`While investigating some analog to digital converters for an IMU`

`project I'm working on, I settled on a TI ADS1278. One reason for`

`using this ADC is that it has such fine resolution that I may be able`

`to eliminate the amplifiers on the input for most inertial sensors!`

`This ADC has impressive specs, but I'm a bit concerned about the`

`integral non-linearity (INL) spec. In a previous IMU design, I've seen`

`the result of this parameter when it is too high, and the symptom is a`

`bit nasty.`

`The ADS1278 spec shows INL as (,0.0003,0.0012)% FSR, where FSR =`

`2Vref. I'm not sure if this is 2 *times* Vref, or if Vref is 2*Volts*`

`in this measurement. To avoid figuring this out right now, I'll speak`

`in terms of bits, which is what I am concerned with anyway.`

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`FSR = 2^24 bits (24 bit adc), so that means that the INL is`

`0.000003*2^24 = 50. since 50 is close to 2^6, that means we are`

`(typically) giving up the lowest 6 bits of the converter to INL. This`

`still leaves 18 bits of converter range, but only if the application`

`is designed so that the sensors rail out near the full voltage of the`

`converter. Making this happen may require an input amplifier. Worst`

`case, INL is 0.000012*2^24 = 201, ~ 8 bits, leaving me with a 16 bit`

`ADC. Again, if I leave out the amplifier, I could wind up with 12-14`

`bit conversion, which is hardly worth the effort of placing this part.`

`On to my question: Tim and I discussed this, and he mentioned that the`

`INL could be calibrated out. After some thought, I don't think this is`

`possible. I thought problem with INL is that the output may show a`

`particular bit pattern for more than one input voltage. Could someone`

`(maybe Tim?) explain how this works and how we can calibrate this out?`

`Secondly, has anyone actually done this? What was it for?`

Thanks, D _______________________________________________ psas-avionics mailing list psas-avionics@lists.psas.pdx.edu http://lists.psas.pdx.edu/mailman/listinfo/psas-avionics