This article is intended to address when and if a bypass capacitor is required for an analog temperature sensor. Because system cost is so critical in many of today’s industrial designs such as telecommunications, factory automation, medical and instrumentation, understanding why a temperature sensor is required is important.
When engineers use an analog temperature sensor in an industrial system design, they often find themselves asking a similar question: What size bypass capacitor do I need? The next logical question is: Do I need one at all? To quote the legendary analog expert Bob Pease, “It depends!”
Engineers do notice recommendations for bypass capacitors in analog temperature sensor datasheets. However, these really are intended to be general recommendations or guidelines. If a bypass capacitor is called out in the datasheet, it is usually there to preserve the stability of the device. Therefore, it should be considered at a minimum.
In addition, some newer analog temperature sensors do not recommend a bypass capacitor. What to do then? While it may be hard to believe, the bypass capacitor is not required.
Laboratory tests have proven that the output of analog temperature sensors that specifically do not recommend a bypass capacitor can handle transient loads. These types of loading are often found when driving a switch or an analog-to-digital converter (ADC) input.
In addition, further tests show that the output does not experience the typical oscillation issues caused by the power supply lead inductance. An example of this is having a 10’ wire run between the 0.1 uF bypass capacitor and the device’s VDD pin.
The two circuits shown in figure 1 are used to determine the stability performance of the temperature sensor to the lack of a bypass capacitor. Specifically, figure 1a (CL=10 pF scope probe) has a minimum output load capacitance. Conversely, the circuit of figure 1b shows the temperaturesensor with the maximum allowable load capacitance (CL=1 nF), without requiring a series resistor in series with its output. Series resistors often are used to buffer an amplifier output from load capacitance and to improve stability of the system. The drawback here is that the additional resistance can cause voltage-drop errors when current is drawn from the output.
For this case, the resistor is not used to prevent another variable from skewing the results. Also, notice that in both cases, the power supply pin of the temperature sensor is connected through 10’ of cable (banana jack) to the 5 V power supply source. A 0.1 uF bypass capacitor was placed directly at the power supply source. In order to test the stability of the temperature sensor, a square wave is coupled through a series capacitor (CIN) into the output of the temperature sensor. As shown in the oscilloscope photos of figure 2, the response of the temperature sensor to the input square wave is monitored. In both cases we chose CIN along with the amplitude of VIN. By doing this, an actual transient can be observed on the device’s output.
As can be seen in figure 2, a slight initial ringing occurs on the temperature sensor’s output. However, the output stage recovers nicely in either case — no sustained oscillations are observed. Therefore, it is acceptable for the temperature sensor to run without a bypass capacitor.
Bypassing depends on another factor: the system’s power supply noise. In general, it is quite common for analog circuits to require a bypass capacitor for this reason. This is because it lowers the supply noise. In most analog circuits, the power supply rejection ratio (PSRR) degrades as the supply noise frequency increases. As an example, the analog temperature sensor’s sensitivity to supply noise is shown in figure 3.
The graph shows the attenuation of the power supply input noise between 500 Hz and 1 MHz. You can see how it couples to the temperature sensor’s output with no-load and a 1 nF load capacitor. The higher load capacitor helps to filter the higher frequencies as it creates a low-pass filter with the source impedance of the temperature sensor’s output amplifier.
Based on figure 3, the power supply noise is attenuated somewhat. However, it may not be enough to meet a designer’s system requirements, especially in a system with around 2 to 200 kHz, where a -8 to -4 dB peak is observed.
For a thorough analysis, it is best to understand the source impedance of the supply, the noise level and the frequency spectrum. It also is important to understand the capacitor’s characteristics at the frequencies being filtered. Industrial system designers have two choices:
- They can allow the supply noise to be coupled to the output of the temperature sensor and then filter it.
- Or, they can choose to filter the noise at the source.
In either case, a simple RC filter is all it takes. Even so, some experimentation will be required to optimize the values for a specific system design.