When accurate values are required in an application, digital sensors are superior to analog instruments. Particularly in automotive engine test benches, accurate pressure measurements are a must.
We take a look at the basic design of both analog and digital sensor measuring chains, and also typical error influences. The differences in the wiring and signal evaluation lead to error model calculations that are clearly different from each other. Thus in our example, with lower investment costs, a digital measuring chain can achieve an overall accuracy of 0.1 per cent while also being more resistant to external interference by design.
Whenever accurate measured values are required in an application, the advantages of digital sensors, compared to analog instruments, become obvious. When talking about digital sensors, we mean sensors with an integrated analog-to-digital conversion, which uses a digital interface to transmit the measured value (e.g. CANopen or USB) with the pressure value transmitted as a numeric value. An analog sensor, however, has no built-in analog-to-digital conversion and transmits its signal as an analog current or voltage signal, e.g. 4 V mA to 20mA, or 0 V to 10 V.
Therefore, in applications where high accuracy is required, for example in test stands for propulsion technology, it is advisable to use digital sensors. This avoids further sources of error that exist in analog instruments, over and above the signal conditioning, as a result of the analog signal transmission. Figure 1 shows the schematic design of a typical analog pressure sensor. By the deformation of a diaphragm under a pressure load, a resistance change occurs in the resistance bridge fixed to the diaphragm.
This change in resistance is converted into an electrical signal, amplified and transformed into a standard signal. The compensation of the sensor-specific errors (zero error, span error, non-linearity) is also made through analog circuit technology, for example, resistance networks. With digital sensors, how- ever, the electrical signal of the resistance bridge is directly converted into a digital value and the subsequent compensation is instead made mathematically in a microprocessor (see Figure 1).
Here, depending on the required accuracy, non-linear errors of any order can be compensated and accuracies up to 0.05 per cent can be achieved at low costs. By using a μC, an active temperature compensation is also possible, eliminating any temperature error within a defined temperature range.
This compensated digital signal now exists in the pressure transmitter as a numerical value and then can be output via any digital protocol (e.g. USB, CANopen, etc.). During the onward transmission of this digital pressure signal, it is now immune to interferences which might cause a further deterioration in the accuracy.
If we compare the complete analog measuring chain with its digital counterpart, the advantages of digital sensors become even clearer. Figure 2 shows the schematic structure and at which point external interferences, such as EMC or temperature, introduce additional errors.