PVT Tolerant Transconductor for Low-Voltage Highly-Selective High-Frequency Filter of MRI Front-end Receiver

Abstract

MRI machine has evolved tremendously over the years from its inception in order to render high-quality 3D images, best among its companion imaging systems. Of the whole system, RF receiver is the most crucial for its noise performance, which is expected to provide high SNR at all operating conditions. At Philips, MRI’s RF receiver is based on direct-digitization architecture which has replaced the bulky data-acquisition system with an integrated on-coil receiver. It allows a lot of signal processing to be done digitally which was earlier done in analog-domain, thus improving the SNR and dynamic range to more than 100 dB. However, this has increased the design restrictions of the remaining analog front-end. The high-frequency bandpass filter employed to select the signal band around the receiver’s resonance frequency has not only become highly selective (Q 400, 6th order) to narrow down the signal bandwidth and reject the rest with a high stopband attenuation but also incorporates a high passband gain (60 dB), unlikely to see otherwise. Operating at a high frequency of 100’s of MHz, with such a response, the filter can become drastically sensitive to the shift in its components with varying operating conditions. At Philips, the filter uses transconductor as one of its building blocks and filter frequency response is heavily dependent on the transconductance of the unit cell as they are employed in large number in the circuit. The transconductor cell, at Philips, shows 15% deviation with the process, supply and temperature variations. This leads to more than 10% variation in the passband gain of the filter. Operating at low supply voltage, such a deviation in MRI filter can deteriorate its output SNR and dynamic range by 10dB.

In this thesis, a new inverter-based self-biased transconductance circuit has been proposed, which shows a deviation of ±2.5% in transconductance under the process, temperature and supply variations. It limits the passband gain of the filter to 60 dB±2% and bandwidth shows a deviation of less than 0.1%. However, the center frequency is matched precisely with the resonance frequency of the receiver by the external components of the filter. Given, this matching is the basis for the working principle of MRI receiver. With the proposed transconductance circuit, the filter shows an SNR of 112 dB, THD of -63 dB and consumes a power of 0.8 mW. The circuit has been implemented in 40 nm TSMC process and simulated for a temperature range of 0°C -85°C at the power supply of 1.1 V±10%, post-extraction.