• State
  • Locked Locked
  • Replies 1 reply
  • Subscribers 46 subscribers
  • Views 115 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ADS122S14: Using a TI ADS122S14 (or alternative) as a precision resistance meter

James Winter
Prodigy 20 points

Part Number: ADS122S14

I have an unusual application where I need to measure the resistance of heater banks in a system.  The heaters will be run live, and then a couple of times a day - be electronically isolated from the supply and a resistance measuring system switched in - to make sure that the heaters are still working correctly and are the same resistance as when they were initially commisioned.

This is a replacement for a system that was originally designed using a DMM chipset made by Cyrustek using ES51932.  This legacy device measured resistance and sent the result out via RS232.  We need to design a replacement system that does the same - using an ADC and a processor, to a sufficient resolution.

The solution needs to have the dynamic range to cover 30 ohms to 10 killo ohms

We need to handle two extremes. At one end, we must detect a 6W failure in a full 1200W load, which is a shift of only 0.17 ohms  to 0.22 ohms somewhere accross the range (depending on voltage, controller can operate between 200-230VAC). At the other end, we may be measuring single 6W heaters where the base resistance is as high as 8.8 killo ohms.

The solution must resolve 0.01R steps without ‘flicker’ accross the entire range.

Do you have any application notes on using ADC's as resistance meters, or can you suggest a means of achieving this (we are not fixed on this ADC - and can consider alternatives).

Many Thanks

 

James

 

  • 0
    TI__Mastermind 49475 points

    Hi James,

    Welcome to TI E2E forum.

    To meet your specific 0.01 resolution requirement across a wide dynamic range (30 to 10kΩ), a ratiometric resistance measurement architecture is the most robust approach. A ratiometric design eliminates errors from the excitation source drift by using the same current to generate both the signal voltage and the ADC's reference voltage. Use the internal IDACs (Programmable Current Sources) of a 24-bit ADC like the ADS122S14 to drive a series circuit containing a precision reference resistor (Rref) and your heater bank (Rheat).

    One fixed setting cannot cover 30Ω to 10kΩ, you can use a high current (e.g. 1mA) and small Rref with a high gain setting on the ADC to resolve the small 0.01Ω steps. For the high range (10kohm), switch to a lower current (e.g. 100uA) to avoid saturating the ADC inputs. Running the ADC at a low data rate for lower noise. If it is possible, run the ADC at 20sps or 25sps to utilize simultaneous 50/60Hz rejection.

    Refer to A Basic Guide to RTD Measurements. The sections of "3-wire or 4-wire ratiometric resistance measurement" for RTDs circuit are identical to your needs.

    Let me know if you have any further questions.

    Best regards,

    Dale