11..3.3.

Reading and Calculating of Digital-to-Analog Converter

Due to the manufacture process drift, there is an offset voltage in the FS511 ADC such that an offset

value is existed in the ADC output. In order to eliminate the offset value, FS511 provides three operation modes,

which can be set by CYS <1:0> of control register NETD. The ADO output and calculation are different in

different operation modes, and they are described in the following subsection.

ADC Output ADO

Set CYS<1:0>=00, the ADC inputs are short circuited, and we can find the negative offset voltage of the ADC

from ADO[23:0].

Set CYS<1:0>=11, we can find the equivalent voltage of the input signal from ADO[23:0].

Set CYS<1:0>=01, and the ADO[23:0] output is the value of an ideal ADC. This mode is suitable for high

resolution measurement.

When CYS<1:0>01, the output rate of ADO[23:0] is the first zero frequency,

1

Z

f

, of COMB as described in

Equation 3. When CYS<1:0> =01, the output rate equals

2

1

Z

f

.

11..3.4.

The Conversion of the Digital Codes and Equivalent Voltage

The output of the FS511 ADC is ADO[23:0], which is a 24-bit 2’s complement number. ADO[23] is the sign bit;

0 represents a positive number, and 1 represents a negative number. The decimal point lies in between

ADO[22] and ADO[21].

If ADO[23:0]=0010_1000_0000_0000_0000_0000, the equivalent floating point number is:

Equation 4

625

.

0

125

.

0

5

.

0

2

0

......

2

0

2

0

2

1

2

0

2

1

0000

_

0000

_

0000

_

0000

_

1000

_

10

.

00

22

5

4

3

2

1















































ADO

If ADO[23:0]=1101_1111_1111_1111_1111_1111, the equivalent floating point number is:

Equation 5

5000002384

.

0

2

1

......

2

0

2

0

2

0

2

1

(

)

0001

_

0000

_

0000

_

0000

_

0000

_

10

.

00

(

1111

_

1111

_

1111

_

1111

_

1111

_

01

.

11

22

4

3

2

1













































ADO

From Equation 1, if gain

'

G

equals 1 and reference voltage

ref

V

=1.00000V, the value of ADO,

0010_1000_0000_0000_0000_0000, can be used to calculate the measured voltage as:

V

62500

.

0

625

.

0

1

V

00000

.

1

'











x

ref

x

D

G

V

v

。

If ADO=1101_1111_1111_1111_1111_1111, the measured voltage can be calculated:

V

50000

.

0

5000002384

.

0

1

V

00000

.

1

'















x

ref

x

D

G

V

v

。

However, due to the manufacture process drift

'

%

1



V

, and make

ref

V

not to be exact 1.00000V. Therefore, we have to calibrate the ADC.