MCP3461RT-ESLASHST datasheet(25/118 Pages) MICROCHIP

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DS20006404C-page 25

MCP3461/2/4R

2.1

Noise Specifications

Table 2-1 and Table 2-2 summarize the noise

performance of the MCP3461/2/4R devices. 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 Continu-

ous Conversion mode, with the differential input

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

The noise performance is also a function of the

measurement duration. For short duration measure-

ments (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 conse-

quence 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 may be more practi-

cal to choose one of these representations depending

on the desired application.

In Table 2-1, 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, the noise is expressed in Effective

Resolution (ER). The Effective Resolution 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

Effective Resolution 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

performed. 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 Effective Resolution 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 may 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% occur-

rence 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 Effective Resolution) when

the AZ_MUX setting is enabled. However, the output

data rate is significantly reduced (see Figure 5-5 and

Table 5-6).

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

is not significant for the noise performance, and there-

fore, the noise values can be extracted from the

Gain = 16x columns. Effective Resolution 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).

ERRMS

2VREF



GAIN RMS (Noise)



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







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

ERpk pk

–

2VREF



GAIN Peak-to-Peak Noise



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







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