Understanding FieldPoint Module Specifications


Specifications of FieldPoint I/O modules are listed at the back of the operating instructions. This document explains the terminology of the specifications and answers the most common questions about them.


FieldPoint Specifications Vocabulary

Bandwidth – The signal frequency range that can pass through the analog input circuitry.

Effective Resolution – The measurement resolution of the module, taking into account both the resolution of the ADC and other factors such as quantization errors and RMS noise.

Gain Error – The degree to which the gain varies from the ideal, specified in percent or ppm of reading. This type of error is multiplied with the measurement.

Gain Error Drift – The amount that the gain drifts for every °C that the environmental temperature differs from the nominal temperature. For example, for an analog voltage input module, if the typical specification is at 25 °C, the gain error drift is 20 ppm/°C, and the environmental temperature is 27 °C, the additional gain error of the reading is 40 ppm.

Input Current Limit – The maximum amount of current that flows into the module based on limiting circuitry. Regardless of the amount of current supplied, it will be limited to a specified amount to prevent damage to the input.

Input Delay Time – The maximum amount of time required for an input signal to pass through the optical isolation circuitry of the module.

Input Impedance – The amount of resistance and reactance between a channel input and channel common. For example, if a module has an input impedance of 100 ohms, the resistance between channel input and channel common is 100 ohms.

LSBleast significant bit. The smallest detectable change in input value. This term is commonly used when determining offset.

For example, if a module has a resolution of 16 bits, an offset error of 6 LSB, and a ±15 V range,

Note: If the module has overranging, include the overrange in offset calculations.

Maximum Conversion Rate – The speed at which the module can update all of its output channels.

Maximum Working Voltage – The maximum voltage differential that can exist between any terminal on the terminal base and the ground in the backplane of the module (the network module ground level).

Noise – A measure of the amount of unwanted signal added by the analog output circuitry.

Nonlinearity – The amount of variance over the measurement range of the module.

Normal-Mode Rejection (NMR) – The degree of rejection in decibels (dB) of an undesired normal-mode noise voltage (often 50 or 60 Hz) on an input channel. Normal-mode rejection can be achieved by filters that reject 50 or 60 Hz signals, or by averaging to remove a broader range of frequencies.

Offset Error – The amount of additional voltage or current that can be introduced by the analog input circuitry. Offset error is added to measurements and is typically noted in least significant bits (LSBs) or in standard units.

Offset Error Drift – The amount that the offset drifts for every °C that the environmental temperature differs from the nominal temperature. For example, for an analog voltage input module, if the typical specification is at 25 °C, the offset error drift is 1 mV/°C, and the environmental temperature is 27 °C, the additional offset error of the reading is 2 mV.

Overcurrent Protection – Circuitry that protects the module from damage caused by current that exceeds the input range.

Overvoltage Protection – Circuitry that protects the module from damage caused by voltage that exceeds the input range.

Overranging – The ability to measure values beyond the nominal input range. Modules with overranging have the ability to measure a little more than the stated nominal range. For example, the [c]FP-AI-110 module has a ±60 mV range that can measure ±65 mV. The additional 5 mV is the overrange. When doing error calculations, use the entire range (including overrange) in the calculations. (Therefore, use ±65 mV for [c]FP-AI-110 offset and gain calculations).

ppmparts per million. A measure of resolution. To convert a value in ppm to the corresponding unit, multiply the ppm by 10–6. For example,

2 ppm = 2 ´ 10 –6 = 0.000002

or 0.0002% of the whole. Therefore, 2 ppm of 5 V is 10 mV.

Relay Type – Relay types are single-pole single-throw (SPST), single-pole double-throw (SPDT), latching, and nonlatching. An SPST relay simply closes or opens a circuit. An SPDT relay switches a common (COM) terminal between a normally open (NO) and a normally closed (NC) terminal. Latching relays maintain their latest state even when powered down. Nonlatching relays return to their normally closed state when powered down.

Resolution – The number of bits that the analog-to-digital converter (ADC) uses to represent an analog signal. The higher the resolution, the greater the number of divisions the range is broken into, and therefore the smaller the detectable voltage change for a given input range and gain. Common resolutions for FieldPoint are 12 bits, giving 212, or 4096, divisions for the specified range, and 16 bits, giving 216, or 65,536, divisions.

Slew Rate – The maximum rate of change per unit of time for an analog output channel.

Transient Overvoltage – The highest voltage transient that can be input to the module for up to 60 seconds without affecting other modules on the bank.

Type of ADC – Different applications require different types of analog-to-digital converters (ADCs) for optimal performance. For example, an AC signal is usually best measured with a delta-sigma modulating ADC. Common types of ADCs include successive approximation, flash, half-flash, integrating, and delta-sigma modulating.

