A-3-1-2 T4 Noise and Resolution [T-Series Datasheet]

ADC Noise and Resolution

T-series devices use an internal analog-to-digital converter (ADC) to convert analog voltage into digital representation.  The ADC reports an analog voltage in terms of ADC counts, where a single ADC count is the smallest change in voltage that will affect the reported ADC value.  A single ADC count is also known as the converter's least significant bit (LSB) voltage.  The ADC's resolution defines the number of discrete voltages represented over a given input range.  For example, a 16-bit ADC with a ±10 input range can report 65536 discrete voltages (2^16) and has an LSB voltage of 0.305 mV (20 V ÷ 2^16).

The stated resolution for an ADC is a theoretical, best-case value assuming no channel noise.  In reality, every ADC works in conjunction with external circuitry (amplifiers, filters, etc.) which all possess some level of inherent noise.  The noise of supporting hardware, in addition to noise of the ADC itself, all contribute to the channel resolution.  In general, the resolution for an ADC and supporting hardware will be less than what is stated for the ADC.  The combined resolution for an in-system ADC is termed effective resolution (aka ENOB).  Simply put, the effective resolution is the equivalent resolution where analog voltages less than the LSB voltage are no longer differentiable from the inherent hardware noise.

The effective resolution is closely related to the error free code resolution (EFCR) or flicker-free code resolution.  The EFCR represents the resolution on a channel immune to "bounce" or "flicker" from the inherent system noise.  The EFCR is not reported in this appendix. However, it may be closely approximated by the following equation:

EFCR = effective resolution - 2.7 bits

The T4 and the T7 offer user-selectable effective resolution through the resolution index parameter on any one AIN channel.  Internally, the ADC hardware uses modified sampling methods to reduce noise.  Valid resolution index values are:

• 0-5 for the T4
• 0-8 for the T7
• 0-12 for the T7-Pro

Increasing the resolution index value will improve the channel resolution, but doing so will usually extend channel sampling times.  See section 14.0 AIN for more information on the resolution index parameter and its use.

Noise and Resolution Data

High-Voltage Channels (AIN0-AIN3)

Low-Voltage Channels (Applicable FIO & EIO)

* Default command-response Resolution Index for the T4.