A printed DAC achieving 0.4LSB maximum INL at 7b resolution level

Circuits based on printed organic thin-film transistors (OTFTs) can be used to create large-area and mechanically-flexible applications. However, the accuracy of these circuits is strongly limited due to the relatively high variability in the characteristics of the OTFTs, typical of low-cost and low-temperature manufacturing techniques based on printing. The circuit performance in terms of linearity and resolution can be improved when other integrated components such as resistors are available. Printed resistors are expected to have better matching than printed OTFTs, since fewer process variables can affect the performance of a resistor. In this work, we show a printed 4-bit DAC which achieves a maximum INL of 0.05LSB, equivalent to 0.4LSB at 7b resolution level. The resistor-based DAC is fabricated in a process flow based on printing which integrates top-gate P- and N-type OTFTs together with resistors on a plastic foil [1]. We have implemented a 4-bit counting ADC by integrating the R2R DAC with an auto-zeroed comparator [2] and a counter. The counter converts the clock signal to a BCD code, which is in turn converted to an analogue signal by the DAC. The DAC output is compared to the input signal using the comparator. Whenever the comparator changes its state, the counter is stopped and the present count is the digital output. A flip-flop is exploited to filter the output of the auto-zeroed comparator and send the valid comparison result to the counter. The ripple counter is reset asynchronously at the end of each conversion. Since the linearity of the ADC is limited by DAC [2], we can achieve ADCs with relatively higher resolution. [1] S. Jacob, et al., "High performance printed N and P-type OTFTs for CMOS applications on plastic substrate," ESSDERC, Sept. 2012. [2] S. Abdinia, et al., "A 4-bit ADC manufactured in a fully-printed organic complementary technology including resistors," ISSCC, Feb. 2013.