This thread has been locked.
If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.
Part Number: ADS1248
My customer has the following:
The PGA (per the data sheet) has a tempco associated with supply voltage, data rate and specific gain. For reference see Fig's 17-20: 5V and Fig's 21-24; 3.3V of the data sheet. We set the gain to 16 and 32, use a 3V supply and 2.5V reference. We have a operating temp of -20 to 55 C. Our testing shows the tempco impacting our system performance. Without going into circuit detail, we actually used this ADC to servo a TI DAC output. Works well … at room temp.
ADC calibration: However, our software engineer has read on the e2e forums, that ADC calibration can be painful to constantly tweek the gain cal constants. Question: The curve shown on Fig 23: is that THE gain vs temp curve, or is that a typical curve? If just typical, do we need to do a two point against the PGA, for each ADS1248 to fit and fix this gain skew? Or, do you folks have some recommendations on calibrating out the gain & offset errors over temperature?
I don’t see how I can add a circuit to compensate. This is in a thermocouple measurement circuit. We do know the cold junction temperature. There are also the built in temp sensing diodes. I have checked “e2e” but nothing jumping out at me.
My associated questions:
1) PSRR is 100dB, min. Is this over temperature?
2) I believe one can do a self-cal (either at 25C or at Temp) – as outlined in section 22.214.171.124, page 40 of the datasheet by using
SELFOCAL. yes, there is a manual version by using SYSOCAL, but it sounds like we want to avoid this from a S/W perspective.
Gain error is calibrated with the SYSGCAL (and from what I can tell, this is a self-cal test). Correct?
Best Regards, Mark P
We are glad that we were able to resolve this issue, and will now proceed to close this thread.
If you have further questions related to this thread, you may click "Ask a related question" below. The newly created question will be automatically linked to this question.
In reply to Joseph Wu:
In reply to Mark Pearson:
Mark,I had thought this was a simple thermocouple circuit, but it looks like there are a few more parts to their setup. I'd still like to know what the components are even if they aren't TI parts. It just helps to track down the drift characteristics of the all the components involved. This would also include the op-amp used as the buffer. It would help to have a full schematic to look at where the sources of error come from. If there are other components (series filter resistances, protection diodes that have leakage), there may be other errors to consider.There will be some ratiometric cancellation if the DAC and the ADC have the same reference, but then the DAC also has it's own gain drift which needs to be calibrated out. I'm not sure if the DAC or ADC has the larger drift both seem similar. To start, I think it would be best to have some precision measurements of both the ADC input (or DAC output) and the reference over temperature. This will give some idea if the the ADC measurement is really drifting, or if the DAC output has a larger drift.I would also note that most thermocouples have a fairly large tolerance error. The best thermocouples have an error of ±0.5°C plus a tolerance proportional to temperature. If a precision measurement is necessary, using an RTD is probably a better solution.Joseph Wu
Thanks for checking in Joseph. It turns out that part of the problem was that their design was violating the PGA common mode range:
"Page 5 states an external Vref should be less than your analog supply minus 1. So let’s say you have 3V analog but stick in a 2.5V external reference. What bad things might happen?
Another question. When measuring this external ref, data sheet states you get Vrefext/4 … APRROXIMATELY! Page 35. What is approximately? If I am using the external reference for other stuff and the ADS1248 hardware is setting bit weight based on the actual reference, what is the impact?
Now we have done external ref and made a measurement using the MUX internals. We see maybe 4 – 5 mV of difference between the ADS1248 measured result and me using a DMM. Are we missing something?"
And my answers to the designer:
Ah, yes – with respect to the External VREF Signal level. Well, you won’t be blowing up the ADS1248, but you will be violating the PGA common mode input range.
I would refer you to page 72, section 10.1.3 of the datasheet
See page 27 for the 126.96.36.199 PGA Common-Mode Voltage Requirements.
For the “…you get Vrefext/4 … APRROXIMATELY” question. Yes, I saw this too, though I never really looked into it before. I will look into it for you now.
Question: We see maybe 4 – 5 mV of difference between the ADS1248 measured result and me using a DMM. Are we missing something?
Answer: I don’t know enough about the situation to say with a high certainty. I would need to know where the precision source is (are we measuring the External VREF – or something else). How long are the DMM leads, their impedance, and the input leakage current? How much noise are they picking up (this foes to the fact that many DMMs are measuring RMS voltages and not true DC. I assume a calibration was done on the ADS1248, yes? If so, then the ADS1248 (input referred Vos) would be less than +/-15uV.
I am using an Agilent 34401A. When I measure the external reference, heck it is dead nuts. That is, I measure 1.8V (or 2.5V when using it). Standard DMM probe leads – maybe 3 feet or less. I am struggling with my ability to measure the 1.8 but when we ask the hardware it reports, let’s say 1.795V. Using that for all calculations when the bit weight is actually based on the 1.8 V would cause problems, yes?
Here is the example. Using an LMT85 for our cold junction. I measure the voltage with the DMM. Ask the ADS1248 to report that voltage it is off by maybe 7 to 8 mV. This is using the external reference. Then we use the internal reference. Heck, the ADS1248 reports the correct LMT85 voltage.
From the attached schematic, if I supply a reference into REFP1 and REFN1 as shown, let’s say 1.8V, I would expect the hardware to report 1.8V. Am I doing something wrong? When I review the RTD circuit example in the data sheet, basically same gig, or so I think.
Question: is it OK to connect REFN1 to circuit common (ground). Example?
Answer: Yes – connect the reference GND to REF1N and to VSS (if VSS in not at a different potential). Sure, for an example – see the EVM schematic.
Joseph - if you could comment on the question" When measuring this external ref, data sheet states you get Vrefext/4 … APRROXIMATELY! Page 35. What is approximately?" I would appreciate that.
All content and materials on this site are provided "as is". TI and its respective suppliers and providers of content make no representations about the suitability of these materials for any purpose and disclaim all warranties and conditions with regard to these materials, including but not limited to all implied warranties and conditions of merchantability, fitness for a particular purpose, title and non-infringement of any third party intellectual property right. TI and its respective suppliers and providers of content make no representations about the suitability of these materials for any purpose and disclaim all warranties and conditions with respect to these materials. No license, either express or implied, by estoppel or otherwise, is granted by TI. Use of the information on this site may require a license from a third party, or a license from TI.
TI is a global semiconductor design and manufacturing company. Innovate with 100,000+ analog ICs andembedded processors, along with software, tools and the industry’s largest sales/support staff.