Datasheet LT1115 (Analog Devices) - 8

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