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ADS805 ADC application notes?

Matt Southgate
Prodigy 20 points


I've looked and failed to find any application notes for this part, and relying on the datasheet I am left unsure about configuration of the part.

For a 5v pk-pk input around 2.5v, I know to connect SEL to GND (Table 1).  I want to use the internal voltage references and cannot rely on the steady state of the supply, but it will remain within spec (4.75 - 5.35v).  Referring to FIG2 (P9) of the datasheet:

a.  is VREF connected to VIN- ?

b.  is CM connected to anything?

c.  with potential voltage fluctuations of the supply, can any of the ADC's outputs be used as Vs to feed the voltage shift network R1 and R2 and these will be stable as any voltage shift in the reference will also need to occur at the amplifier?

d.  and if so what values should be used for R1 and R2?

The part cannot be breadboarded easily and I cannot find the information I require in the data sheet.

I would appreciate any help and will be more than happy to supply  more circuit particulars, but I think I've covered the necessary points.

Thanks

Matt

  • 0
    TI__Mastermind 29220 points

    Hi,

    Actually, the table 1 and the text around it regarding the reference operation describes the generation of an accurate reference in the presence of supply variation, or temperature variation, or process variation.  The purpose of the bandgap circuit is to provide a reasonably accurate voltage reference.    See http://en.wikipedia.org/wiki/Bandgap_voltage_reference.

    So the VREF can be easily set to output 1V or 2.5V depending on how the SEL pin is strapped.  The full scale voltage is always 2x the VREF.  For other desired settings of VREF (and thus different settings for what full scale is to be) the VREF can be set to output anything from 1V to 2.5V by use of a resistive voltage divider as described in Table1.  (This is one place where there is a definition of R1 and R2, which is different that the R1 and R2 of figure2)

    Regardless of the definition of full scale being 2V to 5V or anything in between, the common mode of the signal must always be about 2.5V.  This common mode voltage may be taken from the CM pin, but you don't need to use that.  You may generate a 2.5V level for common mode biasing some other way, including a voltage divider off the supply.  The common mode of the input signal has enough tolerance that the supply drift is not a problem for the common mode bias.  Or the REFT and REFB may be used to generate the common mode voltage as in Figures 1 and 2.  The CM pin is there as a convenience.

    if the input is differential, then each leg of the differential signal is to be biased to 2.5V common mode.  If the input signal is single ended, then the unused input should be biased to common mode.  (As in figure 1 and figure 2 of the datasheet.)

    If you are using an amplifier to source the input signal, then the amplifier may be configured to output at a certain common mode level and this is the purpose of the resistors R1 and R2 in Figure2.  The text seems to indicate the values of R1 and R2 will depend on the gain of the amp, and i suppose other amps and other amp topologies are acceptable.

    Often on older devices such as this the available information is just what is in the datasheet, as the original design team may no longer be available.  Sometimes the User Guide for the EVM may contain some additional information, although i note that the User Guide for the EVM for this device is fairly basic.    See http://focus.ti.com/lit/ug/sbau008a/sbau008a.pdf

    Most of our devices do have an evaluation module EVM available for purchase, although the EVM for this device was not publicly released for sale.  We do have a limited stock of them in hand, and could send one to you if you provide a ship-to address.  That may solve your breadboarding comment.

    Regards,

    Richard P.