FS9701B datasheet(25/46 Pages) FORTUNE

25 / 46 page

FS970X

Rev. 4.4

25/46

and

represent respectively the gain and the offset voltage of ADC. They can both be affected during

manufacturing, and vary every single chip

Under most of the applications of ADC, reference voltage is a fixed value

rrf

. ADC converts the variable

voltage

into an equivalent value as

rrf

. Examples are AC/DC voltage measurement and AC/DC currency

measurement of DMM. In this kind of application, reference voltage is fixed; thus, Formula 4 can be simplified

as:

Formula 5........................................................

ref





 '

is a fixed value.

However, in the application of DMM, the most often used type of measurement of resistor is scale-type

measurement. The reference voltage

ref

will vary with resistor, not a fixed value. Formula 5 will not reflect

the actual situation, and needs to be modified as:

Formula 6........................................................

 

ref











Whereas,

 

ref

os v

is the function of reference voltage. In the application of the measurement of scale-type

resistors, we will learn that

 

os R

is related to resistor

FS970X includes two kinds of ADC output: high-resolution, low-speed and low-resolution, high-speed. Under

high-resolution, low-speed output, the offset voltage has been eliminated. Its transfer function converts from

the ideal linear from Formula 3. Under low-resolution, high-speed output, the offset voltage still exists. The

transfer function comes from either Formula 5 or Formula 6, according to different condition. See Chapter 8 for

details.

Function Gain Setup

0.5

0.25

0.1

ADG<0>

ADG<1>

ADG<2>

ADG<3>

VIN

ADC

signal

input

1.0

0.25

ADG<4>

ADG<5>

VREF

ADC

reference

input

Graph 11. FS970X Gain setup.

As shown in Graph 11, the input of FS970X ADC includes four different gain paths. They are independently

controlled by ADG<3:0> (4 bits) of the register. The input of reference voltage includes two different gain

paths. They are independently controlled by ADG<5:4>(2 bits) of the register. All the gain values are

approximation. Precise gain values are only available after calibration.

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