Because of the extremely low amplitude of the input signal, the design of electro-neuro-graph (ENG) amplifiers involves a special care for flicker and thermal noise reduction. The task becomes really challenging in the case of implantable electronics, because power consumption is restricted to few hundreds lW. In this work, two different circuit techniques aimed to reduce flicker and thermal noise, in ultra-low noise amplifiers for implantable medical devices, are demonstrated. The circuit design, and measurement results are presented, in both cases showing an excellent performance, and noise to power consumption trade-off. In the first circuit, a very simple low-pass G m -C chopper amplifier is used for flicker noise cancellation. It consumes only 28 mW, with a measured input referred noise and offset of 2 nV ffiffiffiffiffiffi Hz p , and 2.5 lV, respectively. In the second circuit, a ultra-low noise amplifier, a energyefficient DC-DC down-converter, and low voltage design techniques are combined, for the reduction of thermal noise with a minimum power consumption. Measured input referred noise in this case was 5.5 nV ffiffiffiffiffiffi Hz p at only 380 lW power consumption. Both circuits were fabricated in a 1.5 lm technology.
Cyclostationary operation of the MOS transistor has been proposed in recent years as a technique for reducing the flicker noise at the device level itself. Several works report measured cyclostationary flicker noise reduction, the PSD showing a plateau below the switching frequency, but at much lower frequencies the slope resembles again the original 1/f spectrum. But current models do not correctly address the latter effect. In this work, the PSD of a DC biased transistor is first deduced using only Shockley-Read-Hall (SRH) statistics and the autocorrelation formalism. Then the analysis is extended by means of simulations and using reasonable physical hypotheses, to a cyclostationary bias condition. The results allow explaining reported experimental data in the whole frequency range. Finally the development of a specific integrated circuit aimed at switched flicker noise measurements in different types/sizes of test transistors and at different bias conditions is presented.
A bulk-linearised composite MOSFET is presented, with an extended linear range and an equivalent saturation voltage of up to several hundred mV even in weak inversion. Some preliminary measurements are presented as well as a 300 MΩ equivalent resistor, with almost a 1 V quasi-linear range. A low frequency, 28 dB gain, biomedical bandpass filter/amplifier is also shown, taking advantage of the linearised MOSFETs to consume only 16 nA of supply current.
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