 100 +ISC Population Short−Circuit Current (mA) VS = 5V 50 0 −50 −ISC −100 −150 −40 −25 0 25 50 75 100 −25 −20 −15 −10 125 Temperature (_C) OFFSET VOLTAGE DRIFT PRODUCTION DISTRIBUTION −5 0 5 10 Offset Voltage (µV) 15 20 25 GAIN BANDWIDTH vs POWER SUPPLY VOLTAGE Population Gain Bandwidth (MHz) 95 90 85 80 75 70 −0.10 −0.08 −0.06 −0.04 −0.02 0 0.02 0.04 Offset Voltage Drift (µV/_C) 6 0.06 0.08 0.1 2.5 3.5 4.5 Power Supply Voltage (V) 5.5 "#$ %"#$ www.ti.com SBOS291E − NOVEMBER 2003 − REVISED NOVEMBER 2004 TYPICAL CHARACTERISTICS: VS = +2.7V to +5.5V (continued) All specifications at TA = +25°C, RL = 2kΩ connected to VS/2, and VOUT = VS/2, unless otherwise noted. TRANSIMPEDANCE AMP CHARACTERISTIC 140 Transimpedance Gain (V/A in dB) Circuit for Transimpedance Amplifier Characteristic curves on this page. CF RF CSTRAY OPA380 120 110 RF = 1MΩ 90 80 R F = 100kΩ 70 60 RF = 10kΩ 50 RF = 1kΩ RF = 1MΩ 100 90 80 R F = 100kΩ 70 60 RF = 10kΩ 50 Transimpedance Gain (V/A in dB) Transimpedance Gain (V/A in dB) CF = 18pF 10M 100M TRANSIMPEDANCE AMP CHARACTERISTIC CDIODE = 50pF RF = 10MΩ 120 110 CF = 0.5pF CF = 1.5pF CF = 4pF RF = 1kΩ CF = 12pF CSTRAY (parasitic) = 0.2pF 20 100 1k 10k 100k 1M Frequency (Hz) 10M CDIODE = 20pF RF = 10MΩ 130 120 110 RF = 1MΩ 100 90 R F = 100kΩ CF = 1pF 80 RF = 10kΩ 70 60 40 CF = 2.5pF RF = 1kΩ 50 CF = 7pF CSTRAY (parasitic) = 0.2pF 30 100 100M 1k 10k 100k 1M Frequency (Hz) 10M 100M TRANSIMPEDANCE AMP CHARACTERISTIC TRANSIMPEDANCE AMP CHARACTERISTIC 140 140 CDIODE = 10pF RF = 10MΩ 130 Transimpedance Gain (V/A in dB) Transimpedance Gain (V/A in dB) CF = 5pF 140 130 120 110 RF = 1MΩ 100 R F = 100kΩ CF = 0.5pF 80 RF = 10kΩ 70 60 CF = 2pF RF = 1kΩ 50 40 CF = 2pF 40 30 TRANSIMPEDANCE AMP CHARACTERISTIC 140 90 CF = 0.5pF 100 CSTRAY (parasitic) = 0.2pF 20 100 1k 10k 100k 1M Frequency (Hz) CDIODE 40 30 CDIODE = 100pF RF = 10MΩ 130 CF = 5pF CSTRAY (parasitic) = 0.2pF 30 100 1k 10k 100k 1M Frequency (Hz) 10M 100M CDIODE = 1pF RF = 10MΩ 130 120 RF = 1MΩ 110 100 90 R F = 100kΩ CF = 0.5pF 80 RF = 10kΩ 70 RF = 1kΩ CF = 1pF 60 50 CSTRAY (parasitic) = 0.2pF 40 100 1k 10k CF = 2.5pF 100k 1M Frequency (Hz) 10M 100M 7 "#$ %"#$ www.ti.com SBOS291E − NOVEMBER 2003 − REVISED NOVEMBER 2004 TYPICAL CHARACTERISTICS: VS = +2.7V to +5.5V (continued) All specifications at TA = +25°C, RL = 2kΩ connected to VS/2, and VOUT = VS/2, unless otherwise noted. SMALL−SIGNAL OVERSHOOT vs LOAD CAPACITANCE 50 SMALL−SIGNAL OVERSHOOT vs LOAD CAPACITANCE 50 2.5pF 2.5pF 45 45 10kΩ +5V 35 Overshoot (%) Overshoot (%) 40 RS 30 V OUT O PA 38 0 25 RP = 2kΩ C 20 −5 V 15 10kΩ 35 +2.5V 30 RS C 20 No RS 5 0 RS = 100Ω 0 10 100 1000 10 100 1000 Load Capacitance (pF) Load Capacitance (pF) OVERLOAD RECOVERY SMALL−SIGNAL STEP RESPONSE 3. 2pF VOUT RL = 2kΩ 5 0k Ω VP = −5V +5V V OUT I IN 1.6 m A VP = 0V 2 kΩ 50mV/div 2V/div RF = 2kΩ −2.5V 10 RS = 100Ω 5 VP 0.8mA/div 0 0 VO U T OPA380 25 15 No RS 10 40 IIN Time (100ns/div) Time (100ns/div) LARGE−SIGNAL STEP RESPONSE CHANNEL SEPARATION vs INPUT FREQUENCY 140 RL = 2kΩ Channel Separation (dB) 1V/div 120 2.5pF 10kΩ 2.5V 2kΩ 100 80 60 40 20 − 2.5V 0 Time (100ns/div) 10 100 1k 10k 100k Frequency (Hz) 8 1M 10M 100M "#$ %"#$ www.ti.com SBOS291E − NOVEMBER 2003 − REVISED NOVEMBER 2004 APPLICATIONS INFORMATION BASIC OPERATION The OPA380 is a high-performance transimpedance amplifier with very low 1/f noise. As a result of its unique architecture, the OPA380 has excellent long-term input voltage offset stability—a 300-hour life test at 150°C demonstrated randomly distributed variation approximately equal to measurement repeatability of 1µV. The OPA380 performance results from an internal auto-zero amplifier combined with a high-speed amplifier. The OPA380 has been designed with circuitry to improve overload recovery and settling time over a traditional composite approach. It has been specifically designed and characterized to accommodate circuit options to allow 0V output operation (see Figure 3). The OPA380 is used in inverting configurations, with the noninverting input used as a fixed biasing point. Figure 1 shows the OPA380 in