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ADS122C04: Source impedance effect on accuracy

Stephen Jones
Prodigy 100 points
Part Number: ADS122C04

The offset for the PGA, typical and max are very different, under what conditions does the max offset occur.

Also when bypassing the PGA I assume the S/H cap is driven directly by the inputs. What is the size of the S/H caps and aperture time for 20 sampl,es per sec so I can see what effect the source impedance has. We have a 10K bridge and 4k7 1n/10n input filter, making a source impedance of 10K on each differential input.. We are seeing 200uV offsets when the PGA is bypassed on some boards and only 30uV when used and are not sure if this is bias current, S/H loading etc.

Thanks

  • 0
    TI__Guru** 112775 points

    Hi Stephen,

    The typical specification is at room temperature and the min/max specification covers the entire operating temperature range of the device.  When the PGA is enabled, the PGA is similar to an instrumentation amplifier with high input impedance.  When the PGA is disabled and bypassed, the input impedance is increased using a pre-charge buffer.  This means the impedance is dynamic relative to the actual input voltage.  To demonstrate the behavior characteristic graphs of the various behaviors are shown in the ADS122C04 datasheet.  The input current graphs start with Figure 4 and go through Figure 11.

    Usually with bridge measurements the output signal is small requiring gain.  Adding gain also improves the input impedance.  Similar to the input current graphs there are graphs showing response for offset in Figures 14 through16.  Figure 14 gives the best view showing the offset values for 1620 devices.  Here you can see the process variation over a significant number of devices.  Temperature does affect the offset, but also there is device to device variations as well.

    Some of the higher end precision delta-sigma ADCs will have a built in self-offset calibration.  However, the ADS122C04 does not have this feature but can be accomplished by using the mux setting for internal short and using the result to adjust the offset.  Subtracting the offset code in this way will remove the ADC offset of the input stage and modulator.  This method of offset calibration is discussed in section 8.3.11 of the datasheet.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 100 points

    Thanks, How often does the offset calibration need to be done, once at power up, or every time the gain changes or once a day if the gain does not change?

  • 0 in reply to Stephen Jones
    TI__Guru** 112775 points

    Hi Stephen,

    It should be done at power-up, and changing gain will affect the offset so you should use different values based on the gain setting.  As far as how often it should be done is difficult to say.  The offset will drift with temperature, but depending on the level of noise in the measurement the drift may not significantly change the measurement within the temperature operating range of the system.  I have seen some customers issue a periodic measurement (like once an hour for example) while other customers use the internal temperature sensor.  If the temperature of the ADC moves outside of a window (like +/-5 deg C for example) then the calibration sequence is issued.

    Best regards,

    Bob B