A novel resonant tunneling Delta-Sigma A/D converter having no feedback loop for ultrahigh-speed operation

Abstract

A resonant tunneling logic gate MOBILE (Monostable-Blstable transition Logic Element) is an ultrahigh-speed functional logic gate,. which is based on the monostable-bistable transition caused by the clock in the series-connected RTD circuit [I]. The MOBILE switches at the rising edge of the clock and the output remains in one of the two stable states while the clock is high. This indicates that the MOBlLE works as an ideal 1 -bit quantizer for high-speed analog-to-digital converters (ADCs). Recently, we proposed a simple AZ ADC using MOBILES [2]. The AZ ADC has a significant advantage compared to other types of ADCs; higher resolution can be easily obtained by increasing the sampling rate. It does not require high-accuracy analog components to achieve high resolution. However, the operation frequency and the resolution of our previous proposal was still restricted by the negative feedback loop. In this paper, we will propose a novel AE ADC having no feedback loop, which can make the best use of high-frequency performance of the RTDs. Figure 1 shows the block diagram of the AZ m C . The input analog signal is first converted to the pulse density modulation signal, then it is converted to the Nyquist rate high-resolution (multi-bit) digital signal by a decimation filter. The key component of this is the AX modulator, which converts the analog signal to the pulse density digital signal. Here, we employ a novel AE modulation technique suitable to MOBILEs using a voltage controlled oscillator (VCO) [3]. The configuration of the novel AE modulator is shown in Fig. 2 together with the conventional one. This implementation is based on the fact that the phase of the frequency modulation (FM) signal from VCO, O(t), is the integral of the input signal x(t) as hete,fc represents the output frequency of the VCO when input is zero, and k the frequency sensitivity.Therefore the integrator in the conventional configuration can be removed. Moreover, the negative feedback is inherently embedded in the VCO, because the phase returns to zero when it reaches 2~. Owing to these features the feedback loop can be removed. Using the I-bit quantizer and the XOR the ideal pulse density signal can be obtained. However, it requires extremely high performance quantizer, having ultra short aperture time and high sensitivity at high frequency, for fully developing this concept, so that this technique has been not widely used. Employing the MOBILE, this problem can be solved, and extremely high performance should be expected.We tested this hc modulator concept using the MOBILE fabricated on the InP substrate. Figure 3 and 4 shows the experimental setup and the chip photomicrograph of the MOBILE used in the experiment. The HEMT has the gate length of 0.7 pm and a threshold voltage of about 0 V. The peak voltage, the peak current density and the peak-to-valley current ratio of the RTD are 0.35 V, 6.0~10' Ncm' and 10 at room temperature, respectively. The frequency modulation signal was synthesized by the signal generator and it was fed to the MOBI...