a typical configuration. Power-supply pins should be bypassed with 1µF ceramic or tantalum capacitors. Electrolytic capacitors are not recommended. OPERATING VOLTAGE OPA380 series op amps are fully specified from 2.7V to 5.5V over a temperature range of −40°C to +125°C. Parameters that vary significantly with operating voltages or temperature are shown in the Typical Characteristics. INTERNAL OFFSET CORRECTION The OPA380 series op amps use an auto-zero topology with a time-continuous 90MHz op amp in the signal path. This amplifier is zero-corrected every 100µs using a proprietary technique. Upon power-up, the amplifier requires approximately 400µs to achieve specified VOS accuracy, which includes one full auto-zero cycle of approximately 100µs and the start-up time for the bias circuitry. Prior to this time, the amplifier will function properly but with unspecified offset voltage. This design has virtually no aliasing and very low noise. Zero correction occurs at a 10kHz rate, but there is very little fundamental noise energy present at that frequency due to internal filtering. For all practical purposes, any glitches have energy at 20MHz or higher and are easily filtered, if required. Most applications are not sensitive to such high-frequency noise, and no filtering is required. CF INPUT VOLTAGE RF +5V 1µF λ OPA380 VOUT(1) (0.5V to 4.4V) The input common-mode voltage range of the OPA380 series extends from V− to (V+) –1.8V. With input signals above this common-mode range, the amplifier will no longer provide a valid output value, but it will not latch or invert. INPUT OVERVOLTAGE PROTECTION VBIAS = 0.5V NOTE: (1) VOUT = 0.5V in dark conditions. Figure 1. OPA380 typical configuration Device inputs are protected by ESD diodes that will conduct if the input voltages exceed the power supplies by more than approximately 500mV. Momentary voltages greater than 500mV beyond the power supply can be tolerated if the current is limited to 10mA. The OPA380 series feature no phase inversion when the inputs extend beyond supplies if the input is current limited. 9 "#$ %"#$ www.ti.com SBOS291E − NOVEMBER 2003 − REVISED NOVEMBER 2004 OUTPUT RANGE ACHIEVING OUTPUT SWING TO GROUND The OPA380 is specified to swing within at least 600mV of the positive rail and 100mV of the negative rail with a 2kΩ load with excellent linearity. Swing to the negative rail while maintaining good linearity can be extended to 0V—see the section, Achieving Output Swing to Ground. See the Typical Characteristic curve, Output Voltage Swing vs Output Current. Some applications require output voltage swing from 0V to a positive full-scale voltage (such as +4.096V) with excellent accuracy. With most single-supply op amps, problems arise when the output signal approaches 0V, near the lower output swing limit of a single-supply op amp. A good single-supply op amp may swing close to single-supply ground, but will not reach 0V. The OPA380 can swing slightly closer than specified to the positive rail; however, linearity will decrease and a high-speed overload recovery clamp limits the amount of positive output voltage swing available—see Figure 2. The output of the OPA380 can be made to swing to ground, or slightly below, on a single-supply power source. This extended output swing requires the use of another resistor and an additional negative power supply. A pulldown resistor may be connected between the output and the negative supply to pull the output down to 0V. See Figure 3. OFFSET VOLTAGE vs OUTPUT VOLTAGE 20 VS = 5V 15 VOS (µV) 10 RP = 2kΩ connected to −5V RF 5 λ 0 V+ = +5V −5 RL = 2kΩconnected to VS /2 −10 OPA380 VOUT Effect of clamp −15 RP = 2kΩ −20 V− = Gnd 0 1 2 3 4 5 VOUT (V) VP = −5V Negative Supply Figure 2. Effect of high-speed overload recovery clamp on output voltage OVERLOAD RECOVERY The OPA380 has been designed to prevent output saturation. After being overdriven to the positive rail, it will typically require only 100ns to return to linear operation. The time required for negative overload recovery is greater, unless a pulldown resistor connected to a more negative supply is used to extend the output swing all the way to the negative rail—see the following section, Achieving Output Swing to Ground. 