Noise Simulation

This section walks through a simplified noise calculation for the front-end circuit, providing an estimate to compare with simulated results in TI-TINA. In this circuit example, the resistor noise from the large resistance (R₁ and R₂) is a significant portion of the overall noise of the system. However, the output impedance (R_S) of the signal source (V_G) is usually low so the noise from the combination of R_S, R₁, and R₂ is small. See the following equation for noise calculation with R_S = 100Ω.

Thermal noise from the resistors (R_S, R₁, and R₂):

Thermal noise from the resistors (R₃ and R₄):

Noise from OPA990 amplifier:

Total noise applied to input of the ADC:

The calculated and simulated noise match well (calculated noise = 4.3µV_rms and simulated = 4.38µV_rms as in the following graph). See TI Precision Labs - Op Amps: Noise 4 for detailed theory on amplifier noise calculations, Amplifier Noise Simulations (Op Amps: Noise 5) for detailed theory on amplifier noise simulations, and Calculating the Total Noise for ADC Systems for data converter noise.