Current measurement from constant current source

Question

I have a constant current circuit using an op amp circuit. I am implementing a current of 21 mA to drive a laser. Spec of laser: https://www.thorlabs.com/thorproduct.cfm?partnumber=L650P007 I would like to measure the noise in the current and do FFT and other analysis. I need to measure it at every 20 ms to analyze the noise from the AC mains. Is there any method I could use to do that? I have already tried with following:

• Fluke 8845 DMM
• NI Virtual bench
• Arduino UNO

20ms is not possible. Does anyone have any idea regarding this?

This is the current measurement over time at every 120ms. The maximum I could get was at every 120ms:

Code I used for Fluke measurement:

import time
import serial
import matplotlib.pyplot as plt

# Configure the serial port settings
port = 'COM6'  
baud_rate = 9600  
timeout = 1  

# Open the serial port
ser = serial.Serial(port, baud_rate, timeout=timeout)

ser.write(b"SYST:REM\r\n")  # Put the instrument in remote mode

duration = 1200  # Duration in seconds
interval = 0.020  # Interval between current measurements in seconds (20 ms)

timestamps = []
currents = []

# Specify the file path and name to save the data
file_path = "CXXXXXX/Current/data.txt"

# Open the file in write mode
file = open(file_path, "w")

start_time = time.time()
end_time = start_time + duration

while time.time() < end_time:
    # Trigger a current measurement
    command = "MEASure:CURRent:DC? 1\r\n"
    ser.write(command.encode())

    # Read response
    response = ser.readline().decode().strip()
    current = float(response)

    current_time = time.time() - start_time
    timestamps.append(current_time)
    currents.append(current)

    # Write the current and time values to the file
    file.write(f"{current_time:.3f}s, {current}A\n")
    file.flush()  # Flush the buffer to ensure immediate writing to the file

    print(f"Time: {current_time:.3f}s, Current: {current}A")

    time.sleep(interval)  # Wait for the desired interval before the next measurement

# Close the serial connection
ser.close()

```

Answer 1

The way how you design your code to communicate with the multimeter, it will use "software" timing, i.e. the sample distance will be given by how fast your software communicates to the device and how fast that device's internal communication frontend will make changes happen to its core.

If you want deterministic sample timing, you need hardware timing, i.e. set up the device to integrate each reading for 1 PLC or 20 ms and throw these readings into its output data buffer. The device will then typically trigger itself from an internal clock source.

Then at whatever intervals you wish, read what is available in the buffer, e.g. each second you can read ~50 points.

This is universal and not limited to the DMM.