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QuantAsylum edited this page Jan 6, 2020 · 12 revisions

The QA351 is a low-cost voltmeter providing microvolt resolution and quick programmability. The QA351 should not be expected to replace a $2500 6.5 digit DVM. Instead, the QA351 makes sense in applications where you are primarily concerned with precise, relative measurements over days or months in situations where humidity and temperature are reasonably controlled (for example, an office setting).

The QA351 provides two channels of measurements: A high-resolution main channel (uV resolution) and a lower-resolution aux channel (mV resolution). Both of these channels are differential. The outer shell of the BNC connector is the low side of the measurement, and in the inner conductor is the high side.

The QA351 is NOT designed to measure high-energy circuits. High-energy circuits are circuits that are connected to the electrical grid. The QA351 is a CAT I device, which is the "weakest" designation for measurement devices. This means you must take external measures to ensure transient events (such as a lightening strike to the utility lines outside of your office) are clamped upstream of the DUT and are NOT propagated through the device you are measuring to the QA351.

ATTENTION The maximum voltage you should subject the QA351 to is +/-50V. Potentials in excess of +/-50V will be reported as overflows. Even though the QA351 is isolated, you should ensure the ground potential difference between the device you are measuring and the PC ground is under 100V. From time to time, you should verify the isolation between the QA351 input section and the USB section is in tact. This can be done by disconnecting all cables to the QA351 and checking the impedance with a DVM between the input ground (outer metal of the BNC) and the USB ground is reported as infinite. A more advanced check can be done with an insulation tester. This should report >10Gohm at 500V.

Input Grounds

The QA351 inputs are both differential, meaning that they can measure voltages unrelated to each other. For example, let's say you have a circuit that has a +12V and -12V supply. You can connect the MAIN input from +12V to ground and measure +12V. And you can connect the AUX input to +12V and -12V and measure +24V. The inputs do not need the same ground reference.

While the MAIN channel can deliver exceptional accuracy and resolution, keep in mind that when using the AUX channel the main channel accuracy can be degraded. Any residual AC components in either the MAIN or AUX inputs can also degrade the measurement.

When the full accuracy and resolution of the QA351 is required, then you should "cap" (short) the input to the AUX channel. For your particular setup, you can see the impact of using the AUX channel during high-resolution MAIN channel measurements: In some cases it can make no difference at all. In other cases, it can make a big difference. The biggest contributing factor will the magnitude and presence of AC or other common mode noise.

All inputs should be within the +/-50V range as established by the MAIN input. The MAIN input will set the internal "ground" of the measurement at the MAIN input midpoint. For safety reasons, the other inputs should reside withing 50V of this midpoint. For example, if you apply 10V to the MAIN input, then your "ground" is at that midpoint, or 5V. The input on all other ports should be within +/-50V of that 5V "ground".


Before using the QA351, you should calibrate it. The QA351 is calibrated against a high-quality 2.5V reference on the front-panel. This reference is derived from a Maxim MAX6126. This reference delivers 0.02% initial accuracy (+/-500uV), a 3 PPM/C tempco, and a long-term stability of 20 PPM/1000 hours. The references used on the QA351 are NOT hermetically sealed, and will exhibit sensitivity to humidity. Make sure you understand the impact humidity and temperature changes can have on your measurements.

The calibration is done from the calibration menu, and it must be done for both the MAIN and AUX channel--just follow the steps shown for the selected channel. Once the unit is calibrated, you can verify the calibration by shorting the input and verifying 0V is read, and by connecting the input to the reference and verifying the 2.5V reference voltage is read.

Before beginning the calibration, make sure the unit has come up to room temperature. Generally, if being brought in from the outdoors in winter this will take 30 minutes or so. The QA351 will light an LED that indicates "STABLE" after the unit has been operating for 10 minutes. It's a good idea to ensure the until has come up to room temperature (if it's been outside) and that the STABLE LED is active before calibrating.

The calibration output should not be excessively loaded. The front-panel reference output has a 50 ohm series resistor. A direct short for extended periods may damage this circuit and impact the accuracy of the reference. Limit the current pulled from this port to 1-2 mA at most.

Measurement Challenges

Measuring at microvolt levels accurately isn't simply a matter of connecting the probes to your DUT. Accurate metrology at microvolt levels requires a broad understanding of an array of concepts. Most detrimental beyond changes to room and DUT humidity and temperature is thermal EMF. For example, anyplace a difference in materials exists, such as a lead-free solder contact on a plated gold connector or in your probes where the wire joins gold contact, you have a small voltage source that will impact the accuracy of your measurements based on the temperature difference between the two metals. This can easily reach 10's of microvolts. Great expense goes into high-end measuring equipment material choices to help equalize and fight thermal EMF sources. To combat this in an instrument such as the QA351 you must actively manage thermal gradients and other sources that can degrade measurements. This means allowing temperatures to stabilize after touching (warming) connectors, minimizing stray air currents around the QA351, being cognizant of how radio transmitters such as WiFi and cellphones can impact microvolt measurements, and so on.

