DS1873T+ Maxim Integrated Products, DS1873T+ Datasheet - Page 16
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DS1873T+
Manufacturer Part Number
DS1873T+
Description
IC CTLR SFP+ ANLG LDD 28-TQFN
Manufacturer
Maxim Integrated Products
Type
Laser Diode Controllerr
Datasheet
1.DS1873TTR.pdf
(86 pages)
Specifications of DS1873T+
Number Of Channels
1
Voltage - Supply
2.85 V ~ 3.9 V
Current - Supply
2.5mA
Operating Temperature
-40°C ~ 95°C
Package / Case
28-WFQFN Exposed Pad
Mounting Type
Surface Mount
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
Other names
90-1873T+000
SFP+ Controller with Analog LDD Interface
the ADC results. The DS1873’s range is wide enough to
cover all requirements; when the maximum input value
is ≤ 1/2 of the FS value, right-shifting can be used to
obtain greater accuracy. For instance, the maximum
voltage might be 1/8 the specified PFS value, so only
1/8 the converter’s range is effective over this range.
An alternative is to calibrate the ADC’s full-scale range
to 1/8 the readable PFS value and use a right-shift
value of 3. With this implementation, the resolution of
the measurement is increased by a factor of 8, and
because the result is digitally divided by 8 by right-
shifting, the bit weight of the measurement still meets
the standard’s specification (i.e., SFF-8472).
The right-shift operation on the ADC result is carried out
based on the contents of right-shift control registers
(Table 02h, Registers 8Dh–8Fh) in EEPROM. Four ana-
log channels, MON1–MON4, each have 3 bits allocated
to set the number of right-shifts. Up to 7 right-shift oper-
ations are allowed and are executed as a part of every
conversion before the results are compared to the high-
alarm and low-alarm levels, or loaded into their corre-
sponding measurement registers (Lower Memory,
Registers 64h–6Bh). This is true during the setup of
internal calibration as well as during subsequent data
conversions.
The DS1873 offers a fully differential input for MON3.
This enables high-side monitoring of RSSI, as shown in
Figure 4. ADC Round-Robin Timing
Figure 5. MON3 Differential Input for High-Side RSSI
16
______________________________________________________________________________________
ROSA
NOTE: IF THE VCC LO ALARM IS ENABLED AT POWER-UP, THE ADC ROUND-ROBIN TIMING CYCLES BETWEEN TEMPERATURE AND V
IS ABOVE THE V
100Ω
V
CC
CC
ALARM LOW THRESHOLD.
MON3P
MON3N
ONE ROUND-ROBIN ADC CYCLE
Differential MON3 Input
TEMP
DS1873
V
CC
ADC
MON1
t
RR
MON2
Figure 5. This reduces board complexity by eliminating
the need for a high-side differential amplifier or a cur-
rent mirror.
The DS1873 offers a feature to improve the accuracy
and range of MON3, which is most commonly used for
monitoring RSSI. The accuracy of the RSSI measure-
ments is increased at the small cost of reduced range
(of input signal swing). The DS1873 eliminates this
trade-off by offering “dual range” calibration on the
MON3 channel (see Figure 5). 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 automat-
ically disables right-shifting (recalling different gain and
offset settings) when the input signal exceeds the thresh-
old. Also, to prevent “chattering,” hysteresis prevents
excessive switching between modes in addition to ensur-
ing that continuity is maintained. Dual-range operation is
enabled by default (factory programmed in EEPROM).
However, it can easily be disabled through the RSSI_FC
and RSSI_FF bits, which are described in the Register
Descriptions section. When dual-range operation is dis-
abled, MON3 operates identically to the other MON
channels, although featuring a differential input.
Dual-range functionality consists of two modes of opera-
tion: fine mode and coarse mode. Each mode is calibrat-
ed for a unique transfer function, hence the term, dual
range. Table 4 highlights the registers related to MON3.
Fine mode is equivalent to the other MON channels. Fine
mode is calibrated using the gain, offset, and right-shift-
ing registers at locations shown in Table 4 and is ideal
for relatively small analog input voltages. Coarse mode is
automatically switched to when the input exceeds a
threshold (to be discussed in a subsequent paragraph).
Coarse mode is calibrated using different gain and offset
registers, but lacks right-shifting (since coarse mode is
only used on large input signals). The gain and offset
registers for coarse mode are also shown in Table 4.
MON3
MON4
Enhanced RSSI Monitoring (Dual-Range
TEMP
CC
ONLY UNTIL V
Functionality)
CC