DS1864 datasheet(20/72 Pages) DALLAS

20 / 72 page

SFP Laser Controller and

Diagnostic IC

____________________________________________________________________

The gain register is now set and the resolution of the

conversion will match the expected LSB. Customers

requiring nonzero null values (e.g., 0.5V as the example

shows) must next calibrate the input’s offset. If the

desired null value is 0V, leave the offset register pro-

grammed to 0000h and skip this step.

To calibrate the offset register, program the gain regis-

ter with the gain_result value determined above. Next,

force the null input voltage (0.5V for the example) and

read the digital result from the part (Meas1). The offset

value can be calculated using the following formula:

This value is then programmed into the corresponding

offset register.

Enhanced RSSI Monitoring

(Dual-Range Functionality)

The DS1864 offers a brand new feature to improve the

accuracy and range of MON3, which is most commonly

used for monitoring RSSI. Predecessors of the DS1864,

namely the DS1859 and the DS1856, feature program-

mable gain, offset, and right shifting (Scalable Dynamic

Ranging) on each of the MON channels. These three

elements are extremely beneficial when monitoring low-

amplitude signals such as RSSI. The accuracy of the

RSSI measurements is increased at the small cost of

reduced range (of input signal swing). The DS1864

eliminates this tradeoff by offering “dual-range” calibra-

tion on the MON3 channel. This feature enables right

shifting (along with its gain and offset settings) when

the input signal is below a set threshold (within the

range that benefits using right shifting) and then auto-

matically disables right shifting (recalling different gain

and offset settings) when the input signal exceeds the

threshold. Also, to prevent “chattering,” hysteresis pre-

vents excessive switching between modes in addition

to ensuring that continuity is maintained. Dual-range

operation is enabled by default (factory programmed in

EEPROM). However, it can easily be disabled by the

RSSIF and RSSIC bits, which are described later in this

section. When dual-range operation is disabled, MON3

operates identically to the other MON channels,

although featuring a differential input.

Dual-range functionality consists of two modes of oper-

ation: fine mode and course mode. Each mode is cali-

brated for a unique transfer function, hence the term

“dual range.” Table 7 highlights the registers related to

MON3. Fine mode is equivalent to the other MON chan-

nels and is similar to the DS1859 and DS1856. Fine

mode is calibrated using the gain, offset, and right

shifting registers at locations shown in Table 7 and is

ideal for relatively small analog input voltages. Course

mode is automatically switched to when the input

exceeds the threshold (to be discussed in a subse-

quent paragraph). Course mode is calibrated using dif-

ferent gain and offset registers, but lacks right shifting

(since course mode is only used on large input sig-

nals). The gain and offset registers for course mode are

also shown in Table 7. Additional information for each

of the registers can be found in the memory map.

Dual-range operation is transparent to the end user.

The results of MON3 analog-to-digital conversions are

still stored/reported in the same memory locations (68

to 69h, Lower Memory) regardless of whether the con-

version was performed in fine mode or course mode.

The only way to tell which mode generated the digital

result is by reading the RSSIS bit.

When the DS1864 is powered up, analog-to-digital con-

versions begin in a round-robin fashion. Every MON3

timeslice begins with a fine mode analog to digital con-

version (using fine mode’s gain, offset, and right-shift-

ing settings). See the flowchart in Figure 15. Then,

depending on whether the last MON3 timeslice resulted

in a course mode conversion and also depending on

the value of the current fine conversion, decisions are

made whether to use the current fine mode conversion

result or to make an additional conversion (within the

same MON3 timeslice), using course mode (using

course mode’s gain and offset settings

⎯and remem-

ber, no right shifting) and reporting the course mode

result. The flowchart also illustrates how hysteresis is

implemented. The fine mode conversion is compared

to one of two thresholds. The actual threshold values

are a function of the number of right shifts being used.

Table 6 shows the threshold values for each possible

number of right shifts.

The RSSIF and RSSIC bits are used to force fine mode

or course mode conversions, or to disable the dual-

range functionality. Dual-range functionality is enabled

by default (both RSSIC and RSSIF are factory pro-

grammed to “0” in EEPROM). It can be disabled by set-

ting RSSIC to 0 and RSSIF to 1. These bits are also

useful when calibrating MON3. For additional informa-

tion, see the Memory Map.

Fault Management

The DS1864 provides a variety of system alerts to help

automate laser control. These alerts are in the form of

fast-trip comparators, fast-trip alarm and warning

thresholds, diagnostic alarm and warning thresholds,

and configurable laser eye safety and shutdown logic.

Fast-trip comparator values are measured against fast-

trip thresholds to set alarms and to enable fault and

OFFSET

Meas

=− × ⎛

⎝⎜

⎞

⎠⎟

DS1864 Datasheet(PDF) 20 Page - Dallas Semiconductor

DS1864 Datasheet(HTML) 20 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