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ADS1260: Bypass mode, resolution

Part Number: ADS1260

Hi,

I want to measure a signal between 0V - 2V5 with the ADS1260.
For this I use the internal reference (REF(06h) = 10h).
The PGA is set in bypass mode (PGA(10h) = 80h).
The multiplexer is set to AIN0 and AINCOM (INPMUX(11h) = 10h).
AVDD = 5V
DVDD = 3V3
AVSS = DVSS = AINCOM = GND
AIN0 = Signal between 0V - 2V5

Why is my value at 2V5 on AIN0 only at 7FFFFFh and not at FFFFFFh?
Why can I measure negative voltages on AIN0 up to -2V5 in bypass mode?
According to datasheet 9.3.1.1 or figure 48 I should never apply a negative voltage below AVSS - 0V3 to an input, but in chapter 9.3.2 or table 4 it is stated that in PGA mode with gain 1 the input voltage may be +/- 2V5. Do I understand the data sheet wrong or is there an error in the data sheet?

With kind regards

  • Hi Klaus,

    If you are measuring between AIN0 and AINCOM, which are at 2.5V and 0V, respectively, then you are measuring a differential voltage of +2.5V. When you compare this against the ADS1260 internal reference, which is also at 2.5V, you should see a positive full-scale code, or 7FFFFFh. Since this is what you got from the ADC, it appears that the ADC is working correctly.

    To answer your other question, please consider the difference between absolute and differential voltage. The absolute voltage on any analog input cannot be <AVSS, but the differential voltage (that is the voltage between AINP and AINN) can be positive or negative. For example, if you had AINP = AIN0 = 0V and AINN = AINCOM = +2.5V, the ADC always measures the difference between AINP and AINN, which is 0 - 2.5V = -2.5V in this case. However, note that absolute voltage on both AINP and AINN is still >=AVSS.

    -Bryan

  • Hi Bryan,

    am i got this right that the ADS1260 then only has 23bit and a sign bit?

  • Hi Klaus,

    No, the ADC is capable of 24 bit resolution. However, you need to use the full-scale range of the ADC to get this, which is +/-VREF/gain. If you are using a single-ended signal then you are only going to be using +VREF/gain, or half of the input range e.g. 23 bits max

    However, you could bias AINN to something like 1.25V, and apply your 0-2.5V input signal to AINP, which is called a pseudo differential measurement. When the ADC measures AINP - AINN, you will be using the full scale range per the details in my last post about positive and negative differential voltages. This assumes you use a gain of 2.

    -Bryan

  • Hi Bryan,

    but for this I have to apply a precise voltage from the outside, right? Or is there a possibility to use the internal reference voltage (VRef/2)?

  • Hi Klaus,

    Yes this is correct, you need some voltage to bias the AINN pin. This voltage, VBIAS, should allow your signal to swing symmetrically around VBIAS. So yes, if your signal range is 0 to 2.5V, then you can use the ADS1260's REFOUT pin and divide it by 2 to get 1.25V. Or, if you could level shift the input signal by 1.25V so your input swing was from 1.25V to 3.75V, then you could use the undivided REFOUT voltage for VBIAS. Either way, the VBIAS voltage would then be applied to whichever analog input you select for AINN.

    -Bryan