RoadTest: PicoLog Data Logger + Raspberry Pi 4 (included)
Author: fmilburn
Creation date:
Evaluation Type: Test Equipment
Did you receive all parts the manufacturer stated would be included in the package?: True
What other parts do you consider comparable to this product?: The resolution of this data logger is unique in its price class but a comparison with other Pico products and the Digilent DataLogger has been made.
What were the biggest problems encountered?: No significant problems were encountered.
Detailed Review:
Introduction
Thank you element14 and Pico Technology for the opportunity to review the Pico ADC-20 Data Logger and Terminal Board. The ability to use the data logger with a Raspberry Pi is useful and the addition of a Raspberry Pi to the RoadTest is appreciated. The RoadTest contains the following sections:
Unboxing
The instrument arrived undamaged and well packaged in a Newark corrugated shipping box surrounded by packing material. All components described in the RoadTest invitation were inside.
Overview
The Pico ADC-20 data logger is a 20-bit resolution data logger and features both single ended and true differential inputs. The software is highly configurable and can run on Windows, macOS, Linux and Raspbian Buster. For this RoadTest the software will be run on a Raspberry Pi 4 Model B 2GB. Software features will be reviewed and several real world applications tested. The basic lab setup is shown below running the Pico data logger software.
Power for testing is supplied by a Tenma 72-2685 and voltages checked with a Tenma 72-1020 bench multimeter that agrees well with my other multimeters. The Tenma 72-1020 will also be used for temperature comparisons. Not shown is a Keysight DSOX1102G oscilloscope with waveform generator that will be used to generate a slowly changing sine wave for one of the tests.
Market Comparison
In the table below the PicoLog ADC-20 is compared to the Digilent OpenLogger and three other dataloggers from Pico. The ADC-24 priced at $579 costs twice that of the others in the table and is not shown. For those interested, the additional features it has above the ADC-20 include 24 bit resolution, 4 digital I/O and 8 additional channels.
The PicoLog ADC-20 has higher resolution and accuracy than the other instruments in the table. It also features differential input which the others lack. The trade-off is that it also has a lower sampling rate. Provided sampling requirements are no greater than 10 per second the ADC-20 is the superior instrument and it is competitive in price.
What isn't characterized by the table is the quality of the software. I found the Pico software to be more refined and without the bugs and non-responsiveness experienced with the Digilent Datalogger in my recent RoadTest of that instrument. If an instrument with a higher sampling rate than the ADC-20 is required I would preferentially look at the Pico offerings over the Digilent Datalogger.
Getting Started
The ADC-20 comes with a Quick Start Guide that leads the user through installation of the PicoLog 6 software. The Quick Start Guide does not list Raspbian as one of the supported operating systems but once opened the link provided includes Raspbian. The rest of the getting started guide consists mainly of limits, warnings, and cautions. The instructions for using the software are online and comprehensive. The main landing page is here.
Hardware Feature Review
The PicoLog enclosure is robust and comes out of the box ready for use. On the front there is a 25-way D connection for the terminal board. A cable (not supplied) can be used to connect the terminal board or it can be hung directly on the PicoLog. The terminal board is not fully supported when hung off in this manner and I am hunting around for a cable. a minor irritations is that when wires are attached to the terminal blocks they obscure the channel designation on the silk screen of the PCB. At the rear of the enclosure is the permanently installed USB cable and a small LED indicating power. The USB cable has some strain relief. The LED can't be seen unless the unit is picked up and turned around. On the bottom is a sticker with information about the instrument.
The PicoLog ADC-20 data logger and terminal board are shown below hooked up to the Raspberry Pi 4 beside it. The data logger is supplied power by the USB connection to the Pi and is not switched. The power indication LED on the back of the enclosure lights when the software is running and the instrument is recognized. The terminal board works with either the 8 channel ADC-20 or the 16 channel ADC-24. Note the arrangement to connect voltage dividers for sources outside the -2.5 to +2.5 V input range on the terminal board.
The Terminal Board User's Manual has a very good description of how to choose resistors and configure them. However, this requires cutting traces on the board and soldering in resistors. For my own use it would be nice to have the board set up for +/- 5V volts or possibly +/- 10V at times. Ideally Pico might have jumpers so that the resistor divider or direct connection could be made. I have ordered resistors and plan to modify it in future but for the time being I will set up the resistor dividers off of the Terminal Board. More detailed information on the hardware follows.
