ads1248 RTD read problem in temperature test

Dear Yavuz,

I am sorry to hear that you have issues with our ADS1248 however I believe we should find a solution to your problem.

Could you tell me what analog supply voltage and IDAC value you are using?
I assume you are using a gain = 1?
Did you select the reference inputs where you have the Rbias connected to as your reference input in the configuration register?

You should make sure the IDAC's have enough headroom to operate, means you need to meet the IDAC compliance voltage of 0.7V.

For the measurement of the external signal you need to make sure your signal is within the allowed common-mode voltage range of the PGA. If you are using a single-supply, you cannot connect the negative input to GND.

Also in general we recommend to provide some kind of RC filter on the analog inputs. However you will then probably not be able to route the IDAC to the same pin where the RTD is connected to as the voltage drop across the filter resistor will add additional measurement error. You would then have to route the IDAC from another available analog input to the point in front of the filter.

Regards,

Hello Mr. Wurker

We are using 5 V supply voltage ,1.5mA IDAC value and you are right PGA gain is set to 1 in our application.Also, we selected the Rbias connected pin as a refence input in the configuration process.

We think that IDAC voltage compliance value may cause a problem while RTD value increase with the temperature.What do you think about it? If we decrease the IDAC value, will it be a solution?

Dear Yavuz,

what temperature range due you need to measure with the Pt1000?

In your current setup you will run into IDAC compliance voltage issues at around 120°C already. But as your Rbias is only 1.4k anyway I assume you do not need to measure that high temperatures. At a 5V supply you will then be okay.

That means your issue seems to be somewhere else. Do you have a more detailed schematic that you could send me?

Regards,

Dear Mr. Wurker;

I attached schematic view below. Actually, 85 C is the maximum temparature that we want to measure.

Also, in previous e-mail, you said , "If you are using a single-supply, you cannot connect the negative input to GND" but we did it in our application and we didn't see any problem in our measurements. Could you explain what do you mean?

Best Regards

Dear Yavuz,

your schematic looks okay from what I can see.

As I said we usually recommend some differential and common-mode RC filter on the inputs. But you would then probably not be able to measure all five RTDs with one ADS1248 anymore due to the IDAC routing.

You even have the reference capacitor connected. That would have been my next suspicion why the IDACs don't seem to work properly.
Could you explain in some more detail what kind of issues you see when you change the ambient temperature?
Also could you explain why you have two RTDs connected in parallel?

I noted you are using ferrites between AVDD and DVDD. We have seen problems in the past using those ferrites. We therefore recommend to simply use some resistor between the supplies if you need to isolate them in some way.

The PGA inside the ADS1248 requires a certain common-mode voltage for the input signal to stay in. Otherwise you will see additional errors and non-linearities. I wonder why you didn't see any errors so far.
I tried to explain the PGA limitations in more detail in the datasheet in one of our newer devices, the ADS1120. Please have a look at pages 19ff there. Let me know if this clarifies things or not.
If the negative input in your case is connected to GND and the positive inputs wants to measure e.g. a 1V signal, then your common-mode voltage will be at 0.5V. The PGA however requires the common-mode voltage to bet at least 0.6V.

The ADS1120 and ADS1220 have a nice feature. In this devices you can bypass the PGA. You can then measure single-ended signals without problem. Unfortunately those devices do not offer as many analog inputs. But in principle you could use 2x ADS1220 for your implementation.

Regards,

Dear Joachim,

Actually, two RTDs were placed on the PCB for measuring the temperature of two different areas on the board but one of them is DNP(do not place) so it means, there is only one RTD on the board. 

During the temperature test, whose range between -40 C to 55 C , digital data read from the channels ,which the RTDs are connected, seems wrong but digital data read from the channel, which an external signal is connected, is fine. I can't say the actual ambient temperature that the data read from RTDs become wrong but it corrupted during the test.

After setting the temperature to room temperature, it doesn't get better and digital data read from the RTDs still seem wrong. After that, we change the ADS1248 with the new one, then everything become fine. 

Could you explain in some more detail what kind of issues you see when you use ferrites between AVDD and DVDD? 

We analyzed the datasheet of the ADS1120 for the common mode voltage requirements and according to our calculations, it seems, we are in the safe region for the common mode voltage range. 

