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ADS1248 current matching

Neeraj Swarup
Prodigy 30 points
Other Parts Discussed in Thread: ADS1248

Hi

Ques 1-How ADS1248 maintains current matching inside.

Ques 2- ADS1248 has IEXC1 and IEXC2 as two matched current source. In ratio metric method, IEXC1 sees majorly RTD resistance along with RBias and lead resistance. However IEXC2 sees only lead and Rbias resistance.In case the thermal sensor reaches high resistance(like thermistors reaches kohms), the resistance difference between IEXC1 and IEXC2 is too high(IEXC1 sees kohms , IEXC2 sees only few ohms). Can this cause any current mismatch?

Ques 3- Is there any load limitation for connecting to current sources? for maintaining a constant set current(or only voltage compliance Avdd-0.7 should be met)

 

Neera  

  • 0
    TI__Guru* 98025 points
    Neeraj,


    I'll answer your questions in order.

    1. The excitation current sources (IEXC) are matched based on the transitor sizing and layout of the device. There isn't any kind of other switching or matching used.

    2. I think what you are asking about is the output voltage of the IEXC and if the difference between IEXC1 and IEXC2 will add error to the difference between the two currents. What you are looking for is the output impedance of the current source to be very high since the output devices are cascoded. In this case, I don't have any numbers but the IEXC current sources have a high output impedance and should have little difference as long as the output voltage is not too high.

    I would refer you to the graphs in the datasheet for IDAC Voltage compliance on page 22. Figures 47 and 48 show how the current sources behave as output voltage rises.

    3. The only limitation would be the output voltage. Since I think the AVDD-0.7V is a bit misleading, I'd use Figures 47 and 48 as a better guide for the compliance since the higher currents may have a worse compliance voltage.

    If you are concerned about the output current matching of IEXC1 and IEXC2, you can use the fact that each can be routed to any analog input and to the IEXT1 and 2 pins. Use the IDAC1 Control Register to route the currents to the desired pins, and make a measurement. Then use the IDAC1 Control Register to swap the IDAC currents and the pins, make another measurement and average the first and second measurement. This should take out the mismatch error.


    Joseph Wu