MCP3918 datasheet(26/88 Pages) MICROCHIP

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MCP3918

DS20005287A-page 26

 2014 Microchip Technology Inc.

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

sine wave input, and is defined in Equation 4-7.

EQUATION 4-7:

The THD calculation includes the first 35 harmonics for

the MCP3918 specifications. The THD is usually only

measured with respect to the first ten harmonics. THD

is sometimes expressed as percentage. Equation 4-8

converts the THD in percentage.

EQUATION 4-8:

This specification depends mainly on the DITHER

setting.

4.12

Spurious-Free Dynamic Range

(SFDR)

SFDR is the ratio between the output power of the

fundamental and the highest spur in the frequency

spectrum (see Equation 4-9). The spur frequency is not

necessarily a harmonic of the fundamental, even

though this 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 4-9:

4.13

MCP3918 Delta-Sigma

Architecture

The MCP3918 incorporates one delta-sigma ADC with

a multi-bit architecture. A delta-sigma ADC is an

oversampling converter that incorporates a built-in

modulator, which digitizes the quantity of charges

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

quantizer

the

block

that

performs

the

analog-to-digital conversion. The quantizer is typically

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

linearity performance of the ADC (the DAC structure is,

in this case, inherently linear).

Multi-bit quantizers help 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

linearity is as difficult to attain, and its linearity limits the

THD of such ADC.

The 5-level quantizer present in MCP3918 is a Flash

ADC composed of four comparators arranged with

equally spaced thresholds and a thermometer coding.

For improved THD figures, the MCP3918 also includes

proprietary 5-level DAC architecture that is inherently

linear.

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

ADC will show idle tones. This means that the output

will have spurs in the frequency content that depend on

the ratio between the 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 to 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 the

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

They can be localized in the baseband of the converter

and are 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 ADC, while no power is really present at the

inputs. The only practical way to suppress or attenuate

the idle tones phenomenon is to apply dithering to the

ADC. The amplitudes of the idle tones 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 amplitudes of the idle

tones.

THD dB



HarmonicsPower

FundamentalPower

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





log

THD %



100

THD dB



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



SFDR dB



FundamentalPower

HighestSpurPower

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





log

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