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2.7.3 - Typical Analog Input Connections

Typical Analog Input Connections Overview

A common question is “can this sensor/signal be measured with the UE9”. Unless the signal has a voltage (referred to UE9 ground) beyond the limits in Appendix A, it can be connected without damaging the UE9, but more thought is required to determine what is necessary to make useful measurements with the UE9 or any measurement device.

Voltage (versus ground): The analog inputs on the UE9 measure a voltage with respect to UE9 ground. When measuring parameters other than voltage, or voltages too big or too small for the UE9, some sort of sensor or transducer is required to produce the proper voltage signal. Examples are a temperature sensor, amplifier, resistive voltage divider, or perhaps a combination of such things.

Impedance: When connecting the UE9, or any measuring device, to a signal source, it must be considered what impact the measuring device will have on the signal. The main consideration is whether the currents going into or out of the UE9 analog input will cause noticeable voltage errors due to the impedance of the source. See Appendix A for the recommended maximum source impedance.

Resolution (and Accuracy): Based on the selected input range and resolution of the UE9, the resolution can be determined in terms of voltage or engineering units. For example, assume some temperature sensor provides a 0-10 mV signal, corresponding to 0-100 degrees C. Samples are then acquired with the UE9 using the 0-5 volt input range and 16-bit resolution, resulting in a voltage resolution of about 5/65536 = 76 µV. That means there will be about 131 discrete steps across the 10 mV span of the signal, and the overall resolution is 0.76 degrees C. If this experiment required a resolution of 0.1 degrees C, this configuration would not be sufficient. Accuracy will also need to be considered. Appendix A places some boundaries on expected accuracy, but an in-system calibration can generally be done to provide absolute accuracy down to the INL limits of the UE9.

Speed: How fast does the signal need to be sampled? For instance, if the signal is a waveform, what information is needed: peak, average, RMS, shape, frequency, … ? Answers to these questions will help decide how many points are needed per waveform cycle, and thus what sampling rate is required. In the case of multiple channels, the scan rate is also considered. See Sections 3.1 and 3.2.