# THD Testing Procedure

```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.
```