Feature: Software & Tools
Choosing the best amplifier with LTspice simulation and noise analysis
By Hooman Hashemi, Product Applications Engineer, Analog Devices O
ne of the challenges of designing a signal path that includes a small-signal transducer, amplifiers, filtering and converters (ADCs) is to
determine the noise contribution of each component the signal traverses on its way to being digitised and processed. If the design is done correctly, and most of the gain is taken at the front-end, the task is simpler since choosing the lowest-noise front-end ensures the highest signal-to- noise ratio and minimises the impact of noise on the rest of the circuit. But what if such a clear distinction
can’t be made and/or the application necessitates the lowest levels of noise and signal integrity, where noise must be fully optimised?
Noise analysis LTspice is a readily-available analysis soſtware that effectively assesses noise impact on a design and helps optimise it by focusing on a particular component (resistor, transistor, etc.). Te LTspice simulation presents the output noise contribution as a noise density plot; see Figure 1. Tese plots can be integrated, to
easily identify their impact on the circuit over an entire frequency range, as shown in Figure 1 for R3 as the highest resistive contributor at 100nV/√Hz flat-band. LTspice is a very powerful simulation
tool that also applies to components like op-amps. In Figure 2, for example, we analyse an idealised op-amp. Te figure shows the
UniversalOpAmp.asc file built into the LTspice educational library. It shows a simplified op-amp model with five levels of increasing complexity, from the simplest to the most complex, with all outputs plotted simultaneously. Tis is a useful macro model that can be copied into any design and easily edited to determine the impact of each parameter.
Extending the analysis to op-amps With UniversalOpAmp, the voltage noise, current noise and respective corner frequencies can be varied for each noise source. Te resulting output/input noise plots will help quickly identify the best device for the task, simply by knowing the exact noise tolerance of the design. Figure 3 shows this approach where
the circuit of Figure 1 is modified to take the UniversalOpAmp instead. With the noise current “In” set to be a variable
30 September 2023
www.electronicsworld.co.uk
parameter, and the voltage noise term “En” value of 0.1nV/√Hz to be insignificant, with the “.step param” function one can see the result of varied current noise aſter simulation. In this case, the parameter is varied through a list: 0.1pA/√Hz, 1pA/√Hz, 2pA/√Hz, 5pA/√Hz and 10pA/√Hz. It’s worth noting that this technique is
a first-order approximation of an op- amp’s noise characteristic. Behaviour such as increasing noise with frequency, oſten found in FET-input op-amps, is not included in the universal model and must be accounted for separately, once the actual device is simulated or bench tested. It’s also worth noting that in many
applications, such as transimpedance amplifiers (TIAs) where noise performance is usually very important, there is a strong interaction between the amplifier noise and the external components (for example, input capacitance from a photodiode or avalanche photodiode), making any evaluation accurate only if these external components are included and accounted for. Otherwise, the simulated performance will be far from the measured results. See Table 1 for the simulation results summary, which offers easy comparison
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