ADC Accuracy Part 1: Is accuracy different from resolution?

RE: ADC Accuracy Part 1: Is accuracy different from resolution?

Abhilash Naragund — Sat, 07 Jan 2023 20:40:17 GMT

Hello Everyone, I would like to ask you about the selection of resolution of ADC. Do you have any calculation to arrive at n- bit ADC? If so could you share the methodology please? I am working on the measurement of strain/stress/force calculations using Strain gauge sensor project. My idea of connecting: Strain gauge diagonal bridge (I have -0.49mV for maximum loading condition as an output from the strain gauge bridge) > Instrumentation Amplifier > ADC of high resolution so that it can accept the amplified analog voltage(Example:ADS1256) > WiFi enabled Microcontroller. Could you please guide me in this regard further? Your feedback will really helps me in proceeding further with my project. If further clarifications, I am happy to contact you over the Teams meeting. My mail ID: abhilash.naragund@student.uni-siegen.de

RE: ADC Accuracy Part 1: Is accuracy different from resolution?

Jim Gavin — Mon, 24 Apr 2017 19:26:15 GMT

When specifying accuracy, should ADC accuracy be specified in bits, counts or volts (each with a tolerance)? It seems that specifying ADC accuracy in bits is actually specifying a resolution until you add a tolerance. For example, 16-bit ± 2 LSB seems to specify both resolution and accuracy, which both ultimately translate to voltages for a given application.

RE: ADC Accuracy Part 1: Is accuracy different from resolution?

Vinay Agarwal — Sat, 01 Nov 2014 00:58:19 GMT

In order to define the ADC resolution, the entire transfer function of the ADC has to be accounted for and not a limited section of the transfer function. If you assume a perfectly ideal ADC, then in your example a change in 10mV will not always change the digital output code by 1 count (change from 0.895V to 0.905V will not result in output change), whereas a change in 1V will always change output code by 1 count.

I agree to you that the precision of the ADC is dependent on the offset, gain and non-linearity errors which is explained in more details in the part-II of this series (e2e.ti.com/.../adc-accuracy-part-2-total-unadjusted-error-explained.aspx).

You are right about the definition of SINAD being in dB units. This is how the SINAD is specified in ADC datasheets, as I mentioned in the post above. So, if the SINAD (in dB) as specified in ADC datasheets is used in the formula, then ENOB can be calculated.

In sigma-deltas, the averaging can result in higher resolution but this post is based on Nyquist-sampled ADCs which is the basis for the formula for ENOB mentioned in equation 2. However, the basic conceptual difference between accuracy and resolution is applicable to both nyquist-sampled as well as oversampled ADCs.

RE: ADC Accuracy Part 1: Is accuracy different from resolution?

tilmann — Wed, 29 Oct 2014 15:42:25 GMT

I would like more precise definitions.

So the resolution is not " the smallest change in the value of an input signal that changes the value of the digital output by one count" but (for linear ADCs) the average value achieved by input signal range divided by number of steps inside this range. An example: on an ideal 4-bit ADC with a range of 16 V the resolution is 1 V, but you can see a change of the value of the digital output by changing the input only from 0.495 V to 0.505 V, a change of only10 mV.

The missing precision of the transfer function of an ADC is dependent on offset, slope, differential and integral nonlinearity, not on noise. You can use this feature by adding noise to the signal to upgrade the resolution of an (low-resolution) ADC!

The definition of ENOB does not contain any dimensions: ENOB is a dimensionless value, ok. If SINAD is treated as dimensionless then a signal of 1 V with a noise&distortion of 10 mV (what is the amplitude of a sum of statistical independent signals like noise and distortion?) would result in a SINAD of 100 resulting in wrong values. Usually SINAD is given in dB as well as the values 1.76 and 6.02, so it is a logarithmic value: for voltages 20 time the decadic logarithm of signal divided by "noise and distortion" (1 Bel is a factor of 10 for powers).

In a system the achieved resolution is well dependent from software. As example you can see on sigma-delta ADCs that the software algorithms (averaging, filtering) can influence or even define the resolution of the ADC.