.png)
Analogue Input Bits Are Not Accuracy: Write an End-to-End Measurement Requirement
A procurement specification that asks only for a 16-bit input can still produce a measurement unsuitable for the process. Define the smallest useful change, permitted error and operating conditions separately, then evaluate the transmitter, conditioning, input module and software scaling as one chain.
Original conceptual illustration; not a specific product, live software screen or customer installation.
Key takeaways
Bit count describes discretisation, not total accuracy.
Use engineering units throughout the assessment.
Choose improvements from the limiting error source.
1. State the decision the measurement supports
Begin with the operating range and what an operator must decide. A tank inventory trend, a quality record and a control loop can need different sampling and error performance even when they measure the same quantity. Record the decision before comparing input-module bit counts.
Use engineering units for the requirement. Specify a permitted error over the relevant temperature range and the smallest change worth displaying or acting on. Include how quickly that change must be observable. Avoid deriving an acceptance tolerance from the number of decimal places on a dashboard.
2. Calculate the ideal code width
NI explains ideal ADC code width as input range divided by two raised to the number of bits. For a hypothetical ideal 12-bit conversion over 0–10 V, 10/4096 is about 2.44 mV per code. Over a linear 0–100 unit process span, that corresponds to about 0.0244 units per code.
Those figures describe an ideal discretisation step. They are not a complete error allowance or an assertion about a product sold by ProSense. Check the actual usable codes, input range and conversion mode in the exact equipment documentation before applying the calculation.
3. Build the error and noise picture
Input noise, gain and offset behaviour, conditioning and operating conditions can matter more than the ideal code width. A transmitter can introduce an error before the signal reaches the ADC. The scaling and reporting path can then introduce rounding or unit mistakes after conversion.
Keep each contribution on the same engineering-unit basis. Distinguish specifications with guaranteed limits from typical behaviour, and document the conditions attached to them. Do not combine unrelated percentage-of-span and percentage-of-reading figures by copying their numeric values into one column.
Original explanatory diagram. Hypothetical values and proposed checks are described in the text; this is not a product specification or field result.
4. Design a useful acceptance test
In an approved test arrangement, compare known inputs and recorded outputs at representative points across the working range. Record the reference equipment, conditions, settling interval and observations. A steady input helps expose noise; a sequence of changing inputs helps expose scaling and reporting problems.
For example, if the project permits an error of 0.5 process units, passing a display-step calculation of 0.0244 units does not demonstrate compliance. The actual chain must still satisfy the agreed acceptance rule, including the reference uncertainty and any required guard band.
Define the process decision and permitted error.
Calculate ideal code width for the actual range.
List the chain contributions and their conditions.
Agree a reference-based acceptance method.
5. Choose improvements from evidence
More bits can help when quantisation is the limiting issue. They cannot correct a poorly matched range, a noisy signal path or a transmitter error. Conversely, excessive display precision can make normal noise look like useful process movement. Present a justified number of digits and retain adequate raw evidence.
Ask suppliers for the complete input performance specification and the configuration that achieves it. Electrical installation and test access must be planned by competent people for the actual circuit. Send ProSense the process span, acceptable error, response requirement and existing sensor details to support an application-based selection.
Related guide and project resources
Technical references
Need help with your application?
Send the relevant point list, equipment documentation and a sanitised example of the behaviour you need to verify. Confirm the available functions and integration requirements for the exact equipment and deployment.
