100 MΩ source impedance thermal noise is about 1.28 µV/√Hz (= 12.8 fA/√Hz) and it will not be enough to distinguish between DUT and resistor noise. So, we would need a resistor value of at least 10 GΩ to see some of the noise. However, it becomes impossible to measure accurately if the resistor current noise dominates the current noise of the DUT. This gives us the measurement current noise floor ofĪnd it can be subtracted out in postprocessing. Typical bias current of FET input amps is about 1 pA, which equals 0.57 fA/√Hz This is not the case in FET input amps rather, in Figure 2, it looks like a bizarre noise shape that is not well known and is ignored in many simulation models.Ī 10 GΩ SMT resistor from Ohmite (HVC1206Z1008KET) was used to convert current noise to voltage noise at the noninverting pin of the DUT. Some customers-as well as manufacturers-assume that current noise for FET input amps follows a similar shape as bipolar input amplifiers-for example, 1/f or flicker noise component and flat wideband component, as shown in Figure 1. Over the past few years, there has been a growing interest regarding current noise over frequency in amplifiers. Does this calculated number hold up to 1 kHz for every amplifier? Historically, ADI had provided most current noise numbers this way. Is it measured or is it theoretical? Some manufacturers are transparent in how they come up with this number by providing an equation of It isn’t always clear where the current noise specifications come from. Many semiconductor manufacturer data sheets, including ADI’s, specify the current noise of an amplifier in the specification tables, typically at a frequency of 1 kHz. The phenomenon of current noise increasing with frequency is well known to IC design engineers and circuit designers, but it was elusive to many engineers as a result of either too few articles in the field or incomplete information from manufacturers. Why is my design noisier at higher frequencies? Answer:
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