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Hi All,

Can anyone suggest method to measure k-type thermocouple temperature using ADS1220?

After getting the ADC reading from the ADS1220 ,how to convert it into the equivalent voltage and then to its respective temperature?

Regards,

Fakhruddin

• i used equations from NIST

www.keysight.com/.../5306OSKR-MXD-5501-040107_2.htm;cc=US&lc=eng
• Hi Fakhruddin,

When making temperature measurements with a thermocouple (TC), it is not just a single conversion from the voltage reading you get to a temperature. You must also consider the cold junction where the TC connection is made on your board and the TC that is formed at that junction.  Basic information for the TC application can be found in the ADS1220 datasheet starting on page 48.

For a good background on TC measurements, look at the ADS1118 application note:

http://www.ti.com/lit/pdf/sbaa189

Most everything in this application note will also apply to the ADS1220.  You may also find some of these TI Designs helpful as well:

Best regards,

Bob B

• Hi Bob ,Chris

I am having the following issue
Is the following equation is for calculation thermocouple voltage,
equation: 1 LSB = (2 · Vref / Gain) / 2^24 = +FS / 2^23

1) I am getting 0x00FFFFFF continuously.
• Hi Fakhruddin,

Your LSB equation is correct.  However it appears that you are not reading the data correctly.  You show a 32-bit value and the ADS1220 is a 24-bit device.  I'm guessing that you are not properly sign extending the result.  A value of all bytes high can mean that you are reading a code value of -1, or it may mean that the device is either not setup correctly or communication is not working.  CS must remain low throughout the entire communication transaction.  Both AVDD and DVDD supplies must be correctly applied for the device to function.

Can you send me your schematic?  I would also like to know the register settings that you are using, and if you have successfully read the configuration registers from the ADS1220 to verify that your communication is functioning.

Best regards,

Bob B

• Hi Bob,

register values are as follows,

0x00                                        0x08

0x01                                        0x04

0x02                                        0x10

0x03                                        0x00

Regards,

Fakhruddin

• Hi Fakhruddin,

Are you able to verify that the device configuration is properly written?  Can you send me any scope or logic analyzer shots of the communication?  Is your circuit built on a PCB, or is it built using a protoboard?

One thing you can do is monitor the DRDY pin.  The default mode is single-shot, so if you have written the device for continuous conversion mode you should see that the DRDY pin pulses at the specified data rate of 20sps.

When you are attempting to read from the device are you reading directly (just sending 3 bytes of SCLK) or are you using the RDATA command?  One common mistake is CS is not held low throughout the entire communication transaction.  For example, you need to hold CS low for the entire length of the command and data when issuing the RDATA command.  Raising/lowering CS will cause the SPI bus to reset ending the communication.

If you do not see the DRDY pulsing but you believe you have correctly communicated to the device, make sure that the CLK pin has a good and solid connection to GND through the pull-down (you can tie the pin directly to GND).  If noise glitches on this pin, the internal oscillator will be shut down.  The noise will appear to the device that you are attempting to apply an external clock.

Best regards,

Bob B

• Hi Bob,

I am getting DRDY Pulsing and also data but it is incorrect.
If you have example code of ADS1220 for reading and calculation of k-type thermocouple temperature.

Regards,

Fakhruddin
• Hi Fakhruddin,

Previously you had said that the output for DOUT is always high.  Did you ever read back the configuration registers to make sure that you did not have a communications issue by a possible bad connection on the DOUT line?  Can you send me scope/logic analyzer shots of the communication?

What are you using for an input?  Do you actually have a K type TC connected?

Here is a software link to a TI Design (TIDA-00189) that uses an ADS1220 to measure a TC.  You may find some of the code useful:

http://www.ti.com/lit/zip/tidc701

Best regards,

Bob B

• Hi Bob,

I am successfully able to read the thermocouple  tip voltage and also the cold junction temperature.I have even coded the calculations according to the algorithm/steps given in the datasheet to calculate the actual temperature.

I am testing the thermocouple by using bucket of water with the immersion heater immersed into it and thermocouple also immersed into the bucket of water.

I am getting following temperature reading.

1)  First case

Normally I read the temperature 5°C more than the actual & room temperature when the heater is not immersed in the bucket.

