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LT1028AMH Datasheet(PDF) 10 Page - Linear Technology |
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LT1028AMH Datasheet(HTML) 10 Page - Linear Technology |
10 / 20 page 10 LT1028/LT1128 Voltage Noise vs Current Noise The LT1028/LT1128’s less than 1nV/ √Hz voltage noise is three times better than the lowest voltage noise heretofore available (on the LT1007/1037). A necessary condition for such low voltage noise is operating the input transistors at nearly 1mA of collector currents, because voltage noise is inversely proportional to the square root of the collector current. Current noise, however, is directly proportional to the square root of the collector current. Consequently, the LT1028/LT1128’s current noise is significantly higher than on most monolithic op amps. 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 = [en 2 + 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 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 = [0.85 2 + 1.842 + (1.0 × 0.2) 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 LT1028/ LT1128’s voltage noise becomes negligible. As Req is further increased, current noise becomes important. At 1kHz, when Req is in excess of 20k, the current noise component is larger than the resistor noise. The total noise versus matched source resistance plot illustrates the above calculations. 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 > 1k, the current noise term will exceed the resistor noise. When the source resistance is unmatched, the total noise versus unmatched source resistance plot should be con- sulted. Note that total noise is lower at source resistances below 1k because the resistor noise contribution is less. When RS > 1k 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 LT1028/LT1128 are the optimum ampli- fiers 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 in- creased beyond the LT1028/LT1128’s level of usefulness. – + 100 Ω 100k 100 Ω LT1028 LT1128 1028/1128 AI01 S APPLICATI I FOR ATIO – OISE BEST OP AMP AT LOW FREQ(10Hz) WIDEBAND(1kHz) SOURCE RESIS- TANCE( Ω) (Note 1) Best Op Amp for Lowest Total Noise vs Source Resistance 0 to 400 LT1028/LT1128 LT1028/LT1128 400 to 4k LT1007/1037 LT1028/LT1128 4k to 40k LT1001 LT1007/1037 40k to 500k LT1012 LT1001 500k to 5M LT1012 or LT1055 LT1012 >5M LT1055 LT1055 Note 1: Source resistance is defined as matched or unmatched, e.g., RS = 1k means: 1k at each input, or 1k at one input and zero at the other. |
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