# [psas-avionics] Analog to digital conversion

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.
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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.
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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?
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Thanks,
D

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