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LTC2157 Alternate Bit Polarity

Category: Hardware
Product Number: LTC2157-14

When testing the LTC2157-14 with ABP and randomizer turned on the MSB is inverted to what  we expect.  We have to invert the bit that is already being inverted.  Is there a problem with the LTC2157 that isn't in the datasheet? 

  • Hello,

    Ignoring the randomizer for a moment,  ABP mode inverts all the odd bits of which DB13 (or MSB) is one of those bits that get inverted.   It appears that  you state that this is indeed occuring in the transmitted digital output.   On the receive capture side, one would need to invert all the odd bits again (including MSB) to restore the correct digital value representing the sampled waveform.  So it is unclear what you mean by "we have to invert the bit that is already being inverted" based on previous statement.

    Both ABP and randomizer have the same objective in trying to decorrelate the digital data pattern (during transmission) so that any digital noise that is inadvertently fed back to the ADC input appears more noise like.   Considering the 14-bit ADC's SNR of 70 dBFS (vs theoretical 86 dBFS), it would seem that the randomizer itself would be more than sufficient (vs both features enabled) since the LSB will be toggling quite frequently being dominated by noise floor of the ADC.


  • In the bit stream the MSB is inverted to what we expect it to be.  It is almost like the part is in 2s complement when it should be in offset binary.  

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