Connecting 4 3-wire RTD sensors in one ADS1248

Dear Vicenzo,

First of all I would like to thank you for your support!

I am interested in connecting four 3-wire sensors on the same ads1248.

Reading the manual and the application sheets I have the impression that only two 3-wire RTD's can be connected on the same ads1248!

Is it possible to send you my design for verification and check?

Best regards

Nikos

yes of course you can...

if you need to connect 4 RTDs then you would need 2 x ADS1248...

you could also use the trick as for the attached app.notes, but the results you could get woiuld be very sensitive to the matching grade of the two line resistances...

sbaa180.pdf

sbaa201.pdf

slau520a.pdf

KR

Vincenzo

Dear Vincenzo,

I would like to thank you once more for your support!

We would like to use 2 pH sensors and four 3-wire RTD's.

For this reason we are focusing in using 2 ADS1248 for each pH sensor and two 3-wire RTD's using the following topology...

Are there any recomendation in improoving this cirquitry?

BR

Nikos

ADS1248_3wire-RTD & pH.pdf

ADS1248_3wire-RTD & pH (upd1).pdfHi Vicenzo,

Our requirements are one pH sesnor (+/- 0.1 accuracy) - two 3-wire RTD's (0-50 oC 0.1 oC accuracy) sampling rate 5Hz!

The sampling procedure will be

 - pH:      (a) Turn On VREF

                 (b) Read pH (AIN1 - AIN0)

 - RTD1: (a) Turn On IEXC1 & IEXC2

                 (b) Read RTD_1 (AIN3 - AIN2)

 - RTD2: (a) Turn On IEXC4 & IEXC5

                 (b) Read RTD_1 (AIN7 - AIN6)

(-if not RTD is connected you have 0V reference so you can't sense the pH sensor as well)

The ADS1248 VREF will be used for pH amp VREF, so  REFP0 & REFN0 are disconnected from VREFOUT & VREFCOM!

(- tha max REF voltage is VA - AN_GND - 1V, so having R46 = 3.3K means that the max IEXC is much less than 1mA; while if you are planning 1mA IEXC then you have to increase VA (that is decoupled from VD))

Updated to 820R!

(- what's the purpose of R4 / R94, since that is not the normal position for the compensating resistors?)

They are for compansation! (Corrected!)

(- C6 nad C80 must be placed very close to IC47)

C6 & C80 or C74 & C42?

Also, should I open a new topic to ADS 1248? Could you transfer this topic?

Thank you very much!

Nikos

Dear Vincenzo,

In the previous post the schematic have been updated/corrected taking into consideration your comments.

Reading again the appnotes for ADS family I am confused i little bit with the compensation resistor.

I have the feeling that in the upd1 schematic the compensation resistors are not connected properly and the upd2 are closer to the appnotes (like tidu145).

Could you please advice on this issue?

Best regards,

Nikos

ADS1248_3wire-RTD & pH (upd2).pdf

Hi Nikos,

Please allow me to add some comments to your questions as Vincenzo is currently out of the office.

It seems that your design is pretty much in accordance to the TI Design TIDU145 which is based on a Hardware-Compensated Ratiometric approach. I have reviewed your schematics. I assume that you are going to use PT-100 RTDs in your design for a temp range of 0-50°C. Below some design considerations that need to be taken into account:

- Rcomp resistance: Has to be equal to the resistance of the PT-100 sensor at +25°C (mid-scale). In your Design you are using Rcomp = 110 ohm which is fine for a temp range of 0-50°C using PT-100 RTD.

- Rcomp location: Should be placed in series with the negative ADC input with the second current source, I2, flowing through it. This is placed correctly in your design.

- Selecting IDAC values: The magnitude of the current sources directly affects the magnitude of the RTD voltage. While maximizing the magnitude of the excitation current would seem desirable, higher excitation currents create a concern for violating the compliance voltage of the current source as well as errors due to self-heating. Here the recommendation is to start with 250uA to 1mA max.

- Selecting Rref: Select Rref such that Vref is ~40% to 50% of AVDD. In your design AVDD is at 3.3V so that Vref should be at around 1.65V. Rref should be chosen with tight tolerance and low temperature drift: 

• DC errors in Rref directly affect the uncalibrated measurement gain error
• Typical drift for Rref = 5 – 20ppm/°C

- The value of Rref is selected based on the IDAC setting and the desired VREF voltage of 1.65 V @ 3.3V AVDD

- For a 250uA IDAC current Rref = 1.65V / (2x250uA) = 3.3k ohm

- RTD input and reference filters should have matching cutoff frequencies

- PGA Gain: You need to select the highest gain to get the best resolution per °C.  The big concern is not taking the gain so high that the measurement at the highest temperatures is greater than the reference voltage. 

- Further, you need to make sure that the input common-mode voltage is not in violation with respect to the desired gain.

- Turn internal VREF ON, otherwise IDACs will not work.

- Start a measurement only after the input signal has settled. Especially important when MUX'ing channels.

- If a data rate of 20sps or less is used, you can take advantage of the simultaneous 50/60Hz power line cycle rejection on the ADS1248.  This noise can be picked up on the leads of the RTD wire.

Hope this will information will be helpful otherwise please let me know.

Kind Regards,

Mo.

Dear Mo,

Thank you very much for your support!

Indeed my design is very close with TIDU145 which includes a resistor in one of the input channels for hardware compensation. I was wondering for compensation resistor accuracy issues from the very beginning of the design and finally after and Mr Wu advice (latest post) I will remove the hardware compensation resistors (R4 & R94).

I am planning to use 1mA IDAC currents (taking into consideration the temperature range 0-50oC). For this reason in the upd2 schematic the REF resistor reduced from 3K3 to 820R. Do you believe that I have to stay with the initial design using 3K3 REF resistor?

Also I was confused with Vincenzo advice on the initial design "if no RTD is connected you have 0V reference so you can't sense the pH sensor as well". Do you believe that there is an issue in the upd2 design with VREF in pH measurement if  RTD's are not connected?

Best regards,

Nikos