Data Acquisition Board for Pi Pico

RE: Data Acquisition Board for Pi Pico

Jan Cumps — Fri, 15 Dec 2023 14:20:02 GMT

Some sample results, using bulk sampling and averaging

10 single measurements; taken in bulk in 1/86 sec
averaging 215 samples, taken in bulk in 1/4 sec
averaging 860 samples, taken in bulk in 1 sec

All samples in the exercise are taken at the same speed, 860 samples per second. On the yellow line, each data point is one sample. On the red line it represents an average of 215 samples. On the blue line, 860 samples are averaged for each data point.

I think that I have a wrong value in the spreadsheet for the first 215-avg data point. Ignore that.

RE: Data Acquisition Board for Pi Pico

Jan Cumps — Fri, 24 Nov 2023 08:30:04 GMT

Modding time: I connected the ADS1115 READY pin to a GPIO (GP13). To support continuous mode.

I plan to launch an i2c read at each pulse.

Also late late change request for a future version: a little pad connected to pin 2 would make it easier, for modifications, to get at the ADC's pin 2: ALERT/RDY.
It's doable to tag a tiny wire directly to the IC leg, but it's somewhat fiddly.

RE: Data Acquisition Board for Pi Pico

ggabe — Mon, 20 Nov 2023 17:02:01 GMT

[mention:b0bc65b9ecdc4307bd967592f00e340a:e9ed411860ed4f2ba0265705b8793d05] as far as next revision, one more idea: to protect the GND reference of the input, thus the computer's USB port it's plugged into: a current limiter.

RE: Data Acquisition Board for Pi Pico

ggabe — Tue, 14 Nov 2023 16:05:54 GMT

We have unused GPIOs on U7, why not design in the range switching?

Paralleling 1 of 2 resistors with each of R3,6,9,10 could give us 3 ranges: 0.5V, 5V, 25V. A single improved rds(on) version of 4053 would be reasonable. Or, breaking the low cost falling back to 4053 layout compatibility, a TMUX1574 or 74CBTLV3257 could be still inexpensive. Or, someone can choose DNP the range switch completely. We have the space!

RE: Data Acquisition Board for Pi Pico

flyingbean — Tue, 14 Nov 2023 03:08:51 GMT

Great topic. Will make a couple of Pi Pico DAQ boards for my own LabVIEW labs.

RE: Data Acquisition Board for Pi Pico

shabaz — Sun, 12 Nov 2023 01:38:12 GMT

Here's the current PDF schematic. The main changes to the original rev 1.0 board are:

1. Bodge wire issue resolved

2. Op-amp footprints changed to a dual op-amp and a single op-amp

3. EEPROM replaced with the more useful PCA9500

4. Most resistors and capacitors are now 0603 to save space

5. Got rid of the temperature sensor, it can be on a separate board (maybe a thermocouple with ADS1118).

6. Added input protection using BAV199 diodes and zener replacement circuit as discussed by [mention:bffbb58a51e44afa8e6e976a48255202:e9ed411860ed4f2ba0265705b8793d05]

The PCB layout is taking shape. Routing is complete and zero DRC errors, but I still need to tidy up traces and silkscreen.

I figured I would stick the KiCad files on GitHub now even though it's not complete, in case anyone wants to inspect deeper or make changes. I'll leave it like this for a while, in case further changes are needed, and then maybe generate the final rev 1.1 gerbers in a week or two. Note to self: replace the MOSFET footprint it is still the hand-solder SOT-23 footprint, and label the inputs to the ADC for those who wish to experiment with the board as discussed by [mention:acaf6a9338de4eef8f6717d5561ed01d:e9ed411860ed4f2ba0265705b8793d05]

RE: Data Acquisition Board for Pi Pico

Jan Cumps — Fri, 10 Nov 2023 16:08:43 GMT

development challenge:

The RP2040 has two ARM cores. It may be possible to continuously collect samples from the ADC in the second core, and either:

present them raw to core 1, for interaction with the screen / client software
aggregate, average, .... and present aggregated values to the 1st core.

If core 1 deals with user interfacing, UART/USB, ... and does not touch the i2c interface used by the ADC, then the 2nd core can have full speed, unshared access to that IC.

