Thermocouple measurement using LTC2368-24 Noise

Hi moro 

How have you determined the 100uV value when the inputs are connected? Is it RMS or peak-to-peak value?

Zoltan

Hi Zoltan,

the value was read directly from the adc without any aditional procesing. I was reading every 50ms the adc's register and just show the data on the screen.

I am doing something wrong?  

Hi,

It seems that it turned out that your data is shifted by one bit, so the SPI settings can be incomptible. Try to find the proper setting.

Anyway, now you should get the right ADC data if you divide it by 2, so you can test the 33mV reading.

Zoltan

Which data is shifted? The adc 24 bit data or the  16bit Nsample data?

I am a bit confused.

Hi,

moro said:

Which data is shifted? The adc 24 bit data or the  16bit Nsample data?

Both, since the 5 bytes form a single bit stream, you have seen the time diagram above. Find the proper SPI setting (read the article I've shown to learn more about SPI modes) to get the bits in the right place.

Optionally, before doing that, you can divide the data by 2 (you know it is equvivalent to shifting all bits to the right by one) and experiment with different analog input voltages. So use 10k and 100Ohm to get about 33mV, then exchange the resistors to get 3.3V-33mV. Try to get proper readings for these voltages for various averages.
I think now you will be able to get this ADC work properly.
Zoltan

Hi ZoltanDTI 

thanks for your patience.

Its seems the correct setting was the initial one for spi with CPHA=0 where data is read on the rising edge of the SCK.

Another misleading factor was the logic analizer capture speed...

Now also the Nsample data returns 3 ( N-1).

I have uploaded the excel with both scenarios values, the green marked ones are the ones which i think they are correct.

For the 33mV with a multimeter i read 32.5mV, unfortunatly for 330uV i dont have enough scale on my multimeter... i can read only 0.3mV , but since the low side resistor is the smalest one, i presume the real value is somewhere near 325 uV

.XLSX

What do you think about these values?

Hi,

Great! Your data has σ≈87 LSB (17uV) for N=1 (and four your whole circuit), only slighly higher than the datasheet value. You could also get the SPI work properly.

I think only one thing remains. As you can see, the averaged value gives you higher value, although the ADC only computes the average of four samples. This means the even the individual sample values must be higher. I think this can be caused by the different sample timing combined with the source impedance of the voltage divider.

I recommend you to try these:

1. Use 10 Ohm and 100 kOhm, as I have suggested. The internal capacitors of the ADC mean current pulses upon every conversion, and the 100 Ohm can cause considerable voltage drop at such high resolution. The 100nF should also be considered, you can also try to use 3.3nF capacitor, it is smaller, can be charged faster. Read the Input Currents part in the datasheet.

2. When you make averaging, you have bursts of pulses, so the distance between the pulses are much smaller than the distance between the pulses when N=1. According to the above, it can mean difference in the readings. Try to increase the distance between the pulses to allow more time for the capacitors to be charged. Now it seems to be about 2.5us. Use 5us or even longer. The best would be to follow the datasheet's figure 14:

So use evenly distributed ADC start pulses, and read the data only at every fourth ADC start pulse. In such case the mean must be the same for N=1 and N>1.
Good luck!
Zoltan

Hi,

concerning the spi line, in the future i must consider using faster opto isolators with smaller propagation delay....

For now to characterize the circuit i will use direct conection with the microcontroler to avoid any comunication issues.

i have changed the resistor divider with 100k and 10ohm,  unfortunately i dont have 3.3n cap, but i used 4.7n cap.

i increased the time between N samples to 11us. 

After increasing the time between samples, the values looks much closer to reality lets say... 

I have attached a small sample report with values  between N = 1  & N=4 and also tried with  N=128.

There is clearly a improvement using the average function now.

XLSX

Hi,

It is very good! Everything works fine, the 360uV is within specs, the zero scale error is about 10ppm (the datasheet guarantees 20ppm), the noise is also low. I think now you can say it is very close to that what can be expected. Congrats!

Best wishes, Zoltan

Hi,

this sounds encouraging for me. The discussion was quite long.

Unfortunately i am not very experienced with precision ADC, had only projects with regular 10-12bit adc inside various mcu's.

But i took this challenge to "jump" in the microvolts area, where are alot of analog challenges lets say.

Coming back to this standard deviation, its still a bit puzzling how to calculate the deviation for my code.

Lets talk about N = 1, the datasheet gives me 68.5 lsb rms  which is ~13.47uV which is 88.4uV peak. This is clear.

But how do i calculate the deviation from my values?

Hi  

I’m sorry, but I have to say that it can’t be the purpose of this forum to give lectures about the basics of engineering knowledge that can be found in textbooks, on wikipedia, in the excel help,  etc. If you do not know how to calculate the standard deviation or histogram from your samples, then take your time, I’m sure you'll easily find and learn it.

Finally take my advice: if your project is to use thermocouples, forget about this (otherwise very good) ADC, use a low-frequency sigma-delta ADC instead. They have 24 bits of resolution, less noise, automatic calibration, internal programmable gain, multiple channels in a single chip, no problems with galvanic isolators, no analog frontend required, much easier to use. Here you can see a good example: CN0384 -Completely Integrated Thermocouple Measurement System using a Low Power, Precision, 24-Bit, Sigma-Delta ADC.

I think this thread should be closed, all of your relevant problems have been solved.

Good luck for your future work!

Zoltan

Hi,

yes you are right, i will study these in more details

 i will take in considerations your hints about these delta-sigma solutions.

Thank you and have a nice day !

Hi,

yes you are right, i will study these in more details

 i will take in considerations your hints about these delta-sigma solutions.

Thank you and have a nice day !