430

noise Q will get ever more predominant in the output signal. What is known as noise

shaping occurs

). Noise shapingmakes it possible to keep the oversampling

frequency

S

fk

⋅

at the same resolution of n-bit lower than actually necessary. The

theoretical value is at a factor

n2

2 k

⋅

=

for a resolution of n-bit.

Frequency behaviour of the

Σ

/

∆

ADC

If a first order

Σ

/

∆

-ADC is expanded by a second integration and summation level, the

result is a second order

Σ

/

∆

-ADC

. The noise shaping ismore pronounced in

the latter and the ENOB is increased, while there is no change in the oversampling

frequency

S

fk

⋅

.

The reduction in bandwidth of the output signal by the digital low-pass filter alsomakes

it possible to reduce the output data rate to less than the original sampling rate of

S

fk

⋅

(which must be above the Nyquist frequency of

S

f5.0

⋅

). Thus it is possible to forward

only each x-th bit from the output data stream. The only parameter is that the frequency

of themodified output data stream is at least twice that of the frequency of the sampled

useful signal. In practice, thedecimator performs this function.

The decimator processes the output signal of an

Σ

/

∆

-ADC, usually by preparing user-

adjustable chronological mean values from the

Σ

/

∆

data stream, which can be utilised

directly by a microcontroller via a digital bus. The signal resolution by the decimator

depends on the time period during which the mean value is formed. The longer this

period, the better the effective signal resolution in bits but this also increases the

effective converter time. Depending on the task at hand, values from a few µs (for short

circuit current monitoring) to several 10µs (for continuous "normal" current

measurement) can be selected.

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