MCP3911 datasheet(26/72 Pages) MICROCHIP

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MCP3911

DS22286A-page 26

4.11

Total Harmonic Distortion (THD)

The total harmonic distortion is the ratio of the output

harmonics power to the fundamental signal power for a

sinewave input and is defined by the following

equation.

EQUATION 4-6:

The THD calculation includes the first 35 harmonics for

the MCP3911 specifications. The THD is usually only

measured with respect to the 10 first harmonics. THD

is sometimes expressed in %. For converting the THD

in %, here is the formula:

EQUATION 4-7:

This specification depends mainly on the DITHER set-

ting.

4.12

Spurious-Free Dynamic Range

(SFDR)

The ratio between the output power of the fundamental

and the highest spur in the frequency spectrum. The

spur frequency is not necessarily a harmonic of the

fundamental even though it is usually the case. This

figure represents the dynamic range of the ADC when

a full-scale signal is used at the input. This specification

depends mainly on the DITHER setting.

EQUATION 4-8:

4.13

MCP3911 Delta-Sigma

Architecture

The MCP3911 incorporates two Delta-Sigma ADCs

with a multi-bit architecture. A Delta-Sigma ADC is an

oversampling converter that incorporates a built-in

modulator which is digitizing the quantity of charge

integrated by the modulator loop (see Figure 5-1). The

quantizer

the

block

that

performing

the

analog-to-digital conversion. The quantizer is typically

1-bit, or a simple comparator which helps to maintain

the linearity performance of the ADC (the DAC

structure is, in this case, inherently linear).

Multi-bit quantizers help to lower the quantization error

(the error fed back in the loop can be very large with

1-bit quantizers) without changing the order of the

modulator or the OSR which leads to better SNR

figures. However, typically, the linearity of such

architectures is more difficult to achieve since the DAC

is no more simple to realize and its linearity limits the

THD of such ADCs.

The MCP3911’s 5-level quantizer is a flash ADC

composed of four comparators arranged with equally

spaced thresholds and a thermometer coding. The

MCP3911 also includes proprietary 5-level DAC

architecture that is inherently linear for improved THD

figures.

4.14

Idle Tones

A Delta-Sigma converter is an integrating converter. It

also has a finite quantization step (LSB) which can be

detected by its quantizer. A DC input voltage that is

below the quantization step should only provide an all

zeros result since the input is not large enough to be

detected. As an integrating device, any Delta-Sigma

will show in this case idle tones. This means that the

output will have spurs in the frequency content that are

depending on the ratio between quantization step

voltage and the input voltage. These spurs are the

result of the integrated sub-quantization step inputs

that will eventually cross the quantization steps after a

long enough integration. This will induce an AC

frequency at the output of the ADC and can be shown

in the ADC output spectrum.

These idle tones are residues that are inherent to the

quantization process and the fact that the converter is

integrating at all times without being reset. They are

residues of the finite resolution of the conversion

process. They are very difficult to attenuate and they

are heavily signal dependent. They can degrade both

SFDR and THD of the converter, even for DC inputs.

They can be localized in the baseband of the converter

and thus difficult to filter from the actual input signal.

For power metering applications, idle tones can be very

disturbing because energy can be detected even at the

50 or 60 Hz frequency, depending on the DC offset of

the ADCs, while no power is really present at the

inputs. The only practical way to suppress or attenuate

idle tones phenomenon is to apply dithering to the

ADC. The idle tones amplitudes are a function of the

order of the modulator, the OSR and the number of

levels in the quantizer of the modulator. A higher order,

a higher OSR or a higher number of levels for the

quantizer will attenuate the idle tones amplitude.

THD dB

()

HarmonicsPower

FundamentalPower

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

⎝⎠

⎛⎞

log

THD %

()

100

THD dB

()

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

SFDR dB

()

FundamentalPower

HighestSpurPower

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

⎝⎠

⎛⎞

log

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