MCP3464T-ESLASHST datasheet(20/108 Pages) MICROCHIP

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MCP3461/2/4

DS20006180D-page 20

 2019-2021 Microchip Technology Inc.

2.1

Noise Specifications

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

performance of the MCP3461/2/4 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 is measured at room temperature (TA = +25°C)

and increases over temperature. For high OSR settings

(> 512), the thermal noise is largely dominant and

increases proportionally to the square root of the absolute

temperature. The performance on the following tables

has been measured with AVDD = DVDD = VREF = 3.3V

and 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 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

consequence of the noise distribution being Gaussian

by nature (see Figure 2-3 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

practical 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 under parentheses. The peak-to-peak

noise is the difference between the maximum and min-

imum 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

averaging of the performance over a large distribution

of parts over multiple lots. They give the typical expec-

tation 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 quantiza-

tion 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

1LSb.

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%

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 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. Therefore,

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

columns.

Effective

Resolution

performance

degraded by 1 bit for GAIN = 32x and 2 bits for GAIN =

64x compared to the GAIN = 16x performance.

ERRMS

2VREF



GAIN RMS Noise





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





In 2



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

ERpk pk

–

2VREF



GAIN Peak-to-Peak Noise



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





In 2



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

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