10 Figure 3. Amplifier with optional pull-down resistor to achieve VOUT = 0V The OPA380 has an output stage that allows the output voltage to be pulled to its negative supply rail using this technique. However, this technique only works with some types of output stages. The OPA380 has been designed to perform well with this method. Accuracy is excellent down to 0V. Reliable operation is assured over the specified temperature range. "#$ %"#$ www.ti.com SBOS291E − NOVEMBER 2003 − REVISED NOVEMBER 2004 the desired transimpedance gain (RF); BIASING PHOTODIODES IN SINGLE-SUPPLY CIRCUITS The +IN input can be biased with a positive DC voltage to offset the output voltage and allow the amplifier output to indicate a true zero photodiode measurement when the photodiode is not exposed to any light. It will also prevent the added delay that results from coming out of the negative rail. This bias voltage appears across the photodiode, providing a reverse bias for faster operation. An RC filter placed at this bias point will reduce noise. (Refer to Figure 4.) This bias voltage can also serve as an offset bias point for an ADC with range that does not include ground. the Gain Bandwidth Product (GBW) for the OPA380 (90MHz). With these three variables set, the feedback capacitor value (CF) can be set to control the frequency response. CSTRAY is the stray capacitance of RF, which is 0.2pF for a typical surface-mount resistor. To achieve a maximally flat 2nd-order Butterworth frequency response, the feedback pole should be set to: 1 + 2pR FǒCF ) CSTRAYǓ Ǹ4pRGBWC F (1) TOT Bandwidth is calculated by: CF(1) < 1pF f *3dB + Ǹ2pRGBWC F RF 10MΩ OPA380 0.1µF (2) These equations will result in maximum transimpedance bandwidth. For even higher transimpedance bandwidth, the high-speed CMOS OPA300 (180MHz GBW), or the OPA656 (230MHz GBW) may be used. V+ λ Hz TOT VOUT 100kΩ For additional information, refer to Application Bulletin AB−050 (SBOA055), Compensate Transimpedance Amplifiers Intuitively, available for download at www.ti.com. +VBias CF(1) NOTE: (1) CF is optional to prevent gain peaking. It includes the stray capacitance of RF. RF 10MΩ Figure 4. Filtered reverse bias voltage CSTRAY(2) TRANSIMPEDANCE AMPLIFIER Wide bandwidth, low input bias current, and low input voltage and current noise make the OPA380 an ideal wideband photodiode transimpedance amplifier. Low voltage noise is important because photodiode capacitance causes the effective noise gain of the circuit to increase at high frequency. The key elements to a transimpedance design are shown in Figure 5: the total input capacitance (CTOT), consisting of the photodiode capacitance (CDIODE) plus the parasitic common-mode and differential-mode input capacitance (3pF + 1.1pF for the OPA380); +5V λ CTOT(3) VOUT OPA380 RP (optional pulldown resistor) −5V NOTE: (1) CF is optional to prevent gain peaking. (2) CSTRAY is the stray capacitance of RF (typically, 0.2pF for a surface−mount resistor). (3) CTOT is the photodiode capacitance plus OPA380 input capacitance. Figure 5. Transimpedance Amplifier 11 "#$ %"#$ www.ti.com SBOS291E − NOVEMBER 2003 − REVISED NOVEMBER 2004 TRANSIMPEDANCE BANDWIDTH AND NOISE (a) Limiting the gain set by RF can decrease the noise occurring at the output of the transimpedance circuit. However, all required gain should occur in the transimpedance stage, since adding gain after the transimpedance amplifier generally produces poorer noise performance. The noise spectral density produced by RF increases with the square-root of RF, whereas the signal increases linearly. Therefore, signal-to-noise ratio is improved when all the required gain is placed in the transimpedance stage. Total noise increases with increased bandwidth. Limit the circuit bandwidth to only that required. Use a capacitor, CF, across the feedback resistor, RF, to limit bandwidth, even if not required for stability if total output noise is a concern. Figure 