Application Main Screen

The main screen of the QA351 is shown below:

The two measurement channels are shown, stacked atop each other. The buttons for each channel accomplish the following:

+/-50V This sets the maximum input of the high resolution channel. This channel exhibits 10M ohm input impedance with a +/-5V input range in the low range with about 1uV of resolution. In the high-range, the channel exhibits 2.2Mohm input impedance with a +/-50V input range and 10uV of resolution. The AUX input doesn't have this button, because it's range is +/-50V always (with a 2M ohm input impedance).

Toggle Graphs This will enable or disable the displayed graphs for that particular channel.

Reset This button resets the collected statistics and re-starts collection process. The number of points to be collected is specified in the Config menu for each channel. Each channel may have its own settings.

Config This button will open a configuration menu, as shown below:

In the channel settings dialog, you can edit the Y axis (dislay in PPM, mV or uV), the depth of the history (in samples) and also customize the channel name and math. This allows you to use an external sense resistor, for example, to display current.

Data Logging

Logging is enabled in the File menu. When logging is toggled, you will be asked to specify the log file and also the logging interval (5 seconds or more). Inside the log, you will see output similar to the following:

#QA351 Data Log
#Logging file Created on 11/14/2019 11:07 AM
#Logging Interval: 5 seconds
#Main Calibrated: True
#Aux Calibrated: True
#System Time, time since start (seconds), Main Value (volts), Aux Value (volts), QA351 temperature (C)
11/17/2019 11:07:14 AM, 0.00, 2.499995, 0.000, 23
11/17/2019 11:07:19 AM, 5.02, 2.499997, 0.000, 23
11/17/2019 11:07:24 AM, 10.04, 2.499997, 0.000, 24
11/17/2019 11:07:29 AM, 15.06, 2.499997, 0.000, 23
11/17/2019 11:07:34 AM, 20.08, 2.499997, 0.000, 23

The output above shows that every 5 seconds (or whatever your specified logging interval is), there will be a time stamp, a delta time (from logging start), and the readings of the main channel, aux channel, and the QA351 temperature.

The QA351 temperature should be considered somewhat coarse: There is 2-4 degrees of offset error with +/- one degree of noise. But the temperature is monotonic with no missing codes.

Application Logging

When developing applications that interface to the QA351, it can be helpful to see a log of the interface activity. Every time the QA351 is launched, it restarts an application log file 'QA351_Log.txt' that is located in the c:\users\USERNAME\Documents\QuantAsylum\QA351 directory. Inside this file you can see your interactions with the webserver, including errors. If the application isn't responding as you expect, then open the log and you should be able to see why the command was rejected (remember URLs are case sensitive by default).

When you open this file, if you open it as 'read only' in your favorite text editor, then you should be able to watch the file update on the fly. Your text editor might ask "The file has changed on disk, do you want to re-open?" or something similar. Using this technique makes it easy to track each API interaction as it happens.


When a new version of the QA351 is installed, upon launch it will verify the firmware in the QA351 matches the firmware expected by the new install. If not, you will be prompted to download new firmware to the QA351. During the download, do not unplug the QA351. The update should take under 1 minute. After updating, you should re-plug the QA351 and re-launch the QA351 application.

REST Interface

The QA351 application provides a REST interface, making it easy to query measurements from an external program. The REST API can be viewed in a browser by starting the QA351 application and going to the URL below:


This will bring up the application interface. For example, you can query the voltage on the high-resolution MAIN channel with the following:


This will return a JSON response similar to the following:

{ "Value":"2.49999165672787" }

Parsing the returned value is a common task in just about all languages. For an example of parsing a JSON response for the QA401H in Python, see the blog post located here

REST Troubleshooting

If you can't see the API when you go to the link above, it could be that the REST server failed to start. Or, it did start but your firewall settings are preventing applications from seeing the server. If you are sure that the firewall on your machine has port 9351 opened, then run the QA351 application with admin rights (right click on the app and select "run as admin") and see if that allows a browser to open the link above. If that solves the problem, then you can either run the app with admin rights when you want to enable the REST API, or you can enable the port generally using a command line solution. To do this, open a command prompt with admin privileges and type the following:

netsh http add urlacl url=http://*:9351/ user=DOMAIN\\user

Replace the ‘DOMAIN’ and ‘user’ text with the appropriate domain and user name for your account. To find your domain name, open the control panel, selection “System and Security” then “System” and your will see an entry showing the Full Computer Name.

If you want to see what existing reservations are present on your machine (which should also give a hint about the domain and user settings on your machine for other ports), then you can type:

netsh http show urlacl

To later remove the permission added above, you would enter:

netsh http delete urlacl url=http://*:9351/

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