How many rtd's can i connect to LMP90080?

Hello,

If you are using Topology #2 shown in Figure 66 of the datasheet you can connect 2 RTDs to the LMP90080.  One RTD could connect to IB1m VIN0, VIN1, VIN6/REFP2, and VIN7/REFN2.  The second RTD could connect to IB2, VIN2, VIN3, VREFP1, and VREFN1.

Mike

Thank you Mike

With Regards

Vijay Kumar.M

If I connect 2 3-wire RTDs in this manner, can I still use VIN4/VIN5 to connect a thermocouple and if so how would VREF work?

Thanks,

Tracy

Hi, 

       I am working in LMP90080. I have connected the 3-wire RTD configuration using single current source which is mentioned in the page no 46 and figure no 66.I got the calibration and everything worked fine.

     a) But actually  i have to use 4 number of 3-wire RTD in my project.I intend to connect the fisrt 2 lines of all four RTD to the four differential input available in the LMP90080 and the 3rd wire of all four RTD to the VREFP1.I have connected the RREF between VREFP1 and VREFN1. Is it possible to connect 4 no of RTDs in this way..?

      b) I fear because when i tried at first by connecting first 2 wire of the RTD between VIN0 and VIN1 and the reference resistor RREF between VREFP1 and VREFN1  i did not get any voltage across RTD(using single current source).I connected the third wire to VREFP1.I get the reading only when I use VREFP2 and VREFN2.

In this case is it possible to establish the connection which is mentioned in a?

What possibly wrong i am doing in b.?

Hi Anbu,

you cannot connect more than 2 3-wire RTD in 1 source configuration to the LMP90080. You can connect 3 3-wire RTD in 2 sources configuration, but it is mandatory to connect the REF_RESISTANCE between VREFP2 and VREFN2.

If you connect the REF resistance between VREFP1 and VREFN1, the current generated by the internal current source cannot reach it.

regards,

Domenico

Hi Domenico,

    Thank you for your quick reply.I need your help to understand the RTD configuration which is mentioned in the DS of LMP90080.

Actually,I have done the project to connect 16 thermocouples using 4 LMP90080, by connecting 4 thermocouples to 4 differential inputs available in each LMP90080.VREFP1 was used as the reference voltage.

Now, i have to modify the project for RTD.I want to connect atleast 12 RTDs using 4 LMP90080 available.But i am facing problem with current sources.I want to use 3-wire RTD configuration for better accuracy.

I have attached a file inwhich the circuit connection for 3 3-wire RTD is drawn with single current source using multiplexer. It is mentioned in the DS that is for 3-wire RTD using single current source, 2 channel readings are needed.I thought the calculation for the 3-3 wire would be like below.
For RTDA,
CH0 = VIN0 - VIN1 = IB1 (RLINE1A + RTDA)
CH1 = VIN1 - VIN6 = IB1 (RLINE3A)
For RTDB,
CH0 = VIN2 - VIN3 = IB1 (RLINE1B + RTDB)
CH1 = VIN3 - VIN6 = IB1 (RLINE3B)
For RTDC,
CH0 = VIN4 - VIN5 = IB1 (RLINE1C + RTDC)
CH1 = VIN5 - VIN6 = IB1 (RLINE3C)
Beacause I assume that,when a channel is not selected, the third line for that channel would be opened.So, the calculation in the above would be possible or not.?

If I use 3 3-wire RTD using 2 current source then i will need a demux to switch those 2 current sources to 3 RTDs right..?

Thanks & Regards

Anbu

Hi Anbu,

the calcualtion you have done is correct if you use the external MUX.

Alternatively, if you do not want to use the MUX, you can conect 2 3-wire RTDs in single channel configuration. In this case you have to use a different current source for each RTD (they cannot be ON at same time). It is possible becasue the LMP90080 has 2 internal current sources.

So just to summarize how many RTD you can connect to the LMP90080/LMP90100:

n.1 - 3-wire RTD double sources

n.2  - 3-wire RTD single source

n.3 - 3-wire RTD single source with 1 external mux

n.3 - 3 wire RTD double source with 2 external muxes

regards,

Domenico

Hi Domenico,

Then I will go with 3 3-wire RTD with single current source using mux.

I need some clarification in using the current source in LMP90080 using mux.In order to reduce the number of mux in my project, I want to mux the current source from one LMP90080 to another LMP90080.A file has been attached in this post in which I have drawn the circuit to interpret the connection for that.

In the circuit, I connected the current source IB1 of ADC1 to a RTDA in the same adc.The current source IB2 of ADC1 is given to RTDB, RTDC in the same adc and also to RTDA, RTDB of ADC2.
I am using SPI communication to select the ADCs connected.The power to the ADCs is given commonly.

In this case, whether it is possible to mux the current source from one LMP90080 to another LMP90080 or I have to use the external current source to do that?

Thanks & Regards

Anbu

Hi Anbu,

I do not see any problem. Make sure to acquire the data with the ADC when the signals have set. Each time you switch the current from one RTD to another one each circuit will take some time to set, this time depends on the parassitic capacitance present on the board and at the output of the MUX.

regards,

Domenico

Hi Domenico,

      Thanks you for the support, you had given me in the last week.I really appreciate your help.

Actually I connected the current source from the 1st ADC and had given it to the 2nd ADC(Connected the RTD in the 2nd ADC) through mux.I read the voltage and it was working.

I will look into the switching time of the mux I am using.

Thanks and Regards

Anbu

     

Hello Domenico,
I got the design for the RTD with what we had discussed in this psot.I got a problem now.
I connected RTD1 to one of the four lmp90080s VIN0 and VIN1, third line to the VREF2.I also connected RTD2 to VIN2 and VIN3, thirdline to the VREF2.
When I read channels seperately(wiht anyone RTD not connected), I read correct values from ADC. But when I connected both the RTD and tried to read both simultaneously, I read incorrect values.
I mentioned the register settings as below,
Registers value

RESETCN 0xC3
BGCALCN 0X00
RESTART 0x01
AUXCN 0x0A
CH_SCAN 0x80 FOR CH0
CHX_INPUTCN 0x53 "
CH_SCAN 0x89 FOR CH1
CHX_INPUTCN 0x65 "
I used delay wherever possible.

Please help me with this situation.I checked the everything but i could not find what went wrong.
(I also posted a seperate thread for this on last saturday for which I didnt get any reply so far.)


Thanks and Regards
Anbu