DS1864 datasheet(19/72 Pages) DALLAS

19 / 72 page

SFP Laser Controller and

Diagnostic IC

____________________________________________________________________

To calculate the voltage measured from the register

value, first calculate the LSB weight of the 16-bit register.

The LSB weight is equal to the full-scale voltage span

divided 65528. Next, convert the hexadecimal register

value to decimal and multiply it times the LSB weight.

Example: Using the factory default VCC trim, what volt-

age is measured if the VCC register value is C340h?

The LSB for VCC is equal to (6.5528V - 0V) / 65528 =

100.00µV. C340h is equal to 49984 decimal, which

yields a supply voltage equal to 49984 x 100.00µV =

4.9984V. Table 5 shows more conversion examples

based on the factory trimmed A/D settings.

The factory-programmed LSB for VCC is 100µV. The

factory-programmed LSB weight for the MON channels

is 38.147µV.

Voltage Monitor Calibration

(Gain, Offset, and Right Shifting)

The DS1864 has the ability to scale each analog volt-

age’s gain and offset to produce the desired digital

result. Each of the inputs (VCC, MON1, MON2, MON3)

has specific registers for the gain, offset, and right shift-

ing (in memory Table 04h (Table 01h in DS1859 config-

uration)) allowing them to be individually calibrated.

To scale the gain and offset of the converter for a spe-

cific input, one must first know the relationship between

the analog input and the expected digital result. The

input that would produce a digital result of all zeros is

the null value (normally this input is GND). The input

that would produce a digital result of all ones (FFF8h) is

the full-scale (FS) value. The expected FS value is also

found by multiplying FFF8h by the LSB weight.

The right-shifting operation on the A/D converter output

is carried out based on the contents of Registers Right

Shift1 and Right Shift2 in EEPROM. Each of the three

analog channels (MON1 (Bias Current (IBI)), MON2

(Transmitted Power (TXP)), and MON3 (Received Power

(RIN)) is allocated 3 bits to set the number of right shifts.

Up to 7 right-shift operations are allowed and will be

executed as a part of every conversion before the result

is loaded in the corresponding measurement registers

62h to 69h. This is true during the setup of internal cali-

bration as well as during subsequent data conversions.

Example: Since the FS digital reading is 65528 (FFF8h)

LSBs, if the LSB’s weight is 50µV, then the FS value is

65528 x 50µV = 3.2764V.

A binary search is used to calibrate the gain of the con-

verter. This requires forcing two known voltages on the

input pin. It is preferred that one of the forced voltages is

the null input and the other is 90% of FS. Since the LSB

of the least significant bit in the digital reading register is

known, the expected digital results can be calculated

for both the null input and the 90% of full-scale value.

An explanation of the binary search used to scale the gain

is best served with the following example pseudo-code:

/* Assume that the null input is 0.5V */

/* Assume that the requirement for the LSB is 50

µV */

FS = 65528 * 50e-6;

/*3.2764V */

CNT1 = 0.5 / 50e-6;

/* 1000 */

CNT2 = 0.9 X FS / 50e-6;

/* 58968 */

/* So the null input is 0.5V and 90% of FS is 2.94876V */

Set the input's offset register to zero

gain_result = 0h;

/* Working register for gain calculation */

CLAMP = FFF0h;

/* This is the max A/D value*/

For n = 15 down to 0

begin

gain_result = gain_result + 2^n;

Write gain_result to the input's gain register;

Force the 90% FS input (2.94876V);

Meas2

= A/D result from DS1864;

If Meas2 >= CLAMP

Then

gain_result = gain_result - 2^n;

Else

Force the null input (0.5V)

Meas1 = A/D result from DS1864

If [(Meas2-Meas1)>(CNT2-CNT1)]

Then

gain_result = gain_result - 2^n;

end;

Write gain_result to the input's gain register;

Table 4. Voltage Monitor Factory Default

Calibration

SIGNAL

+FS

(V)

+FS

(hex)

-FS

(V)

-FS

(hex)

VCC

6.5528V

FFF8

0000

MON1

2.4997V

FFF8

0000

MON2

2.4997V

FFF8

0000

MON3

2.4997V

FFF8

0000

Table 5. Voltage Monitor Conversion

Examples

SIGNAL

LSB

WEIGHT µV)

VALUE (HEX)

INPUT

VOLTAGE

(V)

VCC

100.00

8080

3.2896

VCC

100.00

C0F0

4.9392

MON1

38.147

AA00

1.6601

MON2

38.147

1880

0.2392

MON3

38.147

9CF0

1.5326

DS1864 Datasheet(PDF) 19 Page - Dallas Semiconductor

DS1864 Datasheet(HTML) 19 Page - Dallas Semiconductor

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72

Datasheet Download

Link URL