THD Testing Procedure If you have amplifier schematic below, how to test your THD (Total Harmonic Distortion) of amplifier. VDD 5 M5 M9 M8 IM8 M6 IM9 IM6 IM5 1 6 M10 M11 M1 VIN- M2 IM2 IM1 7 VOUT VIN+ 2 RC CC 3 4 M12 M13 8 IM12 9 M3 M4 M7 IM13 IM4 IM3 IM7 Rb VSS Self-Biasing Circuit 2-Stage Amplifier THD is measured by applying sinewaves to the negative input of amplifier. Then parallel 1GΩ resistor and 1pF capacitor between output and negative input of amplifier. Between the negative input of amplifier and input signal, parallel 1GΩ resistor and 1pF capacitor again and connect positive input of amplifier to analog ground(VAG=2.5V). For 1KHz THD, apply 3.5Vpp sinewaves centered about analog ground at frequency of 1KHz. For 2KHz THD, apply 3.5Vpp sinewaves centered about analog ground at frequency of 2KHz. I run the transient simulation to 5ms with step at 500ns. The testing schematic of THD can be connected as follows: Real Ground Analog Ground VDD VAG=2.5V 1G VDD VAG DC=5V OUT DC=2.5V SIN VSS 1p CL=10pF 1G VSS=0 1p The output swing can also be measured by the peak-to-peak voltage of output node of amplifier. The spice netlist and simulation results are listed below. The THD result in is calculated by FFT function in HSPICE. Since the THD results from Hspice is not in dB, you have to convert to the do as follows: 1.1714m THD _ 1KHZ 20 log 10( ) 98.63dB 100 You can also check the output waveform of THD.