6a shows the transimpedance circuit without any feedback capacitor. The resulting transimpedance gain of this circuit is shown in Figure 7. The –3dB point is approximately 10MHz. Adding a 16pF feedback capacitor (Figure 6b) will limit the bandwidth and result in a –3dB point at approximately 1MHz (seen in Figure 7). Output noise will be further reduced by adding a filter (RFILTER and CFILTER) to create a second pole (Figure 6c). This second pole is placed within the feedback loop to maintain the amplifier’s low output impedance. (If the pole was placed outside the feedback loop, an additional buffer would be required and would inadvertently increase noise and dc error). R F = 10kΩ C STRAY = 0.2pF λ VBIAS (b) RF = 10kΩ CSTRAY = 0.2pF CF = 16pF λ ǒRDIODE ) RFǓ 2pRDIODER FǒC TOT ) C FǓ VOUT OPA380 VBIAS (c) RF = 10kΩ CSTRAY = 0.2pF Using RDIODE to represent the equivalent diode resistance, and CTOT for equivalent diode capacitance plus OPA380 input capacitance, the noise zero, fZ, is calculated by: fZ + VOUT OPA380 CF = 21pF R FILTER = 100Ω λ OPA380 VOUT CFILTER = 796pF (3) VBIAS Figure 6. Transimpedance circuit configurations with varying total and integrated noise gain 12 "#$ %"#$ www.ti.com SBOS291E − NOVEMBER 2003 − REVISED NOVEMBER 2004 500 CDIODE = 10pF Integrated Output Noise (µVrms) Transimpedance Gain (dB) 110 See Figure 6a 80 −3dB BW at 1MHz 50 See Figure 6c 20 See Figure 6b −10 100 CDIODE = 10pF 400 419µV See Figure 6a 300 200 See Figure 6b 100 86µV See Figure 6c 30µV 0 1k 10k 100k 1M 10M 1 100M 10 100 Frequency (Hz) 1k 10k 100k Frequency (Hz) 1M 10M 100M Figure 7. Transimpedance gains for circuits in Figure 6 Figure 9. Integrated output noise for circuits in Figure 6 The effect of these circuit configurations on output noise is shown in Figure 8 and on integrated output noise in Figure 9. A 2-pole Butterworth filter (maximally flat in passband) is created by selecting the filter values using the equation: Figure 10 shows the effect of diode capacitance on integrated output noise, using the circuit in Figure 6c. C FRF + 2C FILTERR FILTER (4) For additional information, refer to Noise Analysis of FET Transimpedance Amplifiers (SBOA060), and Noise Analysis for High Speed Op Amps (SBOA066), available for download from the TI web site. with: 79µV 1 80 2p ǸRFR FILTERCFC FILTER (5) The circuit in Figure 6b rolls off at 20dB/decade. The circuit with the additional filter shown in Figure 6c rolls off at 40dB/decade, resulting in improved noise performance. Integrated Output Noise (µVrms) f *3dB + CDIODE = 100pF CDIODE = 50pF 60 CDIODE = 20pF 0 Output Noise (nV/√Hz) 35µV 30µV 27µV 20 See Figure 6c 300 50µV CDIODE = 10pF CDIODE = 1pF 0 CDIODE = 10pF 1 10 100 1k 10k 100k Frequency (Hz) 1M 10M 100M 200 Figure 10. Integrated output noise for various values of CDIODE for circuit in Figure 6c See Figure 6a 100 See Figure 6b See Figure 6c 0 1 10 100 1k 10k 100k 1M 10M 100M Frequency (Hz) Figure 8. Output noise for circuits in Figure 6 13 "#$ %"#$ www.ti.com SBOS291E − NOVEMBER 2003 − REVISED NOVEMBER 2004 BOARD LAYOUT Minimize photodiode capacitance and stray capacitance at the summing junction (inverting input). This capacitance causes the voltage noise of the op amp to be amplified (increasing amplification at high frequency). Using a low-noise voltage source to reverse-bias a photodiode can significantly reduce its capacitance. Smaller photodiodes have lower capacitance. Use optics to concentrate light on a small photodiode. Circuit board leakage can degrade the performance of an otherwise well-designed amplifier. Clean the circuit board carefully. A circuit board guard trace that encircles the summing junction and is driven at the same voltage can help control leakage. See Figure 11. One method of improving capacitive load drive in the unity-gain configuration is to insert a 10Ω to 20Ω resistor in series with the load. This reduces ringing with large capacitive loads while maintaining DC accuracy. DRIVING FAST 16-BIT ANALOG-TO-DIGITAL CONVERTERS (ADC) The OPA380 series is optimized for driving a fast 16-bit ADC such as