MIC3000 