I have obtained 12-bit ADC output data from the TI ADS7042 EVM kit. I am trying to calculate the SNR, THDm, SINAD (in dB), and other parameters. The output code ranges from 0 to 4095. ADC-output2.txt
I use the below code to check the SNR and THD.
import numpy as np
from scipy import signal
import matplotlib.pyplot as plt
import scipy.io
def preprocess_adc_data(adc_data, v_ref, adc_resolution):
adc_data_np = np.array(adc_data)
adc_data_np = adc_data_np - np.mean(adc_data_np) #
# 7 term Blackman-Harris window
window = signal.windows.blackmanharris(len(adc_data_np))
adc_data_np = adc_data_np * window
return (adc_data_np / (2 ** adc_resolution - 1)) * v_ref
def estimate_input_frequency_from_spectrum(adc_data_voltage, fs):
# Spectrum Calculate
f, Pxx = signal.periodogram(adc_data_voltage, fs)
# Find the frequency corresponding to the maximum value in the power spectrum.
max_index = np.argmax(Pxx)
frequency = f[max_index]
return frequency
def calculate_snr(adc_data_voltage, fs):
f, Pxx = signal.periodogram(adc_data_voltage, fs)
snr = 10 * np.log10(np.max(Pxx) / (np.sum(Pxx) - np.max(Pxx)))
return snr
def plot_signal(adc_data_voltage, fs):
t = np.arange(len(adc_data_voltage)) / fs
plt.figure()
plt.plot(t, adc_data_voltage)
plt.xlabel('Time (s)')
plt.ylabel('Voltage (V)')
plt.title('Restored Signal Waveform')
plt.show()
def plot_spectrum(adc_data_voltage, fs, f_input):
nfft = int(fs)
f, Pxx = signal.periodogram(adc_data_voltage, fs, nfft=nfft)
Pxx_dBc = 10 * np.log10(Pxx / np.max(Pxx))
# Maximum frequency
f_limit = 1e5
idx = np.where(f <= f_limit)
f_limited = f[idx]
Pxx_dBc_limited = Pxx_dBc[idx]
fig, ax = plt.subplots()
ax.plot(f_limited, Pxx_dBc_limited)
ax.set_xlabel("Frequency (Hz)")
ax.set_ylabel("Amplitude (dBC)")
ax.set_title("Spectrum")
# Limit X-axis and Y-axis
ax.set_xlim([0, f_limit])
ax.set_ylim([-160, 20])
# Compute the fundamental frequency and harmonics.
fundamental_freq = int(f_input)
first_harmonic_freq = 2 * fundamental_freq
second_harmonic_freq = 3 * fundamental_freq
third_harmonic_freq = 4 * fundamental_freq
# Locate the positions of the fundamental frequency and harmonics in the spectrum.
fundamental_idx = np.where(f == fundamental_freq)
first_harmonic_idx = np.where(f == first_harmonic_freq)
second_harmonic_idx = np.where(f == second_harmonic_freq)
third_harmonic_idx = np.where(f == third_harmonic_freq)
# Extract the dBc values of the fundamental frequency and harmonics.
fundamental_dBc = float(Pxx_dBc[fundamental_idx])
first_harmonic_dBc = float(Pxx_dBc[first_harmonic_idx])
second_harmonic_dBc = float(Pxx_dBc[second_harmonic_idx])
third_harmonic_dBc = float(Pxx_dBc[third_harmonic_idx])
print("H1：{:.0f} Hz, {:.2f} dBc".format(fundamental_freq, fundamental_dBc))
print("H2：{:.0f} Hz, {:.2f} dBc".format(first_harmonic_freq, first_harmonic_dBc))
print("H3：{:.0f} Hz, {:.2f} dBc".format(second_harmonic_freq, second_harmonic_dBc))
print("H4：{:.0f} Hz, {:.2f} dBc".format(third_harmonic_freq, third_harmonic_dBc))
plt.show()
def read_adc_data_from_file(file_path):
with open(file_path, 'r') as file:
lines = file.readlines()
adc_data = [int(line.strip()) for line in lines]
return adc_data
def calculate_thdn(adc_data_voltage, fs, fundamental_frequency, num_harmonics=7):
f, Pxx = signal.periodogram(adc_data_voltage, fs)
fundamental_power = None
harmonics_power = 0
for harmonic in range(1, num_harmonics + 1):
harmonic_frequency = fundamental_frequency * harmonic
index = np.argmin(np.abs(f - harmonic_frequency))
power = Pxx[index]
if harmonic == 1:
fundamental_power = power
else:
harmonics_power += power
thdn = np.sqrt(harmonics_power / fundamental_power)
thdn_db = 20 * np.log10(thdn)
return thdn_db
file_path = './ADC-output2.txt' # File
adc_data = read_adc_data_from_file(file_path)
v_ref = 3.3
adc_resolution = 12
adc_data_voltage = preprocess_adc_data(adc_data, v_ref, adc_resolution)
sample_rate = 1e6 # 1 MHz
f_input = estimate_input_frequency_from_spectrum(adc_data_voltage, sample_rate)
print("Estimated input frequency: {:.2f} Hz".format(f_input))
# Plot the reconstructed signal waveform and the spectral analysis waveform.
plot_signal(adc_data_voltage, sample_rate)
plot_spectrum(adc_data_voltage, sample_rate, f_input)
# SNR
snr = calculate_snr(adc_data_voltage, sample_rate)
print("SNR: {:.2f} dB".format(snr))
# THD
thdn = calculate_thdn(adc_data_voltage, sample_rate, f_input)
print("THD+N: {:.2f} dB".format(thdn))
From the TI EVM kit, the result is as below: The result from TI software
The result of my code shows below: H1：10009 Hz, -0.00 dBc H2：20018 Hz, -80.43 dBc H3：30027 Hz, -86.12 dBc H4：40036 Hz, -91.65 dBc SNR: -0.03 Hz THD+N: -77.97 dB And the Specturm from Python calculation
Significant differences are observed in the calculated results for SNR. Please kindly help me identify what's wrong with the code? Thank
