TESTING & MEASUREMENT Analog Devices A Simple Way to Measure Temperature
Using One GPIO Digital Interface Chau Tran and Naveed Naeem from Analog Devices say that as the need for sensing capabilities becomes more prevalent in modern applications focused on machine health and other Internet of Things (IoT) solutions, so does the need for simpler interfaces with fewer I/Os and smaller device footprints.
T
he density of devices connected to a single microprocessor or FPGA is continuing to increase, while application
space - and as a result, the number of I/O pins – can become constrained. In an ideal world, all applications would have an ASIC providing a small integrated solution. However, ASIC development is time consuming, costly, and doesn’t provide the a result, more and more applications are using microprocessors or small form factor FPGAs to get product development done in a timely and cost-effective manner.
to-frequency converter that can provide accurate temperature results while only using a single GPIO pin. It will also demonstrate how using a voltage-to-frequency converter can be adapted to a variety of sensing applications.
MOTIVATION Some sensor measurements, such as temperature, humidity, and barometric pressure, are inherently dc and don’t change at rates fast enough - nor do they require resolutions accurate enough - to warrant an ADC and the design considerations that go along with it. Most ADCs require fast, accurate clock generation and timing, a stable voltage reference, a reference buffer with very low output impedance, and analogue front- end circuitry to properly signal condition a transducer output before it can be digitally quantised and used by the system monitoring it.
In the case of sensing ambient temperature,
a discrete application might use a thermistor in a Wheatstone bridge whose output would then and then fed into an ADC. This design is over- engineered and requires more space, power, and computation cycles necessary for an application where the measurement might only need to be made once every 15 seconds. What alternative measurement solution can be used to reduce the number of components
26 May 2023 Irish Manufacturing
The simple temperature-to-frequency converter
chain but still measure an analogue voltage? The solution is a voltage-to-frequency converter, controlled oscillator (VCO) mode can be used to measure an analogue voltage without an ADC.
ambient temperature sensor whose output voltage serves as the input to the LTC6990, resulting in a temperature-to-frequency converter signal chain.
HOW TO CONVERT A TEMPERATURE INPUT TO A FREQUENCY OUTPUT? Today, many modern electronic appliances require an on-board temperature monitoring system. The method of converting analogue signals to pulse-width modulated signals or digital signals is well documented. However, if the measurement solution would require an ADC, there are drawbacks associated with cost, accuracy, and speed. Typically, the more accurate solution. This circuit delivers a low cost, versatile, and
easily interfaceable solution whose accuracy can vary as determined by the needs of the temperature measurement system. temperature sensor by shorting its inputs to
In this circuit where a unipolar supply is used,
–VS = ground (0 V) and an offset voltage must order to bias the output voltage above ground, even if ambient temperatures are negative. The output voltage of the temperature
In VCO mode, the LTC6990 frequency
VCTRL to the LTC6990, Equation 1 can be substituted for VCTRL in Equation 2, and
From here, Tambient can now be solved
for. The voltage units cancel out, which yields
I HAVE A FREQUENCY OUTPUT – HOW IS THIS USEFUL? The beauty of a frequency output is that you can use a single GPIO pin to get a sensor measurement. If the synchronous counter circuit in Figure 2 is used, then the rising edge of a clock will always be observed at its
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