![]() Open this file by selecting the plot window and select View->Spice Error Log. The measurement results are in the “Spice Error Log”. The voltage noise is the same for the A and B grades. I applied a factor of 5 to convert RMS to peak-to-peak. The datasheet gives peak-to-peak values for noise from 0.1 to 10 Hz ( 1/ƒ region) but LTspice calculates RMS. Here is a table comparing the datasheet values to the measured values. MEAS statements are found in the Spice Error Log. For example, is the run with the small input resistor. STEP run is selected by adding or after a source. No problem! LTspice takes care of the calculation for us! As one noise source gets larger than another, it starts to completely dominate. Remember the noise sources add and subtract in an RMS fashion. This is calculated by LTspice for every noise simulation. “V(inoise)” is the output voltage noise of the entire circuit referred back to the input. The red trace shows the difference between the two runs. The purple trace is Step #2 with a 10MΩ input resistor. The green trace is Step #1 with a 1Ω input resistor. Here is a plot showing results from the two-step simulation. Details are below.ĪT 1K - Selects the frequency of the data Used in the log file.įIND - Specifies the measurement, which in this case is just getting a data - The data set to use in the measurement. NOISE - Apply the measurement to a noise simulationĮn1_1k_RMS - Just a name for the result. However, let’s look at one to get the input voltage noise at as NOISE en1_1k_RMS FIND AT 1K No additional calculations are required to compare with the datasheet. There is a small value for the input voltage noise measurement (Step #1) and a large value for the input current noise measurement (Step #2). For example, two runs are done with different values of the input resistor. STEP statements run multiple simulations with different variable values. These are the variable names in curly braces, for example. Measurement conditions are set with statements on the left-hand side.PARAM statements provide values to the variables in the schematic. Here are the LTspice directives used in the simulation. The table is from the Analog Devices ADA4627 datasheet The specifications are used in the comparison table below. The left column is for “B Grade” parts and the right is for “A Grade” parts. Here is the noise section of the ADA4627 datasheet. It was not chosen because the datasheet specs did or did not match LTspice testing. ![]() Then, a check of the datasheet showed the noise is well specified. It has low noise and supports supply voltages from ±5V to ☑5V. The ADA4627 caught my attention from a quick scan of the low-noise op-amp selection table from Analog Devices. Not a lot of work went into choosing this part. With this feature, the ONLY noise source is the op-amp. Add the word “noiseless” as an additional value. Start this editor by holding down the control key and right-clicking on the resistor body. The noiseless attribute can be added using the Component Attribute Editor. This feature is very useful because the extra noise from resistors does not have to be subtracted from the measurement. Why is “noiseless” added to the resistor values? Adding this undocumented attribute to a resistor tells LTspice to ignore the resistor as a noise source. The values for the power supplies and input resistor have been parameterized to make them easy to change and use in. The circuit is a standard non-inverting amplifier with a resistance in series with the non-inverting input to measure bias current noise. The first component to test is a low-noise op-amp. LTspice Noise Simulation Example: Low-Noise Op-Amp An excellent explanation of converting an RMS value to peak-to-peak is in this video from Analog Devices. How accurate are the measurements? Are they useful? To find out, the results are compared to noise specifications in datasheets.Īn excellent summary of op-amp noise and how noise sources combine can be found in this article and app note from Analog Devices. This article shows how to use LTspice to measure the noise of an op-amp and of an op-amp combined with a dual-JFET input stage. In previous articles, we introduced modeling noise with LTspice and simulating noise sources in LTspice. Learn how to measure noise using LTspice for op-amp circuits with handy examples.
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