2) Second case

When I immersed the heater in the bucket, I am getting the actual reading +/-3°C continuously varying.

In the above two cases are tested with following conditions given below:-

a) The heater is connected to the supply.

b) I have connected a dual supply of -2.5V and +2.5V to the analog section (AVDD and AVSS) and , 3.3V & GND (DVDD and DGND) to the digital section of

ADS1220 .I have kept the analog and digital section isolated.

c) I am using a grounded K-Type thermocouple i.e the thermocouple tip is shorted to the shield.

Regards,

Fakhruddhin

• Hi Fakhruddhin,

How are you measuring your cold junction?  It would be very helpful to see the PCB layout with respect to the cold junction, as it appears that there is some error in your measurement.

Another question I have is with respect as how you are determining the 'standard' temperature.  How are you quantifying that you are measuring 5 deg C higher at room temperature?  I have the same question with respect to the water bath.  And how do you know that the water bath has even temperature throughout the bath?  To determine the effectiveness of your system, you should be using calibrated sources.  And you need to make sure that the cold junction is not the cause of the variations.

Best regards,

Bob B

• Hi Bob,

I am measuring cold junction temperature using the internal high precision temperature sensor by making the TS = 1,

register setting for Cold junction temperature is (0x00, 0x06, 0x10, 0x00) and for thermocouple (0x00, 0x04, 0x10, 0x00) in the ADS1220.

Currently I am testing the ADS1220 circuit on a general purpose pcb, since after satisfactorily implementation of ADS1220 we want to integrate this circuit in our system for K-type thermocouple temperature sensing, so the actual PCB is not yet made.

I have a k-Type thermocouple meter whose reading I am considering as a standard.

At room temperature ADS1220 circuit shows temperature  5°C higher than the temperature shown by the thermocouple meter.

The water bath is a smaller bucket just i am using for testing

The cold junction temperature reading is nearly stable no issue , but the thermocouple  voltage varies randomly due to which the ADS1220 circuit is giving +/- 3°C continously varying temperature reading from across the actual temperature (shown by the thermocouple meter).

Do this IC work for K-Type Grounded thermocouple?

The ADS1220 circuit diagram is attached.

Regards,

Fakhruddin

• Hi Fakhruddin,

Yes, you can use the ADS1220 for grounded K-type thermocouple.  A grounded TC is one that has the sheath connected to the junction of the TC wires.  Depending on how/what the sheath connects to there may be a ground loop or additional noise pickup that is created.

The cold junction measurement is very critical.  You may be reading a stable measurement by the internal temperature sensor of the ADS1220, but that does not mean you are reading the correct temperature at the cold junction, nor that the cold junction itself is stable.  The cold junction is the connection point of the TC and the PCB.  The QFN package will yield better results than the TSSOP version for the cold junction measurement due to the package construction and smaller package size.  Essentially you need to get the package itself within the cold junction.  If you take a look at the ADS1118EVM user's guide you can see where the optimal placement would be for the ADC in relation to the cold junction.  Unfortunately I do not have a good example for the ADS1220 at this time.

What I fear you are trying to do is to achieve a high precision system within a non-optimal prototype configuration.  You should first attempt to get a stable measurement.  This instability could be from external noise or from a varying cold junction.  You may also find it beneficial to add some gain once you get a more stable measurement.

Best regards,

Bob B

• Hi Bob,

Can I use the ADS1118 for grounded K-Type thermocouple ?

If yes, please give me the details of the ADS1118EVM board .
Also tell me how can I purchase it and from where.

Best Regards,
Fakhruddin
• Hi Fakhruddin,

Yes, you can use the ADS1118 for the grounded TC measurement.  You can get more information on the web landing pages on TI.com.  Links are created automatically in this post for the ADS1118 and ADS1118EVM to get you to the appropriate pages, or you can search for the info using the web search tools.  There is also a TIPD109 reference design that goes through the design process.  There is also a Launchpad Booster Pack, 430BOOST-1118 that you may find interesting as well.

None of these designs specifically use a grounded TC.  The main point when connecting any sensor to an ADC is to maintain the proper common-mode.  For example, it is possible to have a TC (which is outputting a voltage in millivolts) to have a potential much greater than that if the sensor tip is grounded to a potential that is much different than the ADC ground.

