PGA309EVM-USB: PGA309EVM-USB questions

Hi Joe,

The way the ratiometric temp sensor Vout is calculated is as follows- 

Temp sensor ratiometric Vout = (Vexc / (RbridgeEff + Rt)) * (Rt / Vexc)
Ratiometric Vout = Rt / (RbridgeEff + Rt)
Ratiometric Vout = Rt / [Rbridge*(1 + TC1*Temp + TC2*Temp^2) + Rt], where Rbridge is the nominal value at 0C

If you plot the bridge impedance vs temperature for each pressure, you get five curves. You can then use trendlines in Excel to get a 2nd order polynomial best fit line for each curve, of the form y = a*x^2 + b*x + c. The intercept "c" will shift with pressure, I'm not sure if it's better to use the zero-pressure intercept or the average intercept but we will use the average value. This average "c" value gives us the nominal Rbridge at 0C, which in your case comes out to 434.75056 ohms. The "b" coefficient is TC1*Rbridge and the "a" coefficient is TC2*Rbridge, so we will need the average values of "a" and "b". Dividing these by the calculated Rbridge gives us the TC1 and TC2 values, which for your data come out to 3528.7932ppm and 8.6863603ppm respectively (although the software is limited to only 6 significant figures).

As a quick aside, this all assumes that the temperature coefficients of Rt+ are much lower than the temperature coefficients of the bridge, so make sure that you are using a very low drift precision resistor for this component. A temperature coefficient on the order of 5ppm or better will give you the best results. Once you have your TC1, TC2 and Rbridge values, you can create your sensor file.

Cheers,

Jon

Hi Joe,

Now, regarding the other parts of your question, I tried to use your precal and model files and found some discrepancies between the saved settings and the settings you mention in your Excel document (by the way, thanks for including so many details, it makes debugging much easier!). When I load your precal file, Step 3 is showing a value of 3.399680 instead of 2.12990. Also, in the Temp configuration of Step 7, the External Reference Select of the ADC says Vref instead of Vex.

In addition to those changes, it's important that the proper jumper settings are used on the PG309EVM. I started with the following - 

JMP1 NC, JMP2 don't care so PGA309 uses internal reference mode
JMP3 set to ADS1 to use ADC
JMP7 set to Vout to Prg, JMP8 set to One to Vout to make Three Wire Mode

JMP12 set to Vs for ratiometric mode (more on this in a second)
JMP17, JMP4, JMP5, JMP6 set to Emulate
JMP14, JMP15 set to 100mV (this is actually 120mV but the label says 100mV)
JMP13 set to RT, JMP16 set to Rt+ 

It's important that the appropriate JMP7 and JMP8 settings are used, as well as JMP14 and JMP15. Now, when I ran a calibration with the above, I found the calibration results came out to be quite poor, just as you had seen. You can actually tell something is wrong because when you run the Autocal, the Temp DAC reports the same number for each temperature even though they should be different. What worked for me was setting JMP12 to Vexc instead of Vs. I know the documentation says that using the Vs setting puts you in Ratiometric mode, but what it actually effectively does is disconnect the Vexc pin and use the rail instead, which will cause major issues with your setup.

Click here to see the full schematic for more details

After making that change and re-running the calibration, I checked the EEPROM and saw that the values of GMx, ZMx, and Tx for x>0 were now successfully getting populated. However there was one more problem, when I read the registers the block diagram was showing a strange configuration that didn't make sense. I checked the Registers tab and saw every one was showing the exact same value. I was able to fix this by changing the Output mode on the block diagram from Vout (4-wire) to Vout (3-wire) and then redoing the "read all" operation. Once I did that my registers looked good and the block diagram gave the configuration that I would have expected.

I did some testing with the Sensor Emulator after the cal and found the results looked much better, with error of around 6% or better. I think you might be running into a limitation of the Sensor Emulator though, your curve goes down to below -120mV/V to maybe -125mV/V or so but the bridge output can only go to -120mV/V. Theoretically if it's clipping, that could cause some problems with your calibration accuracy.

I'm attaching the precal, model, and sensor files that I generated. Feel free to reach out if you have any further questions.

7212021_e2e_sensorfile_modified.csv

[PGA309_Settings]
Numb Reg=3
Poly Order=2
Output Mode=1
Vout_High_Target=4.500000
Iout_High_Target=0.000000
Iout_Low_Target=0.000000
Vout_Low_Target=0.500000
Vs=5.000000
Vref=4.096000
Calibrate Nonlin=FALSE
Reg0=0
Reg1=0
Reg2=0
Reg3=3328
Reg4=0
Reg5=0
Reg6=5211
Reg7=0
Reg8=0
Temp0=-1.111000
Temp1=23.889000
Temp2=54.444000

[Filenames]
Pre-Cal Filename=7212021_e2e_precal_modified.txt
Sensor Emulator Filename=7212021_e2e_sensorfile_modified.csv
[Misc]
Comments=modified precal, no nonlin correction
Serial No=8
Model No=0
Model ID=""
Use Sensor Emulator=TRUE

Cheers,

Jon

Hi Joseph,

The short-circuit current limit of the pin is 50mA typical. However, the maximum available current for Vexc depends on the maximum current that the DUT can pull, which depends on how it is powered.

If using the XTR117, with JMP11 in the Loop_Power position, then this current will be limited by the XTR because the PGA will be powered from the Vreg of the XTR. The short-circuit limit from Vreg is about 12mA, but any more than 3.8mA draw and your XTR output can't go down to 4mA so the output range will be clipped. In this operating mode your Vexc current limitation will thus be around 2.6mA or so.

However, if you are in the voltage output mode with JMP11 in the Vdut_Power position, then the USBDAQ will be the power source and the current limitation depends on the settings of that board. It's possible to operate everything off the USB power (JUMP13 to REG and JUMP14 to BUS, or JMP13 to BUS and JUMP17 to BUS) but then you are powering the entire USBDAQ and the PGA off of a single USB connection. The answer to the question of the max current limit then becomes "it depends", because it comes down to the way the user's computer is set up. The USBDAQ makes use of a TUSB3210 USB 2.0 controller. USB 2.0 can theoretically negotiate a current of 500mA, but only 100mA is technically guaranteed. The USBDAQ will basically pull current directly from the connector, but the computer itself decides what that maximum current is. The biggest issue is I don't have a number for the typical Iq of the entire system, so I can't really calculate what the remaining current budget would be. I'd advise not operating in this mode if possible because of all of the unknowns and the limited headroom anyways, you're better off using an external supply.

If using the onboard regulator (JUMP13 in the REG position), you can apply a 6-10V DC power supply to T3 or J15. You'll need to put JUMP14 into the 9V position for this. The regulator can supply 100mA of current to the DUT with only about 60mV of droop, so you'll be able to hit the 50mA typ max short circuit current for Vexc without a problem (as long as your external supply is capable of it). Another way of doing this is connecting an external supply to Vraw (JUMP13 in BUS position, JUMP17 in VRAW), but that supply must be in the 3V-5.5V range and should be very clean. Your best option is just to go the J15 route I mentioned above with the external supply.

TL,DR - the max Vexc short circuit current is 50mA typical, if in XTR mode the effective limit is about 2.6mA, and if in voltage mode the effective limit depends on the USBDAQ settings. To get the full output range of the Vexc you'll want to use an external power supply on J15 or T3 of the USBDAQ

Cheers,

Jon