Datasheet LT1991 (Analog Devices) - 10
| Manufacturer | Analog Devices |
| Description | Precision, 100µA Gain Selectable Amplifier |
| Pages / Page | 28 / 10 — APPLICATIONS INFORMATION. Introduction. Bandwidth. Input Noise. The Op … |
| File Format / Size | PDF / 782 Kb |
| Document Language | English |
APPLICATIONS INFORMATION. Introduction. Bandwidth. Input Noise. The Op Amp. Input Resistance. The Resistors
Model Line for this Datasheet
- LT1991ACDD#PBF LT1991ACDD#TRPBF LT1991ACMS#PBF LT1991ACMS#TRPBF LT1991AIDD#PBF LT1991AIDD#TRPBF LT1991AIMS#PBF LT1991AIMS#TRPBF LT1991CDD#PBF LT1991CDD#TRPBF LT1991CMS#PBF LT1991CMS#TRPBF LT1991HDD#PBF LT1991HDD#TRPBF LT1991HMS#PBF LT1991HMS#TRPBF LT1991IDD#PBF LT1991IDD#TRPBF LT1991IMS#PBF LT1991IMS#TRPBF
Text Version of Document
LT1991
APPLICATIONS INFORMATION Introduction
admittances. Because it has 9 times the admittance, the The LT1991 may be the last op amp you ever have to stock. voltage applied to the P9 input has 9 times the effect of Because it provides you with several precision matched the voltage applied to the P1 input. resistors, you can easily configure it into several different
Bandwidth
classical gain circuits without adding external components. The several pages of simple circuits in this data sheet The bandwidth of the LT1991 will depend on the gain you demonstrate just how easy the LT1991 is to use. It can select (or more accurately the noise gain resulting from be configured into difference amplifiers, as well as into the gain you select). In the lowest configurable gain of 1, inverting and noninverting single ended amplifiers. The the –3dB bandwidth is limited to 450kHz, with peaking of fact that the resistors and op amp are provided together about 2dB at 280kHz. In the highest configurable gains, in such a small package will often save you board space bandwidth is limited to 32kHz. and reduce complexity for easy probing.
Input Noise The Op Amp
The LT1991 input noise is dominated by the Johnson The op amp internal to the LT1991 is a precision device noise of the internal resistors (√4kTR). Paralleling all with 15µV typical offset voltage and 3nA input bias current. four resistors to the +input gives a 32.1kΩ resistance, The input offset current is extremely low, so matching the for 23nV/√Hz of voltage noise. The equivalent network source resistance seen by the op amp inputs will provide on the –input gives another 23nV/√Hz , and taking their for the best output accuracy. The op amp inputs are not RMS sum gives a total 33nV/√Hz input referred noise floor. rail-to-rail, but extend to within 1.2V of VCC and 1V of Output noise depends on configuration and noise gain. VEE. For many configurations though, the chip inputs will function rail-to-rail because of effective attenuation to the
Input Resistance
+input. The output is truly rail-to-rail, getting to within The LT1991 input resistances vary with configuration, 40mV of the supply rails. The gain bandwidth product of but once configured are apparent on inspection. Note that the op amp is about 560kHz. In noise gains of 2 or more, resistors connected to the op amp’s –input are looking it is stable into capacitive loads up to 500pF. In noise gains into a virtual ground, so they simply parallel. Any feedback below 2, it is stable into capacitive loads up to 100pF. resistance around the op amp does not contribute to input resistance. Resistors connected to the op amp’s +input
The Resistors
are looking into a high impedance, so they add as paral- The resistors internal to the LT1991 are very well matched lel or series depending on how they are connected, and SiChrome based elements protected with barrier metal. whether or not some of them are grounded. The op amp Although their absolute tolerance is fairly poor (±30%), +input itself presents a very high GΩ impedance. In the their matching is to within 0.04%. This allows the chip to classical noninverting op amp configuration, the LT1991 achieve a CMRR of 75dB, and gain errors within 0.04%. presents the high input impedance of the op amp, as is The resistor values are 50k, 150k, and 2 of 450k, con- usual for the noninverting case. nected to each of the inputs. The resistors have power limitations of 1watt for the 450k resistors, 0.3watt for the
Common Mode Input Voltage Range
