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ADS8509: offset bias w/ fixed resistor and Vref

Emmett Bradford
Intellectual 425 points
Part Number: ADS8509
Other Parts Discussed in Thread: REF5025

Hello,

I am trying to bias an ADS8509 to have an input range of -0.1 to +4.9V, or similar.

I thought I would achieve this by reducing the 33.2K resistor  in figure 29's  0-5V section.

It doesn't seem to do it.

What is the full voltage range of the internal node that is fed to the CDAC ?

  • 0
    TI__Mastermind 24940 points

    ADS8509ref_ideal.TSCHi Emmett,

    You should be able to achieve your desired offset by decreasing the 33.2k
    I have attached a quick TINA simulation of the internal and external circuit; this shows that the resister should be decreased to about 1k (1.0063K)
    Keep in mind this is an ideal situation, you will have to take into account the surrounding resister's accuracy as well to finalize the value.
    To find the CDAC range, you can sweep Vin from 0 to 5 volts (replace the 1k resistance to 33.2K before hand).

    Regards,

    Cynthia

  • 0 in reply to Cynthia
    Intellectual 425 points
    Hi Cynthia,
    Thanks for the reply.
    The reference buffer cannot source that much current.
    I have verified that by destroying some parts already.
    I got better results by increasing the 100 ohm resistor to ground.
    I'll be back in office Tuesday to look at your simulation.
    Thanks Emmett
  • 0 in reply to Cynthia
    Intellectual 425 points

    HI Cynthia,

    I am not a TINA user, nor do I desire to be.

    Can you tell me directly what the CDAC range is?

    Also, can you give me the network analysis in equation form so that I can put it into excel or something simpler?

    Thanks,

    Emmett

  • 0 in reply to Emmett Bradford
    TI__Mastermind 24940 points

    Sorry for the late reply.

    Yes, we missed the internal buffer current limit (100uA). Please note, the offset calibration is intended to correct for the internal offset of the ADS8509 device and not to perform a change to the actual input voltage range.

    A possible option may be to connect the adjusting resistor to an external reference such as the REF5025 that can handle the current.

    The CDAC range is in the order of  ~314mV to 2.814V, when using VIN=0V to +5V and RADJ=33.2kOhm.

    When using RADJ=1.006kOhm with an external reference, and VIN=-0.1V to +4.9V; the  CDAC range stays as expected at ~314mV to 2.814V.

    Please see attached a power point and an updated simulation file with the  steps used in simulation and the results.  The equation for the complete resistor network gets to be quite long, therefore, not sure if would still be useful to you.ADS8509_Simulation_Summary.pptx

    ADS8509_VIN_VCDAC_Range_5_2_17.TSC

    Regards,

    Cynthia