The remote detection example presented here features high reliability, easy connectivity, and ultra-low power consumption. These circuits are primarily intended for industrial environments that require stable communication and minimal battery maintenance. This solution combines the research progress of low power consumption and high precision amplification in recent years, and has the same low power consumption and high reliability wireless mesh network function. Supporting these solutions are the zero-drift, low input bias amplifiers LTC2063, which run at up to 2 µA, and the LTP5901-IPM, which consume less than 1.5 µA in sleep mode. The power consumption of these devices is low enough to be powered by a battery composed of copper and zinc electrodes (four square inches each), and an electrolyte formed from the contents of a lemon.

Wireless Mesh Network

Measurements implemented and retrieved over wireless networks in industrial environments rarely require high speed, but they often require high reliability and safety, in addition to low-power operation to maximize battery runtime. The LTP5901-IPM forms a node or a SmartMesh® IP Mote in an 802.15.4e wireless network. The LTP5901-IPM integrates a 10-bit, 0 V to 1.8 V ADC and a built-in ARM® Cortex®-M3 32-bit microprocessor for simple programming. This terminal is adopted for security, reliability, low power consumption, flexibility and programmability.

Four detection applications

In general, the following circuit designs do not require advanced rocket knowledge. However, they are neat, efficient, and customized for specific applications. These designs don't need to be complicated, in fact, complicated designs only increase cost and reliability risks.

Each circuit contains a sensor at its input and processes the sensor output to generate an output voltage. Using the LTP5901-IPM 10-bit ADC as the input, each circuit tries to map the input, covering most of the range between 0 V and 1.8 V.

Basic battery voltage detection

Figure 1. Simple battery voltage detection.

Figure 1 shows a typical non-inverting overall gain negative feedback op amp configuration to detect voltage dividers. The ADC range of the LTP5901 input is 0 V to 1.8 V. R1 and R2 reduce battery voltage with minimal quiescent current to prolong battery life. The input bias current of the LTC2063 is so low that even these high resistance values will not affect the accuracy of the final 10-bit ADC. The LTC2063 consumes minimal supply current and offers the advantage of zero drift over time and temperature.

Current Detection

Figure 2. Current sensing circuit.

The great thing about battery powered and isolated electronics is that it can be grounded anywhere. In the most convenient circuit topology, we can sense current without loss of generality, while placing the termination anywhere in relation to the local ground. For unipolar currents, such as 4 mA to 20 mA industrial loops, one can use traditional low-side topologies to safely sense currents related to local ground. Figure 2 shows the current flowing through a very small resistor R2, which generates the sense voltage. This input voltage can be very small due to the amplifier's zero drift, extremely low offset voltage performance, etc. The circuit shown has an increased gain of 101 V/V on the input generated through the 501 mΩ sense resistor. At 20 mA, VOUT is 1.012 V. Other values can be chosen to maximize the use of the ADC's 1.8 V range.

Resistor R4 is relatively low and is a low impedance shunt to the LTC2063's input capacitance. Therefore, the interaction between the larger R1 feedback resistance and the input capacitance will not be stabilizing.

The built circuit is optimized to test the mapping range of a 0 mA to 35 mA current, 0 V to 1.8 V ADC.

Radiometer

Figure 3. Short-circuit irradiance measurements with solar cells.

The circuit shown in Figure 2 can also be used to measure the short-circuit current of a solar cell. In short-circuit current mode, silicon and other solar cells have a highly linear relationship between current and irradiance. The short circuit current is the current of the 0 V solar cell. The circuit in Figure 3 does not guarantee that the solar cell will reach exactly 0 V at maximum current; however, even at 20 mA in full sunlight, the voltage is only 10 mV. A 10 mV level on a solar cell is effectively a short on its IV curve.

We can use a transimpedance amplifier (TIA) as an alternative. The TIA can force the solar cell to 0 V and measure the current. The problem with this circuit is that the op amp is supplying current to the solar cell over the entire irradiance range. If minimizing power consumption is paramount for a remote sensing circuit, then it is not feasible to supply 20 mA to the battery from an op amp.

Considering the need to maintain close to 0 V, a small sense resistor should be used. Detecting small, remote, battery-powered voltages again shows the need for a high-accuracy, low-power power amplifier such as the LTC2063.

This kind of physical layout is what is required for a solar installation, a wireless mesh network that needs to implement zero temperature drift measurements. Fortunately, under short-circuit conditions, silicon photodiodes are relatively stable with temperature. The LTC2063 and LTP5901-IPM, combined with silicon solar cells, provide an ideal solution for a simple and reliable design for large installations with changing ambient temperatures.

Temperature measurement with thermocouples

Figure 4. Thermocouple detection circuit.

Thermocouple voltage can be positive or negative. The circuit shown in Figure 4 combines a micropower reference and a micropower amplifier to detect extremely small positive and negative voltages. Fortunately, if the thermocouple is electrically isolated from the device under test (DUT), it can be placed in any convenient voltage domain. The example in Figure 4 uses the LT6656-1.25 to bias the thermocouple at 1.25 V. The circuit output is a high gain version of the small thermocouple voltage based on a 1.25 V reference. The ADC range of 0 V to 1.8 V is quite reasonable for this configuration. Very high gains of around 2000 V/V cannot be achieved without the use of a zero-drift, low-offset amplifier.

in conclusion

Extremely low power consumption and accurate remote detection is absolutely feasible. The example in this article shows how simple it is to combine a low-power, high-precision amplifier with a programmable system-on-chip wireless mesh node.