So what is ground?  It is really a reference point.  When we hook two devices together we need to make sure that the reference point of both devices is at or near 0V potential where the common-mode voltage requirement is met.  When the grounds are common for the ADS1118, as well as the ADS1220 when the PGA is bypassed, you can use the grounded TC in unipolar supply for AVDD.  In the case of the ADS1220 when the PGA is not bypassed, the restrictions are a little greater due to the common-mode restrictions of the PGA.  This is where the bipolar supply operation is beneficial as the mid-supply point for AVDD is at analog ground.

Here is the real difficult part and that is you will still have the same issues with the ADS1118 as you do with the ADS1220.  Proper PCB layout is critical, a stable cold junction with proper measurement is critical and external noise (potential ground loop and EMI/RFI) can add to the problems.

Best regards,

Bob B

• Hi Bob,

The ADS1118EVM Board has 0402 packages capacitors and resistors for RC filtering circuit. Is it necessary to use 0402 resistors and capacitors for RC filtering circuit or can we use 0603 package resistors and capacitors also?

Best Regards,

Fakhruddin

• Hi Fakhruddin,

You do not need to use 0402 sized devices.  The layout showed that an efficient layout can be achieved within the cold junction area using these devices.  Depending on your design the layout and devices used could be radically different.  Obviously you can squeeze a lot more devices into a smaller area when using 0402 sized devices, but they are a lot harder to hand solder.

Best regards,

Bob B

• Hi Bob,

We have made thermocouple interface board using ADS1118 chip.I have attached the PCB layout of the same .Please review and comment if any.

Regards,

Fakhruddinthermocouple board layout.rar

• Hi Fakhruddin,

You have a very large copper area, with the ADS1118 in the middle.  Consider what may happen if there is a temperature gradient across the board.  This can be caused from a heat source or from a fan blowing across the board.  If the center of the board is very stable and the outer portions of the board change at the point where the TC makes the board connection, the cold junction may vary.  This will give unstable readings.

The best approach is to make sure that the cold junction is thermally isolated and the temperature of the cold junction is measured appropriately.

That said, your board may work if the entire board remains stable with no temperature gradient across the cold junction.

Best regards,

Bob B

• Hi Bob ,

Can you suggest the changes need to be done in the layout so that the cold junction is thermally isolated?

Best Regards,
Fakhruddin
• Hi Fakhruddin,

TIPD109 shows the temperature sensor placement relative to the cold junction.  This board is very small so unfortunately it doesn't show the thermal isolation.  There is an ADS1118 BoosterPack (430boost-ads1118) that shows a thermally isolated area while using the ADS1118.

What is tricky is that you have two connectors and are trying to measure the cold junction for both TCs.  In the end you need to make sure that the cold junction area remains thermally stable across the connections.  Then the temperature sensor must be placed within the same area as the cold junction so that the TCs temperature is properly compensated.  You must take into account any airflow that may move across your PCB that can change the temperature across the connections of the TC.

Best regards,

Bob B

• Hi Bob,

We have planned to use 2 ADS1118 chips seperately for the 2 thermocouple connectors respectively .Also we have made the thermal isolation for each connector with reference to the  ADS1118 BoosterPack .

I have attached the thermocouple PCB.

Please review and comment if any changes to be done so that we can move ahead for fabrication.

Thermocouple.rar

Best Regards,

Fakhruddin.

• Hi Fakhruddin,

That layout is much improved.

Best regards,

Bob B

• Hi Bob,

Are there any further improvements we can make in the board? Please suggest.

Best Regards,
Fakhruddin
• Hi Fakhruddin,

I have never created nor seen a layout that couldn't be improved in some way.  The big trick is in knowing when a layout is good enough.  I've given you all the material and examples that we have so now it is up to you to decide how well your board looks in comparison to the examples.  I cannot tell you how well your board will perform in the end as I really don't know enough about what you are trying to accomplish from a system perspective.

Prototypes will tell you in the end how well your board will perform.  I'm working on one now.  At some point you have to have some level of confidence and go for it.

Best regards,

Bob B