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ADS1220: Input Impedance, TINA-TI Simulation

Ue
Genius 16285 points
Part Number: ADS1220
Other Parts Discussed in Thread: TINA-TI, TS3A44159

Hi all

My customer is using the ADS1220 and has mutliple PT100.

He plans to use the TS3A44259 to multiplex the PT100 to the ADS1220 and he has the following questions:

What is the input impedance of the ADS1220 ?

Is there a TINA-TI model to simulate this system / the ADS1220 ?

Thank you and best regards

Ueli

  • 0
    TI__Guru** 112765 points

    Hi Ueli,

    Unfortunately there is no TINA simulation model for the ADS1220.  As far as the input impedance, it is very difficult to specify a specific value as the input current to the ADS1220 depends on a number of factors including whether or not the PGA is enabled, as well as the operational temperature of the device.  Instead of giving a specific value of impedance, it was decided that a more effective means of showing the input current would be graphical representation. These graph are in the Typical Characteristics curves starting with Figure 15 on page 12 of the ADS1220 datasheet and continue through Figure 26.

    I'm not quite sure which mux is going to be used as the number you posted is not a valid part number.  Also it is not clear as to how many RTDs are to be connected and how many wires each RTD contain.  If we assume a 3-wire RTD, the configuration for multiple RTDs would be similar to Figure 16 on page 30 of A Basic Guide to RTD Measurements.

    http://www.ti.com/lit/an/sbaa275/sbaa275.pdf

    The difference would be that the current would come from a single set of pins going to the mux, and then from the mux to the individual RTDs. There should be no issue with the input impedance of the ADS1220.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    TI__Genius 16285 points

    Hi Bob

    Thank you for the quick reply and makes sense, as the ADS1220 can be used with / without PGA this leads to two different scenarios.

    I am sorry for the typo, the customer is looking at the TPS3A44159 and wants to mux 2 PT100, 

    However I dont know the configuration (2-wire / 3-wire / 4-wire)

    Best regards

    Ueli

  • 0 in reply to Ue
    TI__Genius 16285 points

    ... should read TS3A44159  we should go for shorter pare numbers ... :-)