MCP3460R datasheet(21/103 Pages) MICROCHIP

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DS20006975A page 21

MCP3460R

2.1

Noise Specifications

Table 2-1, Table 2-2, Table 2-3 and Table 2-4

summarize the noise performance of the MCP3460R

device. The noise performance is an analog gain

function of the ADC (digital gain does not change the

noise performance significantly) and the OSR, chosen

through the user interface. With a higher gain, the input

referred noise is reduced. With a higher OSR setting,

the noise is also reduced as the oversampling

diminishes both thermal noise and quantization noise

induced by the Delta-Sigma modulator loop.

The noise value generally increases when temperature

is higher as thermal noise is dominant for all OSR

larger than 32. For high OSR settings (> 512), the

thermal noise is largely dominant and increases

proportionally to the square root of the absolute

temperature. The performance in the following tables

has been measured with the device placed in

Continuous Conversion mode, with the differential

input voltage equal to VIN = 0V, default conditions for

the register map and MCLK = 4.9152 MHz.

The noise performance is also a function of the

measurement

duration.

For

short

duration

measurements (low number of consecutive samples),

the peak-to-peak noise is usually reduced because the

crest factor (ratio between the RMS noise and

peak-to-peak noise) is reduced. This is only a

consequence of the noise distribution being Gaussian

by nature (see Figure 2-5 for noise histogram example

and fitting with an ideal Gaussian distribution). The

noise specifications have been measured with a

sample size of 16384 samples for low OSR values and

have been capped to approximately 80 seconds for the

16384 samples leading to a larger duration. The noise

specifications are expressed in two different values,

which lead to the same quantity. It is more practical to

choose one of these representations depending on the

desired application.

In Table 2-1 and Table 2-3, the RMS (Root Mean

Square) noise is the variance of the ADC output code,

expressed in µVRMS and input referred with

Equation 5-5. The peak-to-peak noise values are in

parentheses. The peak-to-peak noise is the difference

between the maximum and minimum code observed

during the complete time of the measurement (see

Equation 5-5).

In Table 2-2 and Table 2-4, the noise is expressed in

ENOB (Effective Number of Bits). The ENOB is a ratio

of the full-scale range of the ADC (that depends on

VREF and gain) and the noise performance of the

device. The ENOB can be determined from the RMS or

peak-to-peak noise with the following equations.

EQUATION 2-1:

EQUATION 2-2:

Due to the nature of the noise, the performance

detailed in the noise tables can vary significantly from

one measurement to another. They present an averag-

ing of the performance over a large distribution of parts

over multiple lots. They give the typical expectation of

the noise performance, but performance can be better

or worse if a limited number of measurements is per-

formed. For large gain and OSR combinations, if the

noise performance is comparable to the quantization

step (1 LSb), the performance is limited to 0.5 LSb for

the RMS noise and 1 LSb for the peak-to-peak noise

(same limits for ENOB values).

These figures correspond to the resolution limit of the

device as peak-to-peak noise cannot be better than

1 LSb. Similarly, if the intrinsic RMS noise of the device

is much smaller than 0.5 LSb, it can lead to histogram

with either one or two bins, depending on the relative

position of the input voltage versus the possible

quantized outputs of the ADC. If the position is exactly

in between two quantization steps, the histogram of

output noise will have two bins with exactly 50%

occurrence on each. This case gives an RMS noise of

a 0.5 LSb value, which is therefore used as a cap of the

performance for the sake of clarity and a better

representation on the noise tables.

The noise specifications are improved by a ratio of

approximately √2 (or 0.5-bit ENOB) when the AZ_MUX

setting is enabled. However, the output data rate is

significantly reduced (see Figure 5-5 and Table ).

The digital gain added for Gain = 32x and 64x settings

is not significant for the noise performance, therefore

the noise values can be extracted from the Gain = 16x

columns. ENOB performance is degraded by one bit for

Gain = 32x and two bits for Gain = 64x, compared to

Gain = 16x performance.

Note:

All Output Noise performance-related

tables and figures are with reference to

the input (i.e., Input Referred).

ENOBRMS

2VREF



GAIN RMS (Noise)



-----------------------------------------------------







----------------------------------------------------------------

ENOBpk pk

–

2VREF



GAIN Peak-to-Peak Noise



----------------------------------------------------------------------







---------------------------------------------------------------------------------

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