FS970X datasheet(24/46 Pages) FORTUNE

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FS970X

Rev. 4.4

24/46

11. Analog to Digital Converter (ADC)

The operating theory of - ADC

This high-resolution ADC adapts the modulation of delta and sigma. It samples the consecutive analog input

signal by the sampling frequency of far higher frequency width input signal. It will then be converted into

one-bit code by the modulation of delta and sigma. It then filters the high-frequency noise of Σ-Δ converter by

the digital filter in the chip. It becomes high-resolution digital coding to be applied in high-resolution DMM.

Besides, this kind of converter only performs a 1-bit transfer on the analog-end; therefore, it’s with better linear

characteristics. The signal input and reference voltage input will be full differential amplifier input, with good

CMRR, which can reject the common mode signal.

ANALOG

INTEGRATOR

DIGITAL LOW PASS

DECIMATION

FILTER

COMPARATOR

ANALOG

INPUT

out

1, -1, -1, 1, 1, 1,....

ref, -Vref, -Vref, Vref, Vref, Vref,...

DAC

Graph 9.

Σ-Δ ADC concept diagram.

As shown in Graph 9, Σ

-Δ ADC includes an analog differentor , an integrator, and a comparator, a one-bit ADC

and a digital low-pass filter. The analog input signal is taken from the consecutively sampled input, and

deduct it directly from the expected voltage. The difference will then flow into analog integrator, then product a

predicted digital by the comparator. It will then be converted to the expected voltage（+Vref or –Vref ）by ADC,

reversely feed in to the integrator to get a stable negative feedback. The integrator has unlimited gain to DC;

therefore, if the speed of change of input signal is far smaller than the speed of sampling, the average of the

expected voltage signal of theΣ

-Δ converter will be very close to the input signal. It’s considered equivalent

under certain resolution. Thus, the one-bit digital converted from the comparator is equal to the analog signal

value  Vref. Therefore, take the one-bit digital and perform an arithmetical average by the digital filter to get a

high-resolution Σ-Δ digital.

Transfer Function & Non-ideal affect

ADC

ref

Graph10. ADC.

As shown in Graph 10, there are two sets of input and one set of output in ADC – voltage input

, reference

voltage input

ref

, and ADC output

. The ideal transfer function is:

Formula 3........................................................

ref



 '

represents the gain value of the ADC.

However, the ADC in reality is not ideal. The relationship is:

Formula 4 .....................................................

ref















FS970X Datasheet(PDF) 24 Page - Fortune Semiconductor Corp.

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