Could you explain in some more detail how you find the common of voltage to be at least 0.6V for PGA? Because, when we calculated it by using the equations explained in the datasheet of ADS1120, (VSS + 0.2 +(Vin/2)) it seems PGA needs at least 0.45 V. 

Dear Yavuz,

okay, understand. I missed the fact that you had the DNP comment next to the RTD.

So with the newly populated ADS1248 you don't see any issues anymore?
That could mean that either the solder connections were not good or the ADS1248 was damaged. Please let me know in case you see this issue popping up again on the new device.

Regarding the ferrite. It can e.g. happen that the digital supply draws some higher currents during power up. The ferrite will then cause the supply to break down which could lead to a reset on the analog supply. In the end the part will then not power up correctly. My colleague Bob knows much more details about the issue that he saw at some customers that used ferrites.

It seems like you want to measure a signal on AIN6 that swings between 0V and 0.5V, while AIN7 is held at GND, is that correct?
The common-mode voltage is then calculated as (V_AIN6 + V_AIN7)/2.
So when V_AIN6=0V, your common-mode voltage is VCM=0V.
When V_AIN6=0.5V, VCM=0.25V.
However the requirement is VCM(min)=VIN/2 + 0.1V for ADS1248 (it is higher for ADS1120).
VIN in your case is VIN=(V_AIN6 - V_AIN7).
So for the two extreme cases again this means:
V_AIN6=0V -> VCM(min) = 0.1V
V_AIN6=0.5V -> VCM(min) = 0.35V
As you can see your setup does then not meet the common-mode voltage requirement.

Another way to look at it that might be easier to understand is the following:
Let's look at the PGA diagram Fig. 39 in the ADS1120 datasheet. If you select a Gain=1, both R_F resistors will be 0Ohm. If you then connect 0V to AIN_N, amplifier A2 would try to drive OUT_N to 0V as well. However as A2 is not a rail-to-rail output amplifier, it cannot reach 0V and you will thus see some error.

Regards,

Dear Joachim, 

In General, we don't observed problem with the newly populated ADS1248 but as I remember, we observed same problem in one or two of our electronic cards although we change the ADS1248 with a new one. 

We recalculated the common mode voltage for the case of reading an external signal, and we understood the problem that you mention. However, we didn't  observe significant problem or degredation in the performance during our tests. So, could you explain in some more detail what kind of issues we can come across if we don't meet the common mode voltage requirement? And, also could you explain the degree of the problems that we may meet because of the common mode voltage issues.

In addition, we don't want to change the design of the electonic cards for this reason so that ,could you recommend any solution for solving this issue ? Maybe, negative input of the PGA can be driven by the bias voltage of the ADS1248 , what do you think about it? 

Best Regards,  

Dear Yavuz,

okay, then it seems like we need to dig deeper to find the root cause for this issue. However currently I do not know where to start. Can you describe exactly how the output code changes or what you observe when the device is misbehaving?

What exactly happens when you are outside the allowed common-mode voltage range is hard for me to quantify. I would assume the linearity of the ADC degrades and maybe you measure a smaller signal than it is in reality.

I don't know what kind of signal or sensor you have connected to AIN6. But if you can measure this signal with respect to AVDD/2 then this would certainly be an option. You probably only need to do the math in the MCU then.
So you would connect Vbias=AVDD/2 to AIN7. However this is not a very stable voltage as it changes with the supply voltage. Your measurement result would therefore not be very accurate.
A better option would be to route VREFOUT to AIN7 and then measure AIN6 with respect to VREFOUT.
A 3rd option I could think of - this depends on the kind of sensor you are using - is to bias/level shift the sensor and AIN7 to Vbias. This would work as well if the sensor supports it.

Regards,

Dear Wurker,

1) Is it a must to satisfying the common-mode input range ALL of the time? In other words, is it safe that unsatisfying the issue only  during a short period of time like power-up (or similarly power down) ?

2)  Doesn't it help to satisfy the issue by employing three series resistors (as ilustrated in the figure below)?

Best Regards,

Dear Cihangir,

you only need to meet the common-mode voltage requirements when you are performing measurements to achieve the specified performance of the ADS1248. You can certainly have voltage present on the analog inputs that don't meet the common-mode voltage requirements. There is no risk of damaging the device.

Yes, depending on how you choose the resistor values this will be an acceptable solution as well.

Regards,