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ADS1259: ADS1259

Earl Davis
Prodigy 70 points
Part Number: ADS1259

I am using A/D ADS1259. 

* Operating voltage is +5 vdc and ground

*using internal reference +5.5 vdc

*using external power supply for input to IN+ and IN-

I am having problems reading in full scale from +5 to ground. It's like the A/D only reads half the range.

I am writing into the registers the following

Command 0x40, 0x08,  0x85,0x03, 0x17,  0x00,0x00,0x00,  0x00,0x00,0x40

What should the offset be? I have the gain set for gain factor of 1

  • 0
    TI__Guru 52066 points

    Hi Earl,

    Can you clarify what you mean by "using internal reference +5.5 vdc"? The ADS1259 has an internal 2.5V reference, and cannot accept an external voltage reference of 5.5 V, so this was a bit confusing.

    -Bryan

  • 0 in reply to Earl Davis
    TI__Guru 52066 points

    Hi Earl,

    The ADS1259 has a bipolar code range, such that the minimum code corresponds to -VREF and the maximum code corresponds to +VREF. In this case, that is -2.5V and +2.5V since you are using the internal VREF. When you apply 5V to IN+ and 0V to IN-, you are applying a 5V signal, whereas the maximum is only 2.5V. Therefore, once you hit 2.5V, you will see the full-scale code. Any voltage beyond this will also result in the full-scale code.

    You can use the REFOUT feature on the ADS1259 to bias IN-, and then let IN+ swing from 0V to 5V. When IN+ is 0V, the ADC will read a differential voltage of -2.5V. When IN+ = 5V, the ADC will read a differential voltage of +2.5V. This will allow you to use the entire code range. You could also use an external 5 V reference to get more range out of the device.

    Just make sure this will work with any signal conditioning circuitry you may have in front of the ADC, since you mentioned a gain of 1 (whereas the ADS1259 does not have gain).

    -Bryan