Many medical sensors, such as continuous glucose monitoring (CGM) sensors include transimpedance amplifier circuitry for amplifying a sensor current signal. Such sensors often include a high resolution analog-to-digital converter (ADC) to further process the amplified signal. However, high resolution ADCs are typically more expensive and more energy intensive than lower resolution ADCs. Additionally, such sensors typically have low voltage-per-level and voltage-to-current ratios, and thus often exhibit undesirable sensitivity to electrical noise.
As an example, in order for existing glucose monitoring systems to achieve the minimum typically desired resolution of 5 picoamperes/level and cover the full range of current magnitudes produced by a CGM sensor (which, for illustrative purposes, may be approximately 5 microamperes), an ADC with a dynamic range of (5 microamperes)/(5 picoamperes/level)=1,000,000 levels is needed, corresponding to a 20-bit ADC (2^20=1,048,576). The transimpedance amplifier circuitry typically included in such systems often has a maximum output voltage of approximately 3 volts. The voltage-to-level ratio of such a system is then (3 volts)/(1,048,576 levels)=2.9 microvolts/level. Additionally, the value of the feedback resistor included in a transimpedance amplifier is typically selected so that the maximum range of voltage outputs are achieved: for a maximum output voltage of 3 volts and a maximum current of 5 microamperes, the value of the feedback resistor is typically selected to be as close as possible to (3 volts)/(5 microamperes)=600 kiloohms. Assuming that the typical operating range of sensor current is 1 picoampere (much lower than the initial 5 microampere value), the voltage-to-current ratio under typical operation is (1 picoampere)×(600 kiloohms)=0.6 microvolts/picoampere. For systems with voltage-to-level and voltage-to-current ratios this low, special hardware requirements are typically imposed and expensive components are typically used to protect sensitive circuitry from electrical noise.