Variability in device performance is an unfortunate aspect of engineering. Even within a single component many factors can cause distinguishable variability to exist. Production methods for electrical components aim to reduce error and variability, but some sources of variation are impossible to completely alleviate through production practices. Amongst the biggest culprits of inevitable variation is temperature. In integrated circuits, this can cause a circuit to differ in characteristics such as switching speeds due to the change in electron mobility. Temperature can change the expected behavior of discrete components and sensors, which can cause a microcontroller or IC to misread (or not read at all) a crucial signal. For these reasons many systems can benefit from adding a temperature monitoring module to the design.
A temperature correction/monitoring module must, in most cases, be small and low cost. Sensors have various sizes, and a universal error correction module should not be big enough to cause a considerable change in the system’s size. In the Internet of Things (IoT) domain this is particularly important, since most IoT devices are already quite small and cannot spare much extra space. Similarly, an expensive solution would be infeasible for most projects and would likely warrant simply buying a cheaper competing module. GreenPAK devices are an optimal solution, since they are small, low cost, and consume very little quiescent current, which prevents the temperature correction module from substantially affecting battery life.
This application note explores the use of a GreenPAK IC in conjunction with a digital potentiometer, passive components, and an op-amp to measure the temperature change on a system. The GreenPAK IC will monitor the change in voltage compared to a calibration point and will adjust the digital potentiometer to counteract the change in voltage. The number of pulses (up or down) will be stored in the GreenPAK and will be available for an I2C signal to read. Several key design choices will be laid out and further applications using the methodology will be hypothesized.

Below we described steps needed to understand how the temperature compensation has been programmed. However, if you just want to get the result of programming, download GreenPAK software to view the already completed GreenPAK Design File. Plug the GreenPAK Development Kit to your computer and hit the program to design the device.


Changes in temperature can be measured using a voltage divider between a typical resistor and a resistor that is highly sensitive to temperature, aka a thermistor. If this resistor has a Negative Temperature Coefficient (NTC) the resistivity will lower with temperature, causing VOUT in Figure 1 to be a lower voltage.
Figure 1 also shows a digital potentiometer in series with the NTC thermistor. This will be the balancing potentiometer required to satisfy a constant VOUT across temperature. The output voltage equation is shown below: 

Therefore, if the NTC thermistor resistance falls, the digital potentiometer resistivity must increase and vice versa. An important note is that the resistance of a thermistor is nonlinear across temperature. Figure 2 shows typical NTC behavior.

Thus, if an application would like to monitor a large temperature range the system should be designed to account for this nonlinearity, either through additional circuitry or through software. 

System Design


The system design is shown in Figure 3:

n most applications R1 should be very large. This is beneficial since it increases the linearity of voltage change from the digital potentiometer and thermistor. If R1 is very small, then the temperature variation of the NTC voltage divider will be very small:

Alternatively, if R1 is chosen to be very large, temperature change will greatly dictate the relative change in voltage:

However, a caveat to this is that, even if the voltage variation...

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