Campbell Scientific CR3000 CR3000 Micrologger - Page 287

Section 8. Operation mV and -1090 mV. The CR3000 indicates a measurement over-range by returning a NAN (not a number) for the measurement. 8.1.2.5.3 Common Mode Null / Open Input Detect For floating differential sensors, such as thermocouples, nulling of any residual common-mode voltage prior to measurement pulls the H and L input amplifier (IA) inputs within the ±5-V Input Limits. Appending a C to the range code (mV20C, for example) enables the nulling of the common-mode voltage prior to a differential measurement. Another useful feature for both SE and DIFF measurements is the detection of open inputs due to a broken or disconnected sensor wire, to prevent otherwise undetectable measurement errors. Range codes ending with C also enable open detect for all input ranges (see table Analog Voltage Input Ranges with CMN / OID (p. 286) ). On the ±20 mV, ±50 mV, and ±200 mV input ranges, the C range code option results in a 50-µs internal connection of the H and L inputs of the IA to 300 mV and ground, respectively, while also connected to the sensor to be measured. The resulting internal common-mode voltage is ±150 mV, which is well within the ±5V Input Limits. Upon disconnecting the internal 300-mV and ground connections, the associated input is allowed to settle to the sensor voltage and the voltage measurement is made. The ±1000-mV and ±5000-mV ranges are treated in the same way, but with the applied excitation being approximately 1200 mV and 5200 mV respectively. If the associated input is open (floating), the input voltages will remain near 300 mV and ground, resulting in an over-range output (NAN) on the ±20 mV, ±50 mV, and ±200 mV input ranges. Similarly, an open input will result in a voltage of approximately 1200 mV for the ±1000 mV range and approximately 5200 mV for the ±5000 mV range, with a resulting NAN for those ranges. If the associated sensor is connected and functioning properly, a valid measured voltage will result after the input settling associated with open input detect. 8.1.2.6 Offset Voltage Compensation Analog measurement circuitry in the CR3000 may introduce a small offset voltage to a measurement. Depending on the magnitude of the signal, this offset voltage may introduce significant error. For example, an offset of 3 μV on a 1000mV signal introduces an error of only 0.00012%; however, the same offset on a 0.25-mV signal introduces an error of 1.2%. The primary source of offset voltage is the Seebeck effect, which arises at the junctions of differing metals in electronic circuits. Secondary sources of offset voltages are return currents incident to powering external devices through the CR3000. Return currents create voltage drop at the ground terminals that may be used as signal references. CR3000 measurement instructions incorporate techniques to cancel these unwanted offsets. The table Analog Measurement Instructions and Offset Voltage Compensation Options (p. 288) lists available options. 287