## 4 Case Studies
### Case Study of Custom IC System  
One example of a custom built BSN is the ECG chip presented in [7]. It is a 0.13-\(\upmu\)m bulk CMOS sub-threshold (sub-V\({}_{\mathrm{T}}\)) mixed-signal system-on-chip (SoC) that acquires and processes an ECG signal for wireless ECG monitoring. The die photo is shown in Figure 11. The system consists of an adjustable gain instrumentation amplifier (IA), an 8-bit analog to digital converter (A/D), a microprocessor that operates in the sub-threshold region (sub-V\({}_{\mathrm{T}}\)), and a universal synchronous receiver/transmitter (UART) to communicate with an external radio. The SoC uses a sub-threshold digital microcontroller (\(\upmu\)C) for adaptive control of the sub-V\({}_{\mathrm{T}}\) biased analog components and for processing the ECG data. The microcontroller core is a customized variant of the Microchip PIC 16C5X [33]. This base unit has 33 instructions and memory sizes of 24 to 73 bytes of RAM. A simple differential IA topology was chosen for the ECG amplifier. Because the amplitude of an ECG signal varies depending on the placement of the recording electrodes and physiological variations between individuals, the IA has a digitally adjustable gain. The 8-bit A/D digitizes the amplified ECG signal at a 1 kHz sampling rate. The A/D uses a dual-slope, integrating architecture. This architecture was chosen for its simplicity, low power consumption, and its insensitivity to device variation. By adjusting the A/D supply voltage, we can trade off power consumption with resolution. This allows for lower system power, since the A/D power can be reduced when the system does not require the full fidelity capabilities of the A/D. Level converters were used to transition data signals from low voltage domains to higher voltage domains. This is not a simple problem due to the large difference in voltage between a sub-threshold region supply and the nominal V\({}_{\mathrm{DD}}\) of 1.2 V. Most previous implementations of level converters supporting sub-threshold inputs use intermediate voltages to perform the up-conversion over multiple stages. A custom level converter for our SoC can convert from an input voltage of 160 mV up to 1.2 V.  
The microcontroller, based on a PIC architecture, operates from 0.24 V to 1.2 V and consumes as little as 1.51 pJ per instruction at its minimum energy voltage of 0.28 V. The entire SoC (analog front end, ADC, and digital processor) consumes only 2.6 \(\upmu\)W while providing raw ECG data or processed heart rate data. This level of energy efficiency far exceeds the abilities of COTS implementations and makes the idea of an energy harvesting ECG sensor feasible. When only heart rate information is required, the onboard computation of heart rate reduces the wireless channel data rate by a ratio of 500:1, which allows complete beat-by-beat heart rate information to be communicated with much less energy expended in the radio. This chip exemplifies how low energy processing can be used to increase the effective CR in a BSN by extracting the important information from raw data prior to communication. This custom SoC platform takes advantage of the fact that the application is well defined, and therefore platform flexibility can be traded off for optimization in energy efficiency and form factor. This platform utilizes the general strategies of communication _versus_ computation and fidelity _versus_ energy to obtain better energy efficiency. Other examples of custom platforms include [34, 35, 36, 37]. [34] is a 0.5 \(\times\) 1.5 \(\times\) 2 mm\({}^{3}\) size, 5.3 nW intraocular pressure sensor with microprocessor and transmitter that is used to detect glaucoma. It achieves low power by duty cycling, the use of low power clocks, and on board processing. [35] integrates a glucose sensor with a wireless transmitter in a contact lens for diabetes monitoring with a power of 3 \(\upmu\)W that is wirelessly transmitted. It utilizes a sub-\(\upmu\)W low-power regulator and bandgap reference to achieve its low power profile.