ADC-20
Terminal Board
Software Feature Review
The software was installed on the Raspberry Pi 4 supplied for the RoadTest with only one issue. The installation page downloads the latest Version 6.1.15 of PicoLog but the installation instructions below are for version 14. This was reported on the Pico forum. Other than this the directions are clear and the installation went without incident. The software also works on Microsoft Windows 7 and up both 32 and 64-bit versions, macOS 10.9 or later 64-bit only, and Linux 64-bit.
The Pico forum is active and Pico staff appear to be actively helping customers on the site. The software receives frequent updates.
A more detailed overview of the software and how it works is given below.
Basic Setup and Viewing
For the screenshots below the DC voltage was set at 1.1 V on the bench PSU and connected to channel 7 of the PicoLog. The initial window looks like this...
Clicking on the desired channel, in this case channel 7 shown in blue above will open up a window where single ended or differential capture can be selected.
In this case single-ended is selected which then opens up the window for selecting single-ended options...
I tried to enter invalid data into the sample interval window and it would not allow it. While the datasheet states conversions occur up to15 per second, the minimum entry period is 100 millisecond . The advanced options allows the sample mode (average or latest sample) to be selected. The probe or scaling option opens up an optional scaling window. Input range can be +/- 1.25V or +/- 2.5 V.
Pressing the red button starts the recording. The tabs for graph and table are selected to show the data. In the screenshots below the high resolution of the instrument is apparent as it displays 1.1 V DC output from bench power supply. The minimum reading is 1.095895 V and the maximum is 1.095980 V, a difference of 85 uV. In all tests the ADC-20 agreed with the bench multimeter readings to within the specifications of the instruments.
Basic Graph |
Basic Table |
Basic setup and viewing data is demonstrated in the video below.
Graph View
Additional features available when setting up and viewing data such as multiple graphs and annotation are demonstrated in the video below.
Alarms
Setting up and function of alarms is demonstrated in the video below.
Math Channels
Setting up and using scaling and math channels is demonstrated in the video below.
Files and Post-Analysis
For the next demonstration the waveform generator from a Keysight DSOX 1102G oscilloscope was set up to output a 4 V p-p sine wave with a 10 second period to channel 7 of the ADC-20. Data is being recorded every 100 ms which is the limit for the ADC-20. Saving files and post analysis of data is demonstrated.
Note that the video above demonstrates an unexplained issue with the recording of data at this highest rate. The capture rate of the ADC-20 is set at 10 per second so 100 samples are recorded for each period of the sine wave. As shown in screenshot below, there is a plateau or step that occurs. I thought it was a glitch in the plotting software at first, but it is also present in the tabular data. The steps occur 9 times per sine wave period in the frame below. It could be something associated with sample rate being slower than record rate.
PicoLog SDK
The Software Development Kit (SDK) is available for Windows 32 and 64 bit only. According to the Programmers Guide:
The driver exports the function definitions in standard C format, but this does not limit you to programming in C. You can use the API with any programming language that supports standard C calls. It can also be used with programs like Microsoft Excel. The driver supports Windows 7, 8 and 10.
There are examples of use on the Pico web site and Github for many different languages and LabView. The SDK was not tested in this RoadTest.
Error Analysis
The best meter available to me is a Tenma 72-1020 bench multimeter with the following specifications (taken from Tenma User Manual)
As noted above, the specifications for the PicoLog ADC-20 are:
A quick test was done by running the voltage up the scale from -2.5 V to 2.5 V in 0.1 V increments while recording measurements on channel 7 of the ADC-20 and on the Tenma 72-1020. The difference in voltages measured and the percent error were calculated and then plotted in Excel.
ADC |
|---|
ADC-20 Vs. DMM |
Error Difference (V) |
E Error (%) |
The error was calculated by assuming the Tenma value was correct which is only an assumption. In any event the agreement between the two instruments is well within the specifications provided by their manufacturers.
A quick look was made at the deviation between channels by supplying the same nominal 1V to each of the 8 channels. The screenshot below shows the results.
