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LT1991 Datasheet(PDF) 8 Page - Linear Integrated Systems |
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LT1991 Datasheet(HTML) 8 Page - Linear Integrated Systems |
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8 / 24 page  LT1991 8 1991fb Introduction The LT1991 may be the last op amp you ever have to stock. Because it provides you with several precision matched resistors, you can easily configure it into several different classical gain circuits without adding external compo- nents. The several pages of simple circuits in this data sheet demonstrate just how easy the LT1991 is to use. It can be configured into difference amplifiers, as well as into inverting and noninverting single ended amplifiers. The fact that the resistors and op amp are provided together in such a small package will often save you board space and reduce complexity for easy probing. The Op Amp The op amp internal to the LT1991 is a precision device with 15µV typical offset voltage and 3nA input bias cur- rent. The input offset current is extremely low, so match- ing the source resistance seen by the op amp inputs will provide for the best output accuracy. The op amp inputs are not rail-to-rail, but extend to within 1.2V of VCC and 1V of VEE. For many configurations though, the chip inputs will function rail-to-rail because of effective attenuation to the +input. The output is truly rail-to-rail, getting to within 40mV of the supply rails. The gain bandwidth product of the op amp is about 560kHz. In noise gains of 2 or more, it is stable into capacitive loads up to 500pF. In noise gains below 2, it is stable into capacitive loads up to 100pF. The Resistors The resistors internal to the LT1991 are very well matched SiChrome based elements protected with barrier metal. Although their absolute tolerance is fairly poor (±30%), their matching is to within 0.04%. This allows the chip to achieve a CMRR of 75dB, and gain errors within 0.04%. The resistor values are 50k, 150k, and 2 of 450k, con- nected to each of the inputs. The resistors have power limitations of 1watt for the 450k resistors, 0.3watt for the 150k resistors and 0.5watt for the 50k resistors; however, in practice, power dissipation will be limited well below these values by the maximum voltage allowed on the input and REF pins. The 450k resistors connected to the M1 and P1 inputs are isolated from the substrate, and can there- fore be taken beyond the supply voltages. The naming of the pins “P1,” “P3,” “P9,” etc., is based on their relative admittances. Because it has 9 times the admittance, the voltage applied to the P9 input has 9 times the effect of the voltage applied to the P1 input. Bandwidth The bandwidth of the LT1991 will depend on the gain you select (or more accurately the noise gain resulting from the gain you select). In the lowest configurable gain of 1, the –3dB bandwidth is limited to 450kHz, with peaking of about 2dB at 280kHz. In the highest configurable gains, bandwidth is limited to 32kHz. Input Noise The LT1991 input noise is dominated by the Johnson noise of the internal resistors (√4kTR). Paralleling all four resistors to the +input gives a 32.1kΩ resistance, for 23nV/√Hz of voltage noise. The equivalent network on the –input gives another 23nV/√Hz, and taking their RMS sum gives a total 33nV/√Hz input referred noise floor. Output noise depends on configuration and noise gain. Input Resistance The LT1991 input resistances vary with configuration, but once configured are apparent on inspection. Note that resistors connected to the op amp’s –input are looking into a virtual ground, so they simply parallel. Any feedback resistance around the op amp does not contribute to input resistance. Resistors connected to the op amp’s +input are looking into a high impedance, so they add as parallel or series depending on how they are connected, and whether or not some of them are grounded. The op amp +input itself presents a very high GΩ impedance. In the classical noninverting op amp configuration, the LT1991 presents the high input impedance of the op amp, as is usual for the noninverting case. Common Mode Input Voltage Range The LT1991 valid common mode input range is limited by three factors: 1. Maximum allowed voltage on the pins 2. The input voltage range of the internal op amp 3. Valid output voltage APPLICATIO S I FOR ATIO |
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