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Hello,
I have inherited the design below with using the ADS1148 to measure temperature from a "user-selected" 2, 3, or 4 wire RTD, or a thermocouple sensor. The design engineer is no longer around to ask about the thinking behind the design so I am coming here for some clarity. The first Image is the ADS1148 circuit, and the second image is the analog input filter circuit.
For reference, here are the settings we're using for the ADS1148:
IDAC magnitude: 1mA
Data Rate: 20 samples/sec
PGA gain: 1 V/V
IDAC source: IEXC1
R_Bias: 150 Ohm (for RTD inputs) or 47k (for thermocouple inputs), MCU switches between these resistor values.
AVDD: 5 VDC
So with this design there is no connection from IEXC1 to the input measurement or the reference input. So for a 2 wire RTD we must use external jumpers from RTD1_EX+ to RTD1_SENS+ to connect the excitation current source to the input path, and a jumper from RTD1_SENS- to RTD1_EX- to connect the current source to the reference input path. On a 4 wire RTD, we don't need the jumpers. The issue comes in when using a 3 wire RTD with this design/configuration.
I know for a three wire RTD we need two current sources. The ADS1148 tells me to source the current from AIN0 and AIN1 per our application. However, when I do this, I am picking up the 1k-Ohm offset error due to the RC filter on the analog input. This is an issue because a 150 Ohm reference was chosen and a 1k-Ohm plus RTD resistance will give full scale ADC readings because of the offset error. Would this be the correct sources to use for the 2 matching IDAC's for this application? How might I change the hardware or software to support 2, 3, and 4 wire RTD's on the same circuit? Should I be using a larger reference resistor? 2.4k?
Also, I have a question on the analog input filter. We are using 1k-Ohm, 1% tolerance resistors, per instruction from application note SBAA201. However they could be mismatched by up to 10 Ohms, which I believe to be causing some error on the measurements the ADC is getting. The filter is important for the design since the device may be in a noisy enviornment. Would going to a 0.1% or 0.5% tolerance resistor for the filter be recommended?
Thank you for the feedback,
Jonathon D.
Jonathon,
I'm not completely certain of this circuit is put together, but I'll throw out a few comments and you can respond to them.
First, I think the basic idea of how you want to make a 2-, 3- or 4- wire RTD measurement is something like this:
The order of the AINx pins are different than yours, but the idea of how you want to measure this is the same. If R202 is your reference resistor, its too small. At 150Ω it means that the maximum RTD resistance would also be 150Ω. An ADC compares the input voltage against a reference voltage. If the reference is 1V, then the largest differential measurement you can make is ±1V. Using a 150Ω reference is out of the question if you want to measure a 1kΩ RTD because the current is the same for both. Start with the largest RTD resistance you expect based on the temperature range, and that will be the minimum you need for the reference resistance.
Also, this reference resistance must be a precision resistor with high accuracy and low drift because the RTD measurement depends on this reference to be accurate. That also means that any series resistance for the switch is an additional error.
Another point is that you can measure a 3-wire RTD using only one current source. However, it takes two ADC measurements to complete. Using my diagram above, looking at the 3-wire RTD, make a measurement from AIN1 to AIN2. This measures IDAC1*(RRTD+RLEAD1). Leaving the configuration the same, measure AIN2 to AIN4. This measures IDAC1*(RLEAD3). With low ADC input current, no voltage is dropped across RLEAD2. Assuming RLEAD1=RLEAD3, you can subtract the second measurement from the first measurement. This leaves a measurement of IDAC1*RRTD alone.
If you have the ADC in gain of 1, and there are no leakages of IDAC1 from the reference resistance, the output of the ADC reports:
Output code = VIN*2^15/VREF = IDAC1*RRTD*2^15/IDAC1*RREF = RRTD*2^15/RREF
Again, note that any error in RREF, including the switch resistance becomes a gain error.
Going to the thermocouple I'm not sure why you need the 47kΩ resistor. If you intend on driving the resistor for a bias that's fine, but you shouldn't use it for the reference. IDAC currents are do not have good absolute accuracy. The internal reference can be used for the measurement. It would be good to lose the switch resistance error.
As for the filter, you don't need any special resistances. Going to 0.1% or 0.5% won't hurt anything, but it's certainly overkill. As I mentioned through the previous paragraphs, I think you'll be able to make measurements without the error contribution from the filters.
If you have any other questions on topology of the circuit or other aspects of the RTD measurement, feel free to post back. I'm sure I can help (certainly post back before you finalize your circuit).
Joseph Wu
Hi Joseph,
Thank you for the feedback. It is very helpful.
