An organic VCO-based ADC for quasi-static signals achieving 1LSB INL at 6b resolution

Smart sensors embedded in packaging films for food or pharmaceuticals, and read out using RFID protocols, are an important future application of large-area organic electronics. The first steps towards flexible organic electronics were code generators for RFID tags [1]. To realize flexible smart sensors it is also essential to develop analog sensor interfaces and ADCs, which will enable the integration of sensors, data processing and RF communication (Fig. 6.6.1) on the same plastic foil [2]. This paper focuses on the design of an organic ADC for quasi-static signals, like the ones provided by chemical and temperature sensors. Only a few ADCs made with organic TFTs (OTFTs) have been reported [3,4] so far. Their linearity is limited by the poor matching typical of organic technologies and reached an INL of 2.6LSB at 6b resolution before calibration. This paper addresses an ADC whose linearity is not related to the matching of OTFTs or capacitances, but relies on the electrical properties of a transconductor. Even without calibration, the INL is 1LSB and the DNL is 0.6LSB at 6b resolution. The converter is manufactured in a double-gate p-type OTFT technology [5], which provides two gates to control the semiconductor channel, G and TG (Fig. 6.6.1): a voltage applied to the top gate TG produces a linearly proportional shift of the threshold voltage.