the ADS8411. The OPA380 op amp buffers the converter’s input capacitance and resulting charge injection while providing signal gain. Figure 12 shows the OPA380 in a single-ended method of interfacing the ADS8411 16-bit, 2MSPS ADC. For additional information, refer to the ADS8411 data sheet. RF CF RF λ OPA380 VOUT 15Ω ADS8411 OPA380 Guard ring 6800pF Figure 11. Connection of input guard OTHER WAYS TO MEASURE SMALL CURRENTS Logarithmic amplifiers are used to compress extremely wide dynamic range input currents to a much narrower range. Wide input dynamic ranges of 8 decades, or 100pA to 10mA, can be accommodated for input to a 12-bit ADC. (Suggested products: LOG101, LOG102, LOG104, LOG112.) RC Values shown are optimized for the ADS8411 values may vary for other ADCs. Figure 12. Driving 16-bit ADCs CF Extremely small currents can be accurately measured by integrating currents on a capacitor. (Suggested product: IVC102.) Low-level currents can be converted to high-resolution data words. (Suggested product: DDC112.) For further information on the range of products available, search www.ti.com using the above specific model names or by using keywords transimpedance and logarithmic. CAPACITIVE LOAD AND STABILITY The OPA380 series op amps can drive up to 500pF pure capacitive load. Increasing the gain enhances the amplifier’s ability to drive greater capacitive loads (see the Typical Characteristic curve, Small Signal Overshoot vs Capacitive Load). 14 RF R1 VIN OPA380 VOUT (Provides high−speed amplification with very low offset and drift.) Figure 13. OPA380 inverting gain configuration PACKAGE OPTION ADDENDUM www.ti.com 9-Dec-2004 PACKAGING INFORMATION Orderable Device Status (1) Package Type Package Drawing Pins Package Eco Plan (2) Qty OPA2380AIDGKR ACTIVE MSOP DGK 8 2500 Green (RoHS & no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR OPA2380AIDGKT ACTIVE MSOP DGK 8 250 Green (RoHS & no Sb/Br) CU NIPDAU Level-2-260C-1 YEAR OPA380AID ACTIVE SOIC D 8 100 None CU SNPB Level-1-220C-UNLIM OPA380AIDGKR ACTIVE MSOP DGK 8 2500 None CU NIPDAU Level-1-220C-UNLIM OPA380AIDGKT ACTIVE MSOP DGK 8 250 None CU NIPDAU Level-1-220C-UNLIM OPA380AIDR ACTIVE SOIC D 8 2500 None CU SNPB Level-1-220C-UNLIM Lead/Ball Finish MSL Peak Temp (3) (1) The marketing status values are defined as follows: ACTIVE: Product device recommended for new designs. LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect. NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design. PREVIEW: Device has been announced but is not in production. Samples may or may not be available. OBSOLETE: TI has discontinued the production of the device. (2) Eco Plan - May not be currently available - please check http://www.ti.com/productcontent for the latest availability information and additional product content details. None: Not yet available Lead (Pb-Free). Pb-Free (RoHS): TI's terms "Lead-Free" or "Pb-Free" mean semiconductor products that are compatible with the current RoHS requirements for all 6 substances, including the requirement that lead not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, TI Pb-Free products are suitable for use in specified lead-free processes. Green (RoHS & no Sb/Br): TI defines "Green" to mean "Pb-Free" and in addition, uses package materials that do not contain halogens, including bromine (Br) or antimony (Sb) above 0.1% of total product weight. (3) MSL, Peak Temp. -- The Moisture Sensitivity Level rating according to the JEDECindustry standard classifications, and peak solder temperature. Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is provided. TI bases its knowledge and belief on information provided by third parties, and makes no representation or warranty as to the accuracy of such information. Efforts are underway to better integrate information from third parties. TI has taken and continues to take reasonable steps to provide representative and accurate information but may not have conducted destructive testing or chemical analysis on incoming materials and chemicals. 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