D/A ladder calibration using a linear voltage to frequency converter based on the dual input phase accumulator

Abstract

This paper presents a new Voltage to Frequency Converter (VFC) and its use for the calibration of high-resolution high-accuracy converters. The VFC is a PLL based frequency synthesizer that employs a Dual Input Phase Accumulator (DIPA) that behaves as a linear phase detector. The DIPA has two digital inputs, one for the reference (Fref) frequency and one for the VCO frequency (Fout). Additionally, the DIPA has two voltage (analog) inputs, so called Vref and Vin. The DIPA's output is linearly proportional to the integral of the difference (Fref.Vin - Fout.Vref). Therefore output frequency of the DIPA based synthesizer is controlled linearly from the Vin input, since the DIPA based PLL keeps the ratio of the two frequencies Fout/Fref equal to the ratio of the two input voltages Vin/Vref. The accuracy of synthesized frequency of the DIPA based VFC does not depend on accuracy of the capacitors and the other components employed for its implementation, but on its ratio, in contrast to the conventional VFC. Additio-nally, the convergence speed, the phase noise and the performance of the DIPA based analog synthesizer is enhanced compared to these of the conventional PLL based synthesizer. The DIPA based VFC can be used for the calibration of a high-resolution D/A ladder. The frequency ratio can be digitally measured at high accuracy and subsequently the voltage ratio is estimated at the same accuracy.