RTD Board Introduction and Installation
This board allows the measurement of temperatures from Pt100 probes. It can easily be modified to accommodate other kinds of resistive probes by changing the reference resistance for the 0 V point. Having said this, this hasn't been tested and the board is provided publicly as is so it would be up to the user to change the giving resistances.
By default, with the current resistor values for R4 and R5, the board will output (analog outputs from the INA826 which are buffered with an OP Amp) an analog value from 0 V to 5 V where 0 V represents 15 degrees Celsius, and 5V represents 45 degrees Celsius. This can be changed by adjusting the gain (resistor R3) and the initial 0V point (resistor bridge R4, R5).
Texas Instruments made this great white paper on the linearization of RTDs: http://www.ti.com/lit/an/slyt442/slyt442.pdf?&ts=1588981950918 which is great at explaining a lot of the inner workings of this project. They also made a spreadsheet that can help in deciding the values of R3, R4, R5 adjusted to your own needs. You can find it here: http://www.ti.com/lit/zip/SLYT442
The main idea of this circuit is to measure the voltage drop across the RTD Pt100 using a known stable voltage source and compare it to the expected voltage drop at a given temperature. Take the difference with the instrument amplifier (INA826) and then amplify in such a way that 45C is 5V and 15C is 0V.
Once this is done we use a comparator circuit to compare it to a set value after which it will either output +5V (1 = Ok) or 0V (0 = Fail).
The setpoint can be set using the trim pot.
The board has surface-mounted devices (SMD) in mind when the design was done. This decision was made to ensure the compactness of the device, and ease the replacement of parts later on. This means that in order to mount this you will need some specialized equipment such a hot air rework station, solder paste instead of regular solder, and a reflow oven can be handy if you want to batch things, i.e. install all the components on the board and then bake it soldering all of them at once.
From the Gerber Files and ODB++ files, you can generate your own stencil which can be used to apply solder paste in a faster way.
I recommend for a first time user, installing manually (with a hot air rework station) even if slower since it will allow you to become familiarized with the components and workings of the PCB. Indeed you can progressively test the board. You should first install the power supply components, then the INA826 and all its passive components. At this stage, you can already provide power to the board (+15-17V through the barrel jack) and test some of the features with a Pt100 or with a known resistor of around 100 Ohms (that resistor needs to be wired using a 3 wire method!). Then you can proceed with the LM311 and its passives and test that part of the circuit. Finally, you can install the isolation parts (digital isolators and buffers for the analog signals).
Furthermore, you should leave the through hole components for last since these will make soldering the surface mount components way harder.
Check: https://imgur.com/a/YNT4naN This is the first stage of testing. We check voltage values at different points of the circuit and see if they match expectations. I suggest using an oscilloscope probe and connect the oscilloscope's ground (i.e. the building's ground) to the power supply by earthing the power supply. This will mean that you'll have to be more careful probing but it has the advantage you won't have to add a ground to the probe. Since the ground will already be common to both. Watch out that all measurements will be relative to the building's ground!
On the pin 7 (output of the INA826) you expect to see something like this for a resistance of 117 Ohms:
The pin 1 should have a voltage of around 108 mV (this is the voltage drop of the Pt100 at 15C when fed about 1mA + 1 wire voltage drop):
The pin 4 should have a voltage drop due to the current temperature of the Pt100 (or due to the known resistance used for testing - I used 117 Ohms) and one wire voltage drop, so the voltage drop due to the wire will be cancelled.
For 117 Ohms we expect a value on pin 4 of around 118 mV:
The first version of this project had an issue where the pin 4 of the U8 of the -12V power supply was not connecting to Vext. This is now fixed. But if you see that this is not the case on your version, then you can solder a small wire between C32 and R21 which are located extremely close to each other and would connect the right nets together. Just make sure you connect the right part of the R21 and C32 (since they are double-sided components), you should connect the insides of these components.
I recommend not installing the jacks just yet so that you can test the INA826 without the jacks.
Leave them for last. And do not install the R6 and R27 resistors on the v1 version of the project since that connectivity test is not working as of this version. It's unknown if I will add a patch for this, or instead just fall back on the use of one connectivity check CONNGD+.
You could install these resistors, and it won't break anything, you will just have a different behaviour which you should be aware of. If you install these, then when no RTD sensor is connected to the J1s connectors then the voltage on the RTD- net (pin 1 of INA826) will be somewhere around 3.8 V making the INA826 saturate at 12V surely making the LM311 throw a critical error. The original expected behaviour was not to throw a critical error but instead have a CONNGD net go from +5V (ok) to 0V (error) all of this WITHOUT tripping a critical failure on the digital outputs of the CF (U_IsolationOut in main schematic).
Also this happens only if you do not provide +5V externally or if the internal +5V power supply is off.