Update Rate – The speed at which the module reads all of its channels and updates its registers.

Shock and Vibration Specifications

What is the difference between shock and vibration?
A shock is a relatively infrequent, abrupt jar or sudden jolt that an electrical device may experience. Vibration is a more constant level of movement or a periodic sinusoidal movement. Vibration is typically a more practical measure of the tolerance of an electrical device for movement.

What do the shock and vibration specifications mean?
The shock specifications comply with the International Electrotechnical Commission (IEC) standard 60068-2-27, which applies to components, equipment, and other electrotechnical products that, during transportation or in use, may be subjected to relatively infrequent nonrepetitive shocks. The specifications show the amount of gravitational force that the FieldPoint module can withstand for a number of consecutive shocks at a given frequency for a half sine wave.

The vibration specifications comply with IEC standards 60068-2-6 for sinusoidal vibration and 60068-2-34 for random vibration. These standards specify the ability of components, equipment, and other articles to withstand specified severities of sinusoidal and random vibration. Sinusoidal vibration is smoother and more periodic, and random vibration is more unexpected and unpredictable. Random vibration is harsher and more analogous to real-world situations. Each vibration specification ranges from 10 to 500 Hz.

Note: Compact FieldPoint modules were tested for shock and vibration in each of the six orientations (lying on all six sides).

Common Questions about I/O Module Specifications

What is the difference between typical and maximum?
Typical specifications describe the expected performance when the module is first put into use. Typical specifications can also be used after a FieldPoint module has been properly calibrated. Maximum specifications provide a worse-case level of performance after the module has been used over time and subjected to normal operational and environmental conditions.

How are typical and maximum specifications calculated?
Typical and maximum specifications are calculated based on the typical and maximum specifications of the components used to build the module and the role of each of the components in the design.

What is the difference between nominal and overranging?
The nominal range is the preferable range for a given input or output, selected for compatibility with common module applications. Overranging provides the ability to measure or produce a signal beyond the nominal range. For example, overranging is beneficial if an external device states that it outputs 4-20 mA, yet at maximum level the module reads 20.08 mA. Overranging gives you an extended range to account for situations where the maximum and minimum fall outside the specified range. Also, with the overranging feature, a noisy signal near the nominal full scale does not create rectification errors.

How do I calculate gain error?
Multiply the gain error percentage by the value of the signal that is being measured or produced in order to determine the gain error. For example, consider a [c]FP-AI-102 measuring a 50 V unipolar signal. If the typical gain error is 0.1% and the maximum gain error is 0.2%,

Typical = ±0.001 * 50 V = ±0.05 V
Maximum = ±0.002 * 50 V = ±0.1 V

How do I calculate the offset error?
Add the amount of offset error to the value of the signal that is being measured or produced in order to determine typical and maximum values that may be seen. For example, for a [c]FP-AI-102 is measuring a 50 V unipolar signal, the typical offset error is ±0.05 V for the 0–60 V range, and the maximum offset error is ±0.15 V.

How do I calculate the maximum error for a reading?
The maximum error is determined by combining the maximum gain error and the maximum offset error. For example, based on the information above, the maximum error for a 50 V measurement with the [c]FP-AI-102 is

Maximum = |±0.1 V| + |±0.15 V| = 0.25 V

Why don’t the specifications include the total error?
Total error is not included because the specification depends entirely on the value of the measured signal. The input range you select varies depending on the expected maximum signal value. After you select the range, you can combine the appropriate gain error and offset error to determine the total error for your measurements.

How do I calculate the module’s accuracy?
Accuracy of FieldPoint modules can be calculated by determining the maximum difference between the measured signal and the actual value of the signal.

How do I calculate the module’s precision?
Precision is determined by the effective resolution. Based on the effective resolution that the module has for the specified range, the precision of measurements can be determined as follows:

What are the filter settings for and do they affect the accuracy of my measurements?
The filter settings enable you to filter out unwanted frequencies or eliminate noise from input signals. This can improve measurements because it ensures that only the input signal is amplified or digitized. The three available filter settings for applicable modules are 50 Hz, 60 Hz, and 500 Hz. The 50 and 60 Hz filter settings use notch filters, so they eliminate exactly 50 or 60 Hz. With the 50 and 60 Hz filter settings, an input channel can maintain 16 bits of resolution throughout the measurement range. For the [c]FP-AI-110 and [c]FP-AI-111, resolution can drop to 12 bits with the 500 Hz filter.

What is the difference in sinking and sourcing inputs and outputs?
A typical circuit contains a voltage source, a load, and a ground. A sinking digital I/O (input/output) provides a ground for the circuit. A sourcing digital I/O provides a voltage source for the circuit. Please refer to the following KnowledgeBase for more detailed information.