Category: Hardware
Product Number: LTC2986
I'm experiencing an issue with my current design based on LTC2986 that is meant to read temperature using 2 RTDs.
To test my hardware prototype, I decided to read some precision resistors I have for comparison, so I instructed the LTC2986 to report resistance instead of temperature.
If I connect a resistor individually, the readings reported by the LTC2986 seem reasonable. However as soon as I connect 2 resistors simultaneously, the LTC2986 increments the reading result for about 0.8~1 ohm for each of the resistors values.
The schematic is based on the DataSheet Figure 39: 3-Wire RTD Kelvin Current Mode but using the remaining channels to read the second RTD.
I have used the Demo software to validate my design which has the following considerations:
- It's solely for 3 Wire RTD PT100
- One LTC2986 reads 2 RTD
- Channel 2: reads sense resistor, value 1000 ohm, 0.01% 2ppm
- Channel 4: reads 1st RTD
- Channel 8: reads 2nd RTD
- Both channels 4 and 8, use No rotation Sharing, and provide 250uA excitation current.
- The line after the resistor marked RP1 (On the Figure 39) is connected to both RTDs.
- 3-wire RTD Kelvin current excitation mode bit is enabled.
To provide additional protection I also have implemented Transient voltage suppressors (TVSs) following the article:
This issue of having an increased value on the result of the reading when connecting 2 RTDs simultaneously, is repeated with different hardware prototypes, so it seems the problem is related to my design. Here it is the related part to the channels connection of my schematic:
I would appreciate any help on this.
Thank you
