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ads1255(56) input range during system gain calibration

Oleg Voitovich
Prodigy 50 points
Other Parts Discussed in Thread: ADS1256, ADS1255

What is the ads1256 input voltage range during system gain calibration? The ads1256 datasheet said

"System gain calibration can correct
inputs that are 80% of the full-scale input voltage and
larger. Make sure not to exceed the full-scale input voltage
when using system gain calibration."

Can the input voltage be a LARGER then 2*((Vref+)-(Vref-))?

For example in sbaa107 it is said

"This means that with the system GC, the ADC input range can be changed from 60% below to
33% above the nominal input range set by VREF/PGA." for d-s ADC of MSC12xx devices.

Practically my scheme based on ads1256 demonstrates the ability to perform system gain calibration  with input voltage little larger than 2*((Vref+)-(Vref-)) (Vref=4,95; Vin=4,97) if it is not a mistake. And i'd know certain (or approximate) values if it is possible...

Thanks you for help.

  • 0
    TI__Guru* 97775 points

    Peter,

    Yes, the input voltage can be larger than 2*VREF(/PGA).

    The calibration is meant to generally meant to calibrate for the errors between the size of the capacitors sampling the reference and the capacitors sampling the input. However, it can be used to adjust the magnitude of the full scale.

    Much of this can be explained through the diagram in Figure 23, Equation 3, and Table 18.

    For example, assuming the data rate is 30kSPS, and there is no offset, the original ideal FSC is 44AC08h.

    However, if you have 2*VREF=4.95, and you want to set the full scale VIN to 4.97. You would set the input to 4.97 and run the system gain calibration. During calibration, the ideal would get scaled to reflect this. In this example 44AC08h would be scaled by (4.95/4.97) and the ideal FSR value would become 446549h.

    Note that in the MSC12xx products, the input range is +/-VREF/PGA, which is not the same for the ADS1255 in which the input range is +/-2*VREF/PGA. The two products are similar, but I would not rely on sbaa107 to explain the ADS1255.

    Read through my explanation. If you don't understand it or I've misunderstood something, let me know and we can clarify this.

    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 50 points

    Thanks, Joseph.

    I understand what you said and I understand in general how d-s adc's works. But i was not sure in relation to ads1256.

    Today i spent some time with my board and practically results are: adc is able to calibrate system gain with an input signal 4,8% larger than (RefP-RefN)*2.

    For my design it's sufficient. All works fine.