Chopper Modulation Improves OTA Information Transmission

We have investigated information transmission in operational transconductance amplifiers (OTA) using chopper modulation. Previous work showed that the optimal frequency bandwidth for an OTA was much higher than typical operating frequencies. Here we analyze the information transfer rates for a folded cascode amplifier and a chopper modulated folded cascode amplifier using the principles of information theory. The frequency transfer characteristic and intrinsic physical noise source of each device is modeled using process dependent noise parameters and the waterfilling technique is applied to determine the capacity as well as information rates for low frequency signals. Simulations are experimentally verified using circuits fabricated in a commercially available 3-metal, 2-poly 0.5 µm CMOS process.

Operational transconductance amplifiers (OTA) are a basic building block for modern micro and nanoscale sensors. However, as feature sizes scale down, the noise contribution of each MOS device in the OTA increases and the OTA performance suffers from reduced signal to noise ratio. This is an extremely important factor in designing sensors where the input signal is small compared to the circuit noise, a common problem for lab-on-a-chip systems and other densely integrated systems. Examples of these types of integrated sensors are bioamplifiers which amplifiy weak extracellular signals, fluorescence (image) sensors based on fluorophores with weak emission intensities and capacitive sensors which sense the weak capacitive coupling between a cell and substrate we investigated the information transmission of OTAs by modelling input-referred noise and determining the channel capacity for transmission of analog signals through an OTA using an information theoretic approach in which the circuit was treated as a Gaussian communication channel with colored noise . The technique was applied to simple and wide range OTAs. The signal frequency band for optimal use of the OTA was found to be around the second pole of the frequency response, much higher than the typical operating range below the first pole. This suggests that strategies such as signal modulation might provide more efficient use of an OTA from the perspective of information transmission. Here we extend the previous work to analyse the information capacity and efficiency of an amplifier that employs chopper modulation

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