u3Noise.py

"""
Name: u3Noise.py
Desc: See DESC string.

Note: Our Python interfaces throw exceptions when there are any issues with
device communications that need addressed. Many of our examples will
terminate immediately when an exception is thrown. The onus is on the API
user to address the cause of any exceptions thrown, and add exception
handling when appropriate. We create our own exception classes that are
derived from the built-in Python Exception class and can be caught as such.
For more information, see the implementation in our source code and the
Python standard documentation.
"""
import math

try:
    raw_input
except NameError:  # Python 3
    raw_input = input

import u3


DESC = """
This program will attempt to measure the noise of a provided signal on the U3. This is good if there is ever a question of noise from the LabJack or from a signal.

The experiment is performed by taking 128 readings in quick succession. The results are then operated on the get the following values:

Peak-To-Peak Noise: The difference between the minimum and the maximum of the 128 values. Since all readings should be the same, any variation is noise.

Noise-Free Resolution (bits): This represents how many bits are noise free.

Noise-Free Resolution (mV): The smallest value that can be represented noise-free.

Connect your signal and run this program. After, connect something with an obviously low-noise signal (like a battery) and run the program. If you don't have a low-noise signal, you can always jumper two FIOs to Ground and measure the noise as a differential.

You'll find that by comparing the two results, the LabJack is rarely the reason for noisy readings.

On with the test:
"""


def calcNoiseAndResolution(d, positiveChannel=0, negativeChannel=31):
    readings = []

    cmd = u3.AIN(positiveChannel, negativeChannel, QuickSample=False, LongSettling=False)

    for i in range(128):
        readings.append(float(d.getFeedback(cmd)[0]) / 16.0)
    #print readings

    # Peak-To-Peak Noise
    p2pn = max(readings) - min(readings)

    # Noise-free resolution in bits
    if p2pn > 0:
        nfrbits = 12 - math.log(p2pn, 2)
    else:
        nfrbits = 12

    # Noise-free resolution in millivolts
    if d.deviceName.endswith("HV") and positiveChannel < 4:
        vRange = 20.6
    else:
        if negativeChannel != 31:
            vRange = 4.88
        else:
            vRange = 2.44

    nfrres = (vRange / (2**nfrbits)) * (10 ** 3)

    return p2pn, nfrbits, nfrres


print(DESC)

pos = raw_input("Positive Channel (0-15, 30, 31) [0]: ")
try:
    pos = int(pos)
except:
    pos = 0

neg = raw_input("Negative Channel (0-15, 30, 31) [31]: ")
try:
    neg = int(neg)
except:
    neg = 31

# Open first found U3
d = u3.U3()

# Configure all FIO and EIO lines to analog inputs.
d.configIO(FIOAnalog=0xFF, EIOAnalog=0xFF)

results = calcNoiseAndResolution(d, pos, neg)

print("Peak-To-Peak Noise = %s" % results[0])
print("Noise-Free Resolution (bits) = %s" % results[1])
print("Noise-Free Resolution (mV) = %s" % results[2])

d.close()