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LT1115CN8 Datasheet(PDF) 8 Page - Linear Integrated Systems |
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LT1115CN8 Datasheet(HTML) 8 Page - Linear Integrated Systems |
8 / 16 page LT1115 8 1115fa Therefore, to realize truly low noise performance it is important to understand the interaction between voltage noise (en), current noise (in) and resistor noise (rn). Total Noise vs Source Resistance The total input referred noise of an op amp is given by et = [en2 + rn2 + (inReq)2]1/2 where Req is the total equivalent source resistance at the two inputs and rn = √4kTReq = 0.13√Req in nV/√Hz at 25°C As a numerical example, consider the total noise at 1kHz of the gain of 1000 amplifier shown below. Req = 100Ω + 100Ω||100k ≈ 200Ω rn = 0.13√200 = 1.84nV/√Hz en = 0.85nV/√Hz in = 1.0pA/√Hz et = [0.852 + 1.842 + (1.0 x 2.0)2]1/2 = 2.04nV/√Hz output noise = 1000 et = 2.04µV/√Hz At very low source resistance (Req < 40Ω) voltage noise dominates. As Req is increased resistor noise becomes the largest term—as in the example above—and the LT1115’s voltage noise becomes negligible. As Req is further increased, current noise becomes important. At 1kHz, when Req is in excess of 20kΩ, the current noise component is larger than the resistor noise. The Total Noise vs Matched Source Resistance plot in the Typical Performance Characteristics section, illustrates the above calculations. – + 100Ω 100k 100Ω LT1115 LT1115 • AI01 The plot also shows that current noise is more dominant at low frequencies, such as 10Hz. This is because resistor noise is flat with frequency, while the 1/f corner of current noise is typically at 250Hz. At 10Hz when Req > 1kΩ, the current noise term will exceed the resistor noise. When the source resistance is unmatched, the Total Noise vs Unmatched Source Resistance plot should be con- sulted. Note that total noise is lower at source resistances below 1kΩ because the resistor noise contribution is less. When Rs > 1kΩ total noise is not improved, however. This is because bias current cancellation is used to reduce input bias current. The cancellation circuitry injects two correlated current noise components into the two inputs. With matched source resistors the injected current noise creates a common-mode voltage noise and gets rejected by the amplifier. With source resistance in one input only, the cancellation noise is added to the amplifier’s inherent noise. In summary, the LT1115 is the optimum amplifier for noise performance—provided that the source resistance is kept low. The following table depicts which op amp manufactured by Linear Technology should be used to minimize noise—as the source resistance is increased beyond the LT1115’s level of usefulness. Best Op Amp for Lowest Total Noise vs Source Resistance Note 1: Source resistance is defined as matched or unmatched, e.g., RS = 1kΩ means: 1kΩ at each input, or 1kΩ at one input and zero at the other. SOURCE RESISTANCE BEST OP AMP (NOTE 1) AT LOW FREQ (10Hz) WIDEBAND (1kHz) 0 to 400Ω LT1028/1115 LT1028/1115 400Ω to 4kΩ LT1007/1037 LT1028/1115 4kΩ to 40kΩ LT1001* LT1007/1037 40kΩ to 500kΩ LT1012* LT1001* 500kΩ to 5MΩ LT1012* or LT1055 LT1012* > 5M LT1055 LT1055 *These op amps are best utilized in applications requiring less bandwidth than audio. APPLICATIO S I FOR ATIO |
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