I agree that the 150 Ohm reference resistor is too small. I've purchased some 2.4k Ohm, 0.1% tolerance resistors to use instead. The largest temperature we would be expected to read for our application would be 120C. But we must support both a 100Ohm and 1000Ohm RTD, so I think the 2.4k ohm reference makes more sense than the 150 Ohm that was previously chosen.
You bring up a good point about the analog switch resistance. We are actually using a TI SPDT analog switch in our circuit (part number: SN74LVC1G3157) and it looks like the on resistance is about 7 Ohms typically. We hadn't factored that gain error into our calculations. I will consider dropping the switch from the circuit since I am already doing a re-design anyway.
Thank you for the input on the 3 wire RTD using a single IDAC source. To follow up on the info you provided, for the best possible accuracy, would you recommend using a single excitation source and taking two measurements and subtracting out the RLEAD1 (or RLEAD3, assuming they're the same value), or using two excitation sources and swapping/chopping them?
For your example circuit above, a 2-wire RTD I would measure AIN1(positive) to AIN4(negative) and for a 4-wire I would measure from AIN2(positive) to AIN3(negative). Is this correct?
I don't quite understand your statement on thermocouples "IDAC currents are do not have good absolute accuracy". But I think I am aligned with you thinking that using the internal reference and getting rid of the switch could be a better option. The thermocouple would attach to the same terminals as a 2-wire RTD and would be measured from AIN1 to AIN4 in your above example. Is this correct?
Thank you for the feedback on the filtering. I know our current design does not filter the reference at all, but it does filter the input. Would you recommend filtering both the input and reference?
I will work on updating the circuit and will run the changes by you for confirmation before finalizing.
Thank you,
Jonathon D.
Hi Joseph,
Thank you for the detailed and prompt response. It is very helpful to me and I very much appreciate it. I think I am going to use the 2.4k Ohm reference resistor. My thinking: I'd set PGA gain set to 16 (for PT100) or 2 (for PT1000). Leave the IDAC at 1000uA for both PT100 and PT1000. With those settings we would have the Vcm near mid supply and a resolution up to 130C for the PT100 sensor type, and resolution to 500C for PT1000. Which is acceptable per our application.
Thank you for responding to my questions. I believe you have adequately answered them all. I do have a follow up question on the TC measurement though. So going off of the model circuit you provided. I could configure the ADS to measure the TC from AIN0 to AIN1. I would then configure the VBIAS register to apply a mid-supply bias voltage to AIN0. The gain I would chose would be 32? Or more? I'm guessing the max temperature we would want to measure with the TC would be 600C. Most of our users will use K-type thermocouples. Can you confirm this is the proper approach?
Thank you for the info on the ADS114S08, but since this is an existing design, I think that I will stick with the ADS1148 for simplicity's sake on the component procurement and software side.
Here is my proposed updated circuit. I still have to go through and do the component selection for the external low-pass RC filters. Is Application Note SBAA201 a good reference for this exercise? I would appreciate any feedback you may have on the filter component selection and/or the rest of the circuit below:
Thank you,
Jonathon D
Joseph,
380C is far higher than any temperature that we would attempt to measure with this device. So I'm comfortable with that choice of reference resistor. 90% of the applications for our devices we see use PT100 sensors anyways.
Thank you for the heads up on the noise on the VBIAS, I will have to test that out when I have a prototype of the new design.
I have a schematic to review. It is attached. Please provide feedback, if you could. Thank you.
Joseph,
I've made a few changes to the proposed circuit. See attached. I chose a 1kOhm resistor for the filter, over-voltage isn’t a major concern. We are using 20 samples per second as our data rate, so the modulator rate is 32kHz. Therefore, the corner frequency to use in the Cdiff calculation would be 3.2kHz. The resultant Cdiff value I am getting is ~0.024uF. This is assuming a 150 Ohm RRTD (Since most of the time PT100 RTD’s are used). Then choosing a common-mode resistance value of 2400 pF. I am choosing to just use 10uF of bulk capacitance at the reference input, but am going to reserve some space if we decide to put an RC filter on the reference input later.
Also, I’ve added 100 Ohms of series resistance to the IEXCx lines, for over-voltage protection. Which aligns with the ADS1148EVM reference design.
I should also point out that we are using a ferrite bead (L800, L801 on schematic) in series with the termination point, for EMC filtering. See photo below… Will this cause any issues (inaccuracy) to the temperature measurements we take?
Please let me know if you have any further comments to add.2134.67342_Proposal.pdf
Thank you,
Jonathon Dahl