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ADS1278-SP: Differential Input Impedance

Albin Kasper
Prodigy 30 points
Part Number: ADS1278-SP

Tool/software:

The datasheet for the ADS1278-SP gives typical values for differential input impedance, but gives no indication of variability for a fixed modulator frequency. Does any data exist related to the variability of this parameter? Understandably, an op amp can combat the problem of a "not so high" input impedance where a "not so low" output impedance exists. However, op amps introduce their own errors and this is especially so in DC coupled applications. If the input impedance can be reliably predicted or characterized, it can also be calibrated out.

  • +1
    TI__Mastermind 46560 points

    Hello Albin,

    The ADS1278-SP differential input impedance is directly proportional to the inverse of modulator frequency.  Figure 65 in the datasheet provides the formula:

    Zeff = 14kOhm * (6.75MHz/fmod)

    Table 4 provides the relationship between modulator frequency fmod and input clock frequency fclk.

    The above calculated value for a specific modulator frequency is a typical value.  Due to process variation, there can be a range of +/-20% from the typical.

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    Prodigy 30 points

    Keith,

    Do you expect that the +/-20% is a fairly fixed value related to processing? What variations can I expect with temperature and aging?

    Albin

  • +1 in reply to Albin Kasper
    TI__Mastermind 46560 points

    Hello Albin,

    The temperature variation can be found in Figure 39 of the datasheet.  Please note that the input impedance is closely approximated as a linear resistance, but there are non-linearities as well  (input current verses input voltage is not a straight line, but a curve). 

    In order to minimize the effects of the input non-linearities, the differential resistance of the input source should be 100Ohm or lower in order to meet the datasheet specification for INL.  Keeping the source impedance low will also reduce any gain errors as well.  For this reason, we do not recommend using a high input impedance, such as a voltage divider, to directly drive the ADC inputs if DC precision (gain, linearity, offset) are important.

    We do not have any long term drift data specific to input impedance, but a good estimate of input impedance drift over a 10yr period will be approximately equal to the maximum temperature drift.  For example, in high-speed mode, you can expect the input resistance to change by 300Ohm from 25C to 125C.  You can also expect the input resistance to change by about 300Ohm over a 10yr operating period.  This shift would be in addition to the temperature drift change.

    Regards,
    Keith