As can be seen by the summary of the most recent sample on the right hand side of the chart the range is from 995.7727 mV to 995.8442 mV, a difference of 0.0715 uV or 0.0072%. It is also interesting to note that the data groups into two bands. I elected not to do a teardown and am guessing there are two ADCs inside.
Application: Logging of light using light dependent resistors (LDRs)
This application is described in the ADC-20/ADC-24 Terminal Board User's Guide. Six light dependent resistors (LDRs) will be used to measure light level by arranging them in a single-ended voltage divider configuration with 1 Mohm resistors. The following illustration taken from the Pico ADC-20/ADC-24 Terminal Board User's Guide. For this applications, RA was set to 1 Mohm and RB is the LDR.
The demonstration follows:
Application: Logging of temperature using LM35 temperature sensor
Four LM35DZ temperature measurement sensors were set up on channels 1 to 4 of the ADC-20 as described in the Terminal Board User's Guide.
The LM35DZ outputs a voltage of 10 mV for every degree Celsius. Since the temperature of my lab is around 20 degrees Celsius the expected output is about 200 mV. A multiplication factor of 100 is set up in the scaling of the channels so that the output will read degrees Celsius. The tested limit of the LM35 is +/- 1 degree at 25 degrees Celsius but as a check two old analog thermometers and the type K thermometer on the Tenma 72-1020 DMM were also be observed.
Closeup of Sensors |
Overview of Experiment |
The screenshot below shows the outcome of approximately 22 hours of data collection with averaged samples recorded every second.
The results are interesting:
While it looks like the lab has a large variation over the almost 24 hour period it is only a little more than 1 degree C. The lab is in the basement and the thermostat is upstairs. During the day it stays cooler than the rest of the house which warms up from sunlight. At night the central heating kicking in at regular intervals is readily apparent. If I close the door to the lab it gets warmer if the heat is on like it is at the beginning of the experiment. The ADC-20 is very easy to set up for this type of experiment and the software handled the data without issue. The date and time is captured for every sample which is useful.
Application: Measurement of diode IV curve
In this application the ADC-20 was used to generate the an IV curve for a LED and exported in CSV format for plotting in Excel. The bench power supplied DC voltage and a bench multimeter was used as to measure current. The ADC-20 also measured the current in differential mode on Channels 1 and 2 across a resistor. Channel 3 measured the forward voltage (Vf) of the LED at various currents.
Everyone should have a milliohm meter to accurately measure small resistance values :-). The resistor used as a shunt resistor measured 0.9951 ohms.
The LED is an Avago HLMP-1700 red LED with a forward voltage of 1.7 V (typical) to 2 V (max) for a forward current of 2 mA according to the datasheet.
The experiment and the outcome is described in the following video.
The data matches the datasheet for the LED nicely and the initial current flow is readily apparent.
Summary
The Pico ADC-20 has:
It met all specifications in the areas tested. I found the software quick to learn, responsive and suitable for my needs. The graphical display is very nice. The one bug observed was the "step" that occurred when recording at it 100 millisecond intervals on a sine wave with a 10 second period in the Files and Post Analysis testing. The bug wasn't observed when recording was done at 1 second and slower intervals. If the maximum sampling rate of the ADC-20 is too low for your purposes I would recommend looking at some of the other Pico offerings given my positive experience with the ADC-20 and the software. Be aware that the PicoLog 6 SDK only works in Windows. I plan to add resistors to my terminal board so that it can be used at +/- 5V without an external voltage divider for the work I do.
The Pico ADC-20 has found a place on my bench and gets a "thumbs up".
Useful References
https://www.element14.com/community/roadTests/2157/l/PicoLog-Data-Logger-+-Raspberry-Pi-4-(included)
https://www.picotech.com/data-logger/adc-20-adc-24/precision-data-acquisition
Top Comments
The USB specification for a 2.0 host port is to be able to supply 0.5A. It would seem very short sighted to not have sufficient power in the budget and at the same time provide a USB host port. I know…
I updated one of the videos that failed to post on the first attempt. I am also keeping the temperature experiment going to see if issues are encountered when data is collected over long periods of time…
Very good thorough review Frank. The PicoLogs are useful little devices and as you have said the software is very robust and usable. I too find the voltage input a bit limiting. I have an extra termination…