Using Analog Temperature Senso

文章摘要：Load CapacitanceEven when a capacitor isn't needed to compensate for excessive source resistance, it is common practice to place a capacitor across the ADC input pin, as shown in Figure 2. This helps to filter noise that has been picked up on the traces leading to the sensor. Common values are o

Using Analog Temperature Senso,标签：计算机控制技术,工厂电气控制技术,http://www.88dzw.com

Load Capacitance

Even when a capacitor isn't needed to compensate for excessive source resistance, it is common practice to place a capacitor across the ADC input pin, as shown in Figure 2. This helps to filter noise that has been picked up on the traces leading to the sensor. Common values are on the order of 1nF to 100nF. As discussed above, the sampling rate has to be slow enough to allow the capacitor to charge sufficiently between conversions.


Figure 2. A capacitor is often connected across the ADC input to filter noise or help reduce errors due to excessive sensor output resistance. Note that the sampling rate must allow several time constants between conversions.

Ideally, the temperature sensor is able to drive this capacitance without oscillating. (The MAX6605 is stable with load capacitance from 1nF to infinity.) Many other analog temperature sensors become unstable when loaded with a few hundred picofarads of load capacitance; these can sometimes be isolated from the capacitance by the addition of a series resistor with a value of a few hundred ohms. Note that adding a resistor between the sensor output and the ADC input can necessitate slowing the sampling rate further.

Reference Voltage, Temperature Range, and Resolution

The reference voltage used with the ADC affects the useful temperature range and the temperature measurement resolution. As an example, consider the MAX6605 temperature sensor driving a 10-bit ADC with a 2.5V reference. This ADC might be embedded within a microcontroller, or, in applications requiring higher performance, it might be a separate ADC such as the MAX1248.

With a 2.5V reference, each LSB will have a weight of

2.5V/1024LSBs = 2.44mV/LSB

The nominal slope of the MAX6605 temperature sensor's output is 11.9mV/°C, so each LSB will correspond to

(2.44mV/LSB)/(11.9mV/°C) = 0.205°C/LSB

The MAX6605 produces a nominal output voltage of 2.26V at 125°C; thus, its temperature and voltage ranges are compatible with the 2.5V reference.

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