Minimizing offset voltages

Voltage offset can be the source of significant error. For example, an offset of 3 μV on a 2500 mV signal causes an error of only 0.00012%, but the same offset on a 0.25 mV signal causes an error of 1.2%. Measurement offset voltages are unavoidable, but can be minimized. Offset voltages originate with:

Ground currents. See Minimizing ground potential differences.
Seebeck effect Induces microvolt level thermal electromotive forces (EMF) across junctions of dissimilar metals in the presence of temperature gradients. This is the principle behind thermocouple temperature measurement. It also causes small, correctable voltage offsets in data logger measurement circuitry.
Residual voltage from a previous measurement

Remedies include:

Connecting power grounds to power ground terminals (G).
Automatic offset compensation for single-ended measurements when MeasOff = False.
Using MeasOff = True for better offset compensation.
Programming longer settling times.

Single-ended measurements are susceptible to voltage drop at the ground terminal caused by return currents from another device that is powered from the data logger wiring panel, such as another manufacturer's communications modem, or a sensor that requires a lot of power. Currents greater than 5 mA are usually undesirable. The error can be avoided by routing power grounds from these other devices to a power ground G terminal, rather than using a signal ground ( ) terminal. Ground currents can be caused by the excitation of resistive-bridge sensors, but these do not usually cause offset error. These currents typically only flow when a voltage excitation is applied. Return currents associated with voltage excitation cannot influence other single-ended measurements because the excitation is usually turned off before the data logger moves to the next measurement. However, if the CRBasic program is written in such a way that an excitation terminal is enabled during an unrelated measurement of a small voltage, an offset error may occur.

The Seebeck effect results in small thermally induced voltages across junctions of dissimilar metals as are common in electronic devices. Differential measurements are more immune to these than are single-ended measurements because of passive voltage cancellation occurring between matched high and low pairs such as 1H/1L. So, use differential measurements when measuring critical low-level voltages, especially those below 200 mV, such as are output from pyranometers and thermocouples.

When analog voltage signals are measured in series by a single measurement instruction, such as occurs when VoltSE() is programmed with Reps = 2 or more, measurements on subsequent terminals may be affected by an offset, the magnitude of which is a function of the voltage from the previous measurement. While this offset is usually small and negligible when measuring large signals, significant error, or NAN Not a number. A data word indicating a measurement or processing error. Voltage overrange, SDI-12 sensor error, and undefined mathematical results can produce NAN., can occur when measuring very small signals. This effect is caused by dielectric absorption of the integrator capacitor and cannot be overcome by circuit design. Remedies include the following:

Programing longer settling times.
Using an individual instruction for each input terminal, the effect of which is to reset the integrator circuit prior to filtering.
Avoiding preceding a very small voltage input with a very large voltage input in a measurement sequence if a single measurement instruction must be used.