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LTC2986/2986-1

I have couple of questions related to the LTC2986/2986-1 

1)As for the anti-aliasing capacitor, it is recommended to be 100pf in the data sheet and that's the value you have used in the evaluation boards DC2618A and DC2508A, however in the datasheet in pages 1,26 and 30 when you drew schematics for the thermocouple you used 0.1uf. Is it better to use this value with thermocouples or the 100pf would be enough?

2)In the evaluation boards DC2618A and DC2508A you connected the COM to the ground via a capacitor and not directly as mentioned in the data sheet. In the universal protected board DC2507A the COM is directly connected to the ground. Is one of them better than the other?

3)In the datasheet it is mentioned to tie all the VDD together and connect them to 0.1uf and 10uf parallel capacitors. In the  evaluation boards DC2618A and DC2508A you added 2x10uf and 4x0.1uf. Is there a specific reason for such number? if I am gonna added extra capacitors infront of each VDD for better stability would you recommend a specific combination?

3)If I used COG capacitors instead of the recommended X7R for the Q1,2,3 pins, would there be any problem?

4)In the evaluation boards DC2618A and DC2508A, the extra 1ohm resistor and 10uf capcaitor connected to the VREFOUT/VREFP pins, It is mentioned in another question here on the community that it is from an old design, would adding them improve the functionality of the IC or removing them won't affect it at all?

5)In the evaluation boards DC2618A and DC2508A, the VREF_BYP has an extra 10uf capacitor parallel to the 0.1uf recommended capcaitor, does this improve the design or removing it is fine?

6)If I will add multiple single ended thermocouples, would you recommend against joining their free ends together and connect them to a protection resistor then to the ground? or it is preferred to have each leg with its own protection resistor that is connected to the ground

I really appreciate your help and sorry for the long post as I have been trying to collect all my questions to ask them at once
Thank you

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  • Just posting again hoping to get any help soon

  • Hi,

    Apologies for the late reply.

    1. The Anti-Aliasing filters will mainly depend on your application needs, and these are optional.  The most common input circuitry is with 1k to 10k resistance (limited by excitation current for RTDs and thermistors) and a capacitor with 100pF – 0.1µf capacitance. You may start with 100ohm and 100pF to filter out very high-frequency interference without impacting your settling time or open circuit detection significantly. You may refer to this LTJournal article that will help you in designing your RC filters. LT Journal of Analog Innovation V27N1 - April 2017

    2. Regarding your question about the capacitor in COM pin, this is also optional. Since COM pin is an analog input for single-ended configuration, adding a capacitor to ground will form a low pass filter for the COM line.

    3. The 0.1uF and 10uF are the recommended decoupling capacitors in the datasheet. The 10uF will help filter out low-frequency noise, while 0.1uF will filter out any high-frequency noise in your circuit. You can use multiple parallel capacitors with the same value. It will help lower the impedance, and noise will attenuate further.

    4. No problem. COG is also a stable type of capacitor which is not significantly affected by temp or voltage change. 

    5. Datasheet recommendation was to use 1uF capacitor to the VREFOUT to ground. The 1ohm and 10uF are not necessary and not guaranteed to improve the device performance. 

    6. Datasheet requires 0.1uF, but you can add 10uF to add stability on the Vref_byp pin.  Please do note not to apply a voltage or current source to this pin (also in Vrefout pin). This pin must connect to capacitive loads only.

    7.There is a minimum protection resistor requirement to ensure that no more than 15mA flows into or out of your device pin. Refer to page 4 in the LTJournal article to help you design your circuit's protection resistor.

    Regards,

    Marymae

  • I am very thankful for your help, I really appreciate it.

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