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Advanced Audio Recording

R2R-DAC precision enhancement

Typical R2R-DACs suffer from a limited precision when using non selected resistors. Moreover the tolerance of the resistors themselves limits the number bits which could seriously be used to generate an almost analog output. In the design below a common R2R-ladder-DAC is presented which can be optimized up to a precision of 1% using resistors with 5% tolerance only.


precision enhancement of R2R-DACs - Jürgen Schuhmacher

 6 Bits of a digital vector control a R2R-ladder the common way, with an impact between 50% and 1,56% of the particular bit. Because of the fact that the drivin voltage is either GND or VCC, the maximum precision is around the doulbe = 3,2% = 1/32, disregarding the deviation caused by the resistors tolerance.

 

calibrating the DAC

By adding 2 more bits and driving them with also tristate function basically 3 states can be achieved leading to a 1,5% accuracy. Using both calibration bits appropriately a deviation of 4,7% can be compensated which is enough in case of the resistors have maximum tolerance.

This way, 8 bits are used. The former static 2R to GND can be left away.

To finalize the calibration a table has to be established in software determining the output bits to use for a desired value.

 

Conclusion and Summary

 R2R-DAC's precision can be enhanced when compensating the deviation by adding additional calibration bits.

 

© 2002 - Dipl.-Ing. Jürgen Schuhmacher