TESTING & MEASUREMENT Analog Devices
Figure 2 – A 4-bit synchronous counter with LTC6990 output as its clock input
The example Verilog code shows a function that can be used to count the number of periods by using a single GPIO input on an FPGA. The larger the acquisition period, the more accurate the measurement will be. In the case of the following code, a 16-bit counter is used to provide additional resolution. This also assumes the logic that controls the measurement acquisition time is performed at a higher level in the architecture.
Figure 3 – Example Verilog code CLK_IN input.
If the FOUT of the LTC6990 is used as the input clock, then the counter will increment every time a rising edge on FOUT is detected, creating a period counter. If the time interval between each measurement is constant, then the number of periods within a given time interval can be counted and frequency can be
or a lookup table.
By dividing the acquisition time TAcquisitions
by the number of periods counted, we get the period of FOUT. Taking the inverse of this relationship results in Equation 6.
CONCLUSION In this application, a new type of temperature- to-frequency converter is discussed. It provides an accurate, low cost method to measure temperatures. If the temperatures exceed the industrial range of –40°C to +125°C, a thermocouple can be installed at the inputs of the sensor. As a conclusion, the following plot shows the error of the measurement system. It demonstrates the linear relationship between the ambient temperature and the output frequency as well as the accuracy of the system. temperature resolution result, for applications where roughly ±2°C error is acceptable, this provides a cheap and simple interface for measuring temperature. Additionally, the concept of using a voltage-to-frequency converter can be adapted to measure other types of transducer outputs without the need for an ADC.
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Transfer function of the temperature-to-frequency converter
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Figure 5 – Temperature error Irish Manufacturing May 2023 27
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