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?  

Unfortunatly i discovered also the date before and after the opto isolator is diferent because of that delay....

the ADC sends 00 3E AE,  and after the opto, the mcu sees 00 1F 57,    i need to change my setings for the spi. Because of this delay i need to change the data to be validated on the traling edge of the SCK ( CPHA = 1 ).

I will come back in the evening with the corect values, i hope this was my only problem in why the data is strange...

Hi ZoltanDTI 

i removed completely the opto isolators, i noticed that delay from the optical isolators is causing me serious trouble with MOSI and SCK hold/setup times.

Now everything is tied directly to the mcu.

I have attached the correct data now, 400 samples read with N=1 and 400 with N=4.

What do you think?

XLSX

Hi,

I don't see change in the data and the calculated voltage does not seem to match the input voltage. You've used 100 Ohm and 1M for dividing Vref of 3.3V, so the input voltage should be 330uV. The corresponding code is 1678, but even considering the zero scale error (max ±20ppm, ±336 code), your data are far from this.

I do not understand why don't you have the same mean using four averages. Figure 14 shows that you should get the average of four consecutive samples, that should fluctuate around the same mean as the individual samples:

Are the data really the raw values from the ADC?

So until you can't calculate the applied input voltage properly from the ADC output code (at least for N=1), you certainly have somewhere a mistake in reading the data. Maybe in the software.

Zoltan

Hi,

I am wondering, since there is no software power management for supplying the adc,  basically everything is powerd at once ,  and  when the mcu gpio's get initialised  could this trigger these strange values?

Bellow is the 2.5Vdd & 3.3Vref voltage for adc at powerup aswell  the start& busy signals

After the power up at ~560 miliseconds i sent 24 dummy clocks on spi to clear the IO register

for N = 1 ,  SCLK is 250khz,   330uV at input.

This how the raw spi comunication looks like without any software manipulation. The exact same values are read trough software.

Also bellow a picture with the timings between Start-CNV, BUSY and the 1st clock edge

After the busy goes down i clock 24 bits  in frames  of 3 x 8 to readout the data.

And this is how a data frame looks for N = 4 - reading 0x0021CC.

Bellow is how the start-cnv and busy signals look for N=4

After the 4th BUSY falling edge  i send 24 bits to read the averaged by 4 data.

By the way, maybe this sounds stupid.... i presume the adc is already dividing by 4 the data before reading via spi,  I mean i dont need to average this in my sw, right?

At this point i start to go out of ideas.

Hello,

i think i found something strange while checking a data frame.

Here i have ploted also a serial out for debuging the data.

N = 4 and i aparently get a value of hex value of 21BA.

But if i look closely to bits 15-8, judging by the logic voltage levels ( see the zoomed picture bellow) , i actually get 43 in hex

For the rising edge of bit 14 i see the miso goes high delayed by 4 ns from the clock edge, maybe this is why i read this bit as 0 instead of 1.

The same problem for the falling edge of the bit.

The same situation is also for other bits,   the last byte is seen as 0xBA, but based on logic levels seen, i have 0x74.

So my data in reality would be  0x4374 instead of  0x21BA?

Is this 4 ns delay  time normal? 

I am a bit puzzled.

To be honest this is my first time to see a spi device behaving like this.

There are no capacitors or other components on the spi line that can cause such a delay.

Hi,

4ns of delay is absolutely normal, see the datasheet:

If you use the SPI protocol, you may know that if the data is shifted out on the rising edge, then it is sampled on the falling edge (see Introduction to SPI Interface for details), as marked with yellow circles:

So the delay does not mean any problem. Of course, the proper SPI mode should be selected.

I recommend to do the following tests.

1. To be sure that you read the data properly, read 5 bytes of data instead of 3. The last two bytes should represent the number of averages:

Give it a try for different number of averages. Play with this until you read the data correctly.

2. Increase the input voltage to be sure you don't have problems with small errors. So, replace the 1MOhm resistor with 10k, keep the 100 Ohm (do not increase it). Then you have about 33mV at the input. Use N=1 and N=4 as before and see if you can get the correct value.

Good luck!

Zoltan

Hi,

i changed the SPi settings to sample on the falling edge of SCK  ( CPHA  = 1) . This enigma is solved now, at least i have reliable interpretation of the logic data from SPI now....

Before i change the resistors, i  have added the extra 2 bytes to the reading.

With N = 1 for C0..C15 bits i read a value of 0.

But when i change to N = 4  for C0...C15 i get a value of 6 ( see bellow screen shot).

Before i change the resistors, i was wondering why i get a value of 6 samples for average? 

Shouldn't be 4 ?

At least this is my understanding from the picture bellow.

Latter edit ...on i spoted this in the datasheet:

So if N =1 , then i get 0 samples averaged.

But for N=4 shouldnt i get  N-1 averaged samples? 3 ?

Hi,

I interpret the time diagram just as you. So 6 is not OK. But as I said, try this with different number of averages, then you'll get more info about the behavior. Try N=1 too. Are you sure there are no other sampling peaks before this four? Check the signals with an oscilloscope, the logic analyser may have somewhat different levels for the logic values.

Zoltan

p.s.: 3 can be obtained by shifting the bits by one bit to the right. So the SPI settings may be wrong.

Hi,

i have played with diferent N values and i found something intresting,

For N=256 i read C15..C0   0x1FE,  this is giving me 510dec,    but if i divide by 2 this number, it gives me 255, which is  N256-1 like expected.

I noticed the same for other values like...

N=10000 gives 19998dec /2 = 9999 samples.

In my case for N=4, gives me 6 which divided by 2 gives me 3 which is N-1.

The same as for N=16384

I am missing something here ?!?

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

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 !