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According to the World Health Organization, about 155 million people worldwide have diabetes, and that number is expected to rise to 300 million by 2025. Because both high and low blood glucose levels pose serious health risks, prevention and monitoring are important. This implies potential demand for millions of personal portable blood glucose meters (BGMs).
Modern BGMs must be highly integrated in a small form factor for portability. At the same time, these high-performance devices need to measure blood glucose accurately using only a small blood sample to meet patients' daily needs.
Precision multiplexer in the sensor front end
The precision multiplexer at the sensor front end allows switching between multiple sensing channels without requiring multiple fingertip measurements. Although a multiplexer may seem to perform only the simple task of switching between signal channels, it plays a critical role in accurately relaying the tiny currents generated by the test strip to the amplifier chain.
Choosing the right precision multiplexer helps engineers designing compact BGMs address major design challenges such as leakage current, on-capacitance, and size constraints. Understanding these parameters helps engineers optimize designs and can reduce patient discomfort while improving quality of life.
Ultra-low leakage current (I_LEAKAGE)
Test strips react chemically with a blood sample to produce a current signal. That small current is typically converted to an amplified voltage in a transimpedance amplifier (TIA) stage. The sensor current is fed into the TIA through configurable gain settings and then sampled by an analog-to-digital converter (ADC), often inside a microcontroller.
An example shows a 4:1 channel multiplexer selecting four different feedback elements for an op amp to implement configurable gain. External resistors, or using one of the four channels left always open, ensure the amplifier does not operate in an open-loop configuration. Although the figure shows four feedback resistors, the resistor count can be increased or decreased based on required gain settings.
These gain settings are necessary because the strip current can range roughly from 10 μA to 80 μA depending on glucose level. The range can be adjusted to support measurements at different times or for different patients.
Designers need not only low on-resistance (R_ON) but also multiplexers with ultra-low leakage to maximize ADC accuracy.
The reason is simple: when a switch is closed, leakage current from the multiplexer adds to the sensor current and flows through the feedback resistor, producing an error at the op amp output. That erroneous voltage is then sampled by the ADC, resulting in inaccurate displayed glucose readings.
In short, lower multiplexer leakage currents yield more accurate output readings.
Some precision multiplexer devices offer extremely low on-state leakage. For example, the TMUX1104 provides very low on leakage, with typical 3 pA and maximum 50 pA at 25°C. With such low leakage, the device can switch signals from high source impedance inputs to a high input impedance amplifier with minimal offset error.
Low on-capacitance (C_ON)
Another important parameter affecting multiplexer performance in BGM applications is the on-capacitance, C_ON. C_ON affects the multiplexer settling behavior and therefore the system transient performance.
Ignoring the multiplexer C_ON specification can lead to TIA stability issues. The multiplexer C_ON may cause oscillations and degraded transient response in the signal chain. Even if designers do not intentionally add capacitance, the multiplexer C_ON can be large enough to affect the system.
Selecting a multiplexer with low C_ON helps minimize problems from these common circuit challenges. When a multiplexer channel is on, its C_ON appears as a capacitance to ground in the system. Typical C_ON values for analog multiplexers range from a few picofarads up to more than 400 pF. The TMUX1104, for example, has C_ON as low as 35 pF.
If configurable gain control is implemented using four single-pole single-throw (1:1) switches, each channel has its own C_ON. When all channels are on, the C_ON values are in parallel. Keeping one of the four channels always off ensures the amplifier will not be in an open-loop configuration. If a multiplexer with higher C_ON is used, a single 1:1 switch channel can have about 70 pF; with all four channels on, the total feedback C_ON would be 280 pF. By contrast, a device with 35 pF C_ON yields only 140 pF total with all four channels on.
Considerations for compact BGMs
Like smartphones, laptops, and tablets, portable medical devices trend toward smaller, lighter solutions. Patients prefer compact glucose meters that deliver accurate measurements and are easy to carry. This creates a challenge for designers seeking precise, compact solutions.
The TMUX11x family of precision multiplexers supports small leaded and leadless packages and a wide supply range (1.08 V to 5.5 V single-supply or ±2.5 V dual-supply). These devices combine very low on-state leakage, low C_ON, and small package options, making them suitable for high-accuracy portable medical measurements. The TMUX1104 is among the smallest single-channel 4:1 precision multiplexers and offers ultra-low leakage (3 pA typical) and low supply current (about 8 nA), which is advantageous for battery-powered BGMs.
When designing an accurate glucose meter, it is unnecessary to trade off system accuracy or stability. Selecting a precision multiplexer with ultra-low leakage, low C_ON, and a small package for the sensor front end can address those requirements.
