## 3 Signal Conditioning Circuit  
### Power Management Circuits  
The signal conditioning system presented in [13] is a complex laboratory equipment that uses relatively high voltages (+25 V and \(-\)5 V) to work and is not adequate to be integrated in an autonomous irrigation management system powered by energy harvesters [19].  
In this work we present a signal conditioning system that operates from a single 5-V power supply, obtained from an energy-harvesting system with a supercapacitor, based on a DC microgenerator attached to the top of an irrigation micro-sprinkler [12]. The simplified block diagram of the developed power management circuit is presented in Figure 2.  
The main supply voltage is taken from an \"ORed\" connection of the voltages derived from: (a) a supercapacitor (which is charged up to 5.2 V); (b) the voltage at the output pin \(V_{out}\) of \(I_{C5}\), an LTC3108 DC-DC converter from Analog Devices, which can reach 5 V when the steady state of the LTC3108 is reached. The voltage taken from the supercapacitor \(V_{Cstore}\) has two series Schottky diodes \(D_{3},D_{4}\) (BAT43 from STMicroelectronics), while the \(V_{out}\) has only one BAT43 diode \((D_{5})\) in series.  
With this configuration, the supercapacitor is discharged only when the LTC3108 DC-DC converter is not harvesting energy (\(V_{out}<4.8\) V, if we considered the voltage drop of the Schottky diodes as \(V_{D}\approx 200\) mV). The LTC3108 has a quiescent current that can reach up to 9 \(\upmu\)A, so diodes \(D_{1},D_{2}\) are necessary to prevent the internal circuits of the LTC3108 from discharging capacitors \(C_{out}\) and \(C_{Cstore}\) when the microgenerator is not operating.  
The output voltage from the ORed connection is sent to two voltage regulators: \(IC_{1}\), a 3.0-V output low drop-out voltage regulator (LDO) TPS70930 (from Texas Instruments), which has a very low quiescent current, typically 1 \(\upmu\)A; and \(IC_{2}\), a 5.0-V output high-efficiency charge pump DC-DC converter (TPS60130, from Texas Instruments), which can operate from input voltages as low as 2.7 V and provides a 5.0 \(\pm\) 4%-V output voltage.  
The 3.0-V from the LDO is used to power the MSP430F6736 microcontroller (from Texas Instruments). The 5.0-V output from the TPS60130 is sent to two LM2662 Positive/Negative Charge Pumps (from Texas Instruments). The LM2662 is very versatile and provides an output that can either double or invert its input voltage. Therefore, in our circuit, the 5.0 V are doubled by \(IC_{3}\) (generating a 10-V output) and inverted by \(IC_{4}\), generating a \(-\)5 V output.  
To measure soil water content with high precision (usually a 1% resolution in \(\theta_{v}\) is required [20]), it is necessary to apply well-controlled pulses of power to heat the transistor. Unfortunately, the output voltage of the LM2662 is not regulated, presents high ripple, and varies with the temperature and also with load current.  
Thus, to obtain a well-known and stable voltage, we needed to send the 10-V output from \(IC_{3}\) to an adjustable LDO, \(IC_{5}\), a TPS71501 (from Texas instruments), which has a power supply ripple rejection of 60 dB, a line regulation of 61 dB, a load regulation of 60 dB and, and output thermal coefficient \(TC\approx 15\) ppm/\({}^{\circ}\)C. In our circuit, the TPS71501 was designed to furnish an 8.0-V regulated output voltage, by adjusting resistors \(R_{1}\) and \(R_{2}\).