After characterizing both the signal source grounding types and the instrument configurations, we will now discuss which combinations of signal sources and instrument configurations can yield the most accurate results.
Measuring Grounded Signal Sources
A grounded signal source is most accurately measured with a differential or NRSE instrument configuration because an additional ground is not introduced into the entire system. An additional ground added to the system can result in ground loops, which are common sources of noise in measurement applications.
Ground loops occur when two connected terminals in a circuit are at different ground potentials, causing current to flow between the two points. The ground of the signal source can be several volts above or below the ground of the instrument. This additional voltage can cause error in the measurement itself and the flowing current can also induce voltages on nearby wires causing additional measurement error. These errors can appear as scalar or periodic signals added to the measured signal. For example, if a ground loop is formed with a 60 Hz AC power line, the standard power line frequency in the United States and some other countries, the unwanted 60 Hz AC signal can appear as a periodic voltage error in the measurement.
To calculate the measured voltage, V_m, use Equation 4 below:
Equation 4: Measured Voltage With a Ground Loop Present
Using Equation 4 above mathematically gives you the measured voltage when a ground loop is present. If you continue to use the 60 Hz power line example, ΔV_g is a value that changes with time instead of a scalar offset. Therefore, the measured signal looks periodic instead of like a simple offset error for the measured voltage.
Figure 6 shows what a system with a ground loop looks like in schematic form. If you are measuring the voltage source V_s with an instrument using an RSE configuration, you can simplify the schematic on the left of the equation with the schematic on the right of the equation in Figure 6, which agrees with the calculations in Equation 4.
Figure 6: A grounded signal source measured with a ground-referenced system introduces ground loops and measurement error.
To avoid ground loops as shown in Figure 6, ensure only one ground reference exists in the signal source and the measurement system by using a differential or NRSE instrument configuration or by using isolated measurement hardware, which is discussed in the Isolation Types and Considerations when Taking a Measurement white paper of the Instrument Fundamentals Series.
Measuring Floating Signal Sources
You can measure floating signal sources with any of the measurement configurations discussed: differential, GRSE/RSE, or NRSE. Note that when using differential or NRSE measurement configurations with a floating source, you must include bias resistors from each lead, positive (+) and negative (-), to the instrument ground (see Figure 7).
Figure 7: When measuring a floating signal source with a differential or NRSE instrument configuration, bias resistors are needed.
Bias resistors provide a DC path from the instrument amplifier inputs to the instrument amplifier ground. Bias resistors should have a high enough resistance to not load the signal source and to allow the signal source to float with respect to the instrument reference. However, the bias resistors should be small enough to keep the voltage within the range of the instrument. This typically results in bias resistors with a range of 10 kΩ to 100 kΩ to satisfy the conditions. You should always double-check the specifications guide of your device to ensure you use a bias resistor value that is within the suitable range.
If bias resistors are not used in a differential or NRSE configuration when measuring floating signal sources, the measured signals can be unstable or at positive or negative full-scale range of the instrument.
When a GRSE/RSE configuration is used to measure a floating signal source, bias resistors are not necessary. To get the best measurement results when using single-ended instrument configurations, the following is recommended:
- The input signals are equal to or greater than 1 V.
- Signal cabling is relatively short and travels through a noise-free environment (or is properly shielded).
- All input signals can share a common, stable, and known reference signal–generally a point in the system where the voltage is at 0 V.
For a summary of the recommended combinations of signal sources and instrument configurations, refer to Figure 8. Grounding and Measurements
Figure 8: Instrument Configuration Versus Signal Source Type Summary.