RE: Data Acquisition Board for Pi Pico

Jan Cumps — Fri, 10 Nov 2023 15:01:42 GMT

late change request:

A place (4-pin header, test pad, ...) to get at the 4 ADC inputs.
You can then use the same PCB for "generic" ADS1115 projects, by not mounting the opamps.

RE: Data Acquisition Board for Pi Pico

Jan Cumps — Fri, 10 Nov 2023 10:33:56 GMT

Test, with a voltage reference:

input shorted:

input 2.5 V

input 5 V

input -2.5 V

RE: Data Acquisition Board for Pi Pico

shabaz — Mon, 06 Nov 2023 23:28:00 GMT

Tons of space.. The footprints are now 0603 (but nevertheless the large variant of that) and still easy to hand-solder. About 40% free top surface, and the underside is 80% free.

I still need to tidy this up, but no errors on DRC.

RE: Data Acquisition Board for Pi Pico

shabaz — Mon, 06 Nov 2023 18:53:43 GMT

Here are renders of a potential revision 1.1 board. I still need to tidy the traces and the silkscreen, but it successfully routes with no errors.

I won't finalise this revision for a while, so that there is plenty of time for more potential modifications (hopefully small).

The changes are:

1. Fixed the bodge wire problem that was on the rev 1 board

2. Improvement to the STS40 area, to try to improve ambient temperature measuring

3. Swapped the quad op-amp footprint (of which only three op-amps were being used) for a dual and a single op-amp, they are VSSOP-8 and SC70 sized. The VSSOP-8 can suit OPA2325 or many other op-amps. Similarly the SC70 footprint supports a range of op-amps too, but there are a couple of popular pin configurations. I used the configuration where pin 4 is the op-amp output.

4. Replaced the EEPROM chip with PCA9500, in case I/O is needed for future boards. The PCA9500 also has the benefit of having address pins, so now it is possible to stack at least two boards with EEPROMs. More boards are easily possible if I bring out another address pin (I will probably make that adjustment to the PCB).

The PCA9500 is moved from where the rev 1 EEPROM was, to a much more convenient location if the I/O is needed for future front end control etc.

I'm tempted to remove the temperature sensor in a later revision. It may be better to stick it on a different board with any other small ancillary stuff if required. That way, the temperature sensor can be very far away from anything heat producing, and won't be sandwiched between boards.

Probably the passives need to shrink to 0603 in a future board, but for now they are all still 0805.

RE: Data Acquisition Board for Pi Pico

Jan Cumps — Sun, 05 Nov 2023 12:11:03 GMT

I put the front-end on the test bench:

[mention:a35b6a699a624e18af44f2903dfb091c:f7d226abd59f475c9d224a79e3f0ec07]

RE: Data Acquisition Board for Pi Pico

DAB — Sat, 04 Nov 2023 18:00:54 GMT

Nice addition to the Pico.

RE: Data Acquisition Board for Pi Pico

ggabe — Sat, 04 Nov 2023 17:31:29 GMT

Here is an enhancement idea I'm thinking of: expand the input range to +/-60V max, and use gpio controlled gain settings to allow 50V, 5V, 0,5V differential input for full scale. Do that on two channels, with modest ~100K input impedance.

It will require a -5V supply, easily generated by an NXE1S0505. For simplicity, THS4530 fully differential amp per channel. DG411 for gain switching, an optionally ADG5421 for input protection.

RE: Data Acquisition Board for Pi Pico

shabaz — Sat, 04 Nov 2023 17:23:45 GMT

I've attached a zip file containing the KiCad symbol and footprint (KiCad 7 files) for making this shape add-ons.

For reasons I can't recall, it wasn't a good idea to have the board outline (Edge.Cuts) layer in the footprint, so that would need to be done in the PCB Editor, simply following the mechanical layer outline (which if you're on a 0.5 mm grid, will be easy to do). Or for any other style e.g. Pico Flap as [mention:07ad7044180c4e69b9d6764380da6d07:e9ed411860ed4f2ba0265705b8793d05] mentions you could extend further.

Optionally you could put a radius on the corners; the ADC board in the photos has a 0.5 mm radius.