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ADS1148: ADS1148

krishna prasad
Prodigy 80 points

Part Number: ADS1148
Other Parts Discussed in Thread: TMUX1133

We have designed 12 channel RTD interface Board using ADS1148 and Mux CD4053. Schematic is enclosed for reference. The measured  resistance values  are higher by few ohms (approx. 28-30Ohms) than connected resistance values. Can you please suggest what may be cause for this error? Also suggest any other schematic or reference circuits to achieve this 12 channel RTD interface using a single ADS1148.

Regards

Krishna Prasad Rtmm multi board.pdf

  • 0
    TI__Guru** 113765 points

    Hi Krishna Prasad,

    It would appear that the error is being caused by the mux channel resistance creating an additional voltage drop.  This would be similar to lead resistance error but many times larger.  I would recommend reviewing the information in A Basic Guide to RTD Measurements. 

    As far as the circuit goes, the schematic for using a single ADC will work as long as the mux resistance is significantly reduced.  The CD4053 has too large of an on resistance so you would need to choose a different mux with a lower on resistance.  You can use a similar scheme with TMUX1133 which is a 2:1 analog mux with low on resistance (2 Ohm typical) and low leakage.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    TI__Guru** 113765 points

    I should have also mentioned that I saw a ferrite (inductance) between analog and digital ground.  I would advise against doing this.  If a transient should occur across the inductance that creates a voltage greater than 300mV, the ADC can be damaged (see Absolute Maximum Ratings table in the datasheet).  You can leave the footprint in place, but I would recommend making this a 0 ohm direct connection instead.

    Best regards,

    Bob B