چكيده لاتين
Large sensor arrays that perform evaluation and measurement in parallel on a chip have wide applications. Also, the array of electrochemical cells equipped with nanopore is used in DNA sequencing. In this type of application, due to the large number of channels, low current levels and bandwidth of several kilohertz, two-level, low-noise and low-power analog-to-digital converters are needed in each channel to read the nanopore cells. Previously, low-noise and high-precision continuous-time sigma-delta modulators have been used to convert the nanopore current signal into a digital code. Another challenge of readout circuits is the reduction of power consumption, which is of particular importance in an array of sensors. Currently all proposed sigma-delta converters used to read input current levels with picoamp resolution require an op-amp due to the high order of the modulator. In addition, in these modulators, in order to reduce the noise referred to the input node, a capacitor divider is used, which requires an op-amp to implement. Leads to a significant increase in the power consumption of the entire array, as a result of which the temperature of the entire structure increases and the performance of the entire modulator are disturbed.
In this treatise, two continuous-time sigma-delta modulators of the order of two current modes are proposed, which, like other sigma-delta modulators that are used to read the current of the nanopore sensor, use a capacitive divider in order to reduce the noise referred to the sensor input. In the first design, a temperature-independent sigma-delta modulator is presented, which is implemented without using any resistance. In this regard, MOSFETs in the saturated region and in the triode region have been used to implement switching current sources and voltage-to-current converters. Switching current sources are created by controlling the source voltage of the MOSFETs in the saturation region by the current mirror structure. In this treatise, the switching of the proposed switching current sources has been done in such a way that by keeping the current in the range of nanoamps, the optimal rise and fall times and the ENOB value of the modulator do not change. In fact, by using two keys with dual functions, the effect of loading when disconnecting and connecting the key has been removed. In the proposed voltage-to-current converter section, linear and constant resistance is created with the temperature change by controlling the MOSFET gate voltage of the triode. In this case, the change of modulator gain with temperature change from 27 °C to 100 °C has reached 0.3%. In this modulator, the value of ENOB and power consumption with an input current of 600pA and a frequency of 10kHz are equal to 12.9bit and 4.6mW, respectively. In the second proposal, a second-order sigma-delta converter based on Gm-C filter is proposed, which uses a Gm block with 4 transistors instead of several op-amps. In the proposed modulator, the power dissipation is significantly reduced while the noise referred to the input remains close to the previous methods. In this structure, the type of control keys and their dimensions have been designed in such a way that by maintaining the power consumption in the nW range, switching current sources with optimal values and small rise and fall times have been produced. A practical example is also implemented to demonstrate the effectiveness of the proposed method. In the proposed modulator with 180nm technology, according to ENOB, the RMS noise referred to the input is 10kHz bandwidth and the power consumption is 12.16bit, 0.2pA and 8.27μW.