150k resistors and 0.5watt for the 50k resistors; however, The LT1991 valid common mode input range is limited in practice, power dissipation will be limited well below by three factors: these values by the maximum voltage allowed on the input and REF pins. The 450k resistors connected to the 1. Maximum allowed voltage on the pins M1 and P1 inputs are isolated from the substrate, and can 2. The input voltage range of the internal op amp therefore be taken beyond the supply voltages. The naming of the pins “P1,” “P3,” “P9,” etc., is based on their relative 3. Valid output voltage 1991fh 10 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Applications Information Typical Applications Package Description Revision History Typical Application Related Parts
APPLICATIONS INFORMATION Introduction
admittances. Because it has 9 times the admittance, the The LT1991 may be the last op amp you ever have to stock. voltage applied to the P9 input has 9 times the effect of Because it provides you with several precision matched the voltage applied to the P1 input. resistors, you can easily configure it into several different
Bandwidth
classical gain circuits without adding external components. The several pages of simple circuits in this data sheet The bandwidth of the LT1991 will depend on the gain you demonstrate just how easy the LT1991 is to use. It can select (or more accurately the noise gain resulting from be configured into difference amplifiers, as well as into the gain you select). In the lowest configurable gain of 1, inverting and noninverting single ended amplifiers. The the –3dB bandwidth is limited to 450kHz, with peaking of fact that the resistors and op amp are provided together about 2dB at 280kHz. In the highest configurable gains, in such a small package will often save you board space bandwidth is limited to 32kHz. and reduce complexity for easy probing.
Input Noise The Op Amp
The LT1991 input noise is dominated by the Johnson The op amp internal to the LT1991 is a precision device noise of the internal resistors (√4kTR). Paralleling all with 15µV typical offset voltage and 3nA input bias current. four resistors to the +input gives a 32.1kΩ resistance, The input offset current is extremely low, so matching the for 23nV/√Hz of voltage noise. The equivalent network source resistance seen by the op amp inputs will provide on the –input gives another 23nV/√Hz , and taking their for the best output accuracy. The op amp inputs are not RMS sum gives a total 33nV/√Hz input referred noise floor. rail-to-rail, but extend to within 1.2V of VCC and 1V of Output noise depends on configuration and noise gain. VEE. For many configurations though, the chip inputs will function rail-to-rail because of effective attenuation to the
Input Resistance
+input. The output is truly rail-to-rail, getting to within The LT1991 input resistances vary with configuration, 40mV of the supply rails. The gain bandwidth product of but once configured are apparent on inspection. Note that the op amp is about 560kHz. In noise gains of 2 or more, resistors connected to the op amp’s –input are looking it is stable into capacitive loads up to 500pF. In noise gains into a virtual ground, so they simply parallel. Any feedback below 2, it is stable into capacitive loads up to 100pF. resistance around the op amp does not contribute to input resistance. Resistors connected to the op amp’s +input
The Resistors
are looking into a high impedance, so they add as paral- The resistors internal to the LT1991 are very well matched lel or series depending on how they are connected, and SiChrome based elements protected with barrier metal. whether or not some of them are grounded. The op amp Although their absolute tolerance is fairly poor (±30%), +input itself presents a very high GΩ impedance. In the their matching is to within 0.04%. This allows the chip to classical noninverting op amp configuration, the LT1991 achieve a CMRR of 75dB, and gain errors within 0.04%. presents the high input impedance of the op amp, as is The resistor values are 50k, 150k, and 2 of 450k, con- usual for the noninverting case. nected to each of the inputs. The resistors have power limitations of 1watt for the 450k resistors, 0.3watt for the
Common Mode Input Voltage Range
150k resistors and 0.5watt for the 50k resistors; however, The LT1991 valid common mode input range is limited in practice, power dissipation will be limited well below by three factors: these values by the maximum voltage allowed on the input and REF pins. The 450k resistors connected to the 1. Maximum allowed voltage on the pins M1 and P1 inputs are isolated from the substrate, and can 2. The input voltage range of the internal op amp therefore be taken beyond the supply voltages. The naming of the pins “P1,” “P3,” “P9,” etc., is based on their relative 3. Valid output voltage 1991fh 10 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Applications Information Typical Applications Package Description Revision History Typical Application Related Parts
