Trouble setting up ADS1148

Diego,


I'm not sure where the EVM is going wrong. Part of the problem might be that you've made some modifications to the EVM, and there was a mistake in the modification. Second, I'm not sure I understand how your setup is done. In the first paragraph of tje last post you say that the IDAC is set to 1mA and the multimeter readings are correct, but in the second paragraph, you say that you use the EVM software and the measurements are wrong. How did you set up the IDAC when the first measurements were correct? Normally this would be setup with the ADCPro software and if this is done correctly, what did you do setup in the software afterwards?

Regardless, let's try to take this one step at a time. Lets look at the board itself first. Can you post a close-up photo of the board in its current state? I'd like to see what changes you've made and what resistors and caps you've taken out or replaced. Show it on the photo and add comments in Microsoft paint. You can show where you are attaching your reference resistor and the resistor you're using for the RTD. You can also show where you make your measurements.

Second, what settings do you have going for ADCPro? It might help to get a couple of screen shots for the tabs for I/O Config and the Power & Ref. You can also check to see that GPIO isn't used and Cal has a reasonable value in the registers for offset and gain.

As I mentioned earlier, I didn't have any problems when I set this up a couple of weeks ago. I was able to simulate many resistance measurements and it seemed to work just fine. Again, I didn't make modifications to the board to get it to work.


Joseph Wu

Diego,

Thanks for the photo in the pdf. First, I have just a couple of questions. Did you replace C20 with the reference resistor (840 Ohms)? I didn't see it in the pic. It probably would have been better to leave in R15 because that would connect the AVSS to REF1N. With the supply set to 0 to 5V, that would have made AVSS=GND and wouldn't have to connect it with a wire. Also, check to see that SW5 on the MMB3 is set to the right. That DAC switch should be set to OUT. 

However,I don't see anything specifically wrong with the changes that you've made. I can see that you've made a few other modifications, but I don't see anything that's bad.

I ran a similar test to what you have done. There were some differences, so I'll explain them as they come up. First, I selected two resistors. I used 110 Ω to simulate the RTD and the second reference resistor is 820 Ω. Here's what the setup looks like:

The only major change that I had was that, I used VREF0 instead of VREF1. There are pins on J8A that are available for the reference input so that I don't have to desolder devices off the board. I turned on the internal reference and set the IDAC magnitude. Then I set the IDAC currents to go to the AIN pins in the front panel. Here's what the ADCPro looks like for that:

I set up AINP=AIN0 and AINN=AIN1. Then I set one of the IDACs to AIN0, and the other to AIN2 (so that it isn't used).  

Then I set the data rate to 20 SPS and take a measurement. Note that excitation current goes through R4 and will add error to the measurement. So as I take the measurement, I short R4 with tweezers and the following is the result I get after 512 measurements.

This value is pretty close to expected (110/820)*2^23 = 1125301. I also used the multimeter to verify the voltages across the 110 Ω and 820 Ω resistors and they came close to what I expected to the voltages that should be on the devices. I would note that if you are making precision measurments, you'd want precision, low-drift resistors. Also, there's a small amount of leakage from D1, D2, and D3, that might make a small difference.

Again, I'm not sure what's wrong on your setup. I would suspect there's something wrong with the reference since you don't see anything across the reference resistor. I would try making a measurement using REF0 to see if it helps.

Joseph Wu

Diego,


There are two other things that I just noted. First, my EVM is for the ADS1248, not the ADS1148. I don't think this will be different in performance. Second, looks like you have a wire that connects AIN0 to VREF1P (which would be the top of C20). Shouldn't that connection be from AIN1 to VREF1P? That way the current doesn't bypass the 110 Ω resistor.

I thought I'd point out the latter comment. It's a little hard to tell from the picture.


Joseph Wu

Diego,


I'm glad you were able to get things working. I think that some of the problem might be the connection of REF1 and how you have it setup, but I think it's cleaner to use REF0 and start with something new that doesn't require modification.

I do use the 110Ω/820Ω for the calculating the measurement. In a way, it is exactly the same thing. Since the same excitation current goes through both resistors, the current cancels out and you are left with the resistances. Note that in setting up this type of "ratiometric" measurement as it is often called, you need a precision resistor that is low drift to make this measurement. In the end, if you make a measurement of the RTD, you care about what the RTD resistance is, not what the exact voltage is.

Good luck with your measurements. If you have any other questions, feel free to post back.


Joseph Wu