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TINA/Spice/ADS1018-Q1: ADS1018-Q1

Amod Mujumdar30
Prodigy 160 points
Part Number: ADS1018-Q1
Other Parts Discussed in Thread: TINA-TI, ADS1118

Tool/software: TINA-TI or Spice Models

Hi ,

Thank you very much for your support so far. We are working on one requirement as below. 

We have used ADS1118 for interfacing thermocouple  K - Type. Our measuring range is 0 to 300°C. Electronics will be exposed to 0 to 75°C operating ambient. 

We are predicting error budget for this block. Circuit is attached for reference. 

In circuit 2 x 1M resistors are at pull up and pull  down. Two resisotr of 499 ohm for low pass filter. 

Now considering tolerances and PPMs for resistors how we can we calculate deviation of Thermonuclear voltage till ADC input ? Can we simulate this in TINA. 

Can you me pl. suggest on calculations to predict how resistors will impact on thermocouple mV o/p to ADC I/P? These are passive components errors. 

Now what is internal reference value for this chip? I am not able to find this in data sheet. 

I would like to predict ADC counts based upon input voltage at ADC i/p. 

If due to resistors tolerances thermocouple mV deviates ,w e need to predict this . Pl. guide on this. 

Can you pl. help on this ?

Regards,

Amod

S49-Heater Controller-TC.pdf

  • 0
    TI__Guru* 98135 points
    Amod,


    There's a lot to your question. First, you could simulate some of this with TINA, but it's more important to go through your calculations directly.

    I think you should read through the following TIPD. This is similar to your design and has a lot of good information. There is a section on System Error Calculations starting on page 12 that you should be able to apply to your design.

    www.ti.com/.../slau509.pdf

    This design guide should help you calculate the error you might expect for the measurement error. I would read through it carefully to make sure you understand the total error contribution of each section.

    As for your other questions, for the reference voltage, the datasheet combines the reference error with the gain error, which is specifed in the datasheet. As an example, if you are using the ±2.048V range, if you have an gain error of ±0.01%, it is exactly the same as having a reference error of ±204.8uV.

    I had one other note in the schematic that you sent me, generally I avoid putting in inductors in the supply path to the ADCs. With series inductance, you could cause some large voltage spikes in the supply because the L(di/dt) of the digital currents. You do have some other resistances to reduce this, but I would generally avoid it.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 160 points
    Hi Joseph,
    Thank you very much. It is really a great information. Only one doubt. I understood errors due to filters. However how Rpu and Rpd can incorporate errors as if both are equal and drift equally due to temperature then it will not affect level of TC voltage . Is my understanding correct ?
    Further will there be any error due to supply voltage variation ?

    Regards,
    Amod
  • 0 in reply to Amod Mujumdar30
    TI__Guru* 98135 points
    Amod,



    The pull-up and pull-down resistors generally do not have much of an affect on the measurement. The main way that these resistors add error to the measurement is through the biasing current created by these resistors. This biasing current reacting with the thermocouple lead resistance will add an offset voltage to the measurement. For example with two 1MΩ biasing resistors and a 3.3V supply, the current will be 1.65uA. If the total lead resistance is 1Ω, then this is 1.65uV of additional error. If the lead resistance is 10Ω, this is 16.5uV if additional error. Any power supply variation will add to this error, but this is still likely a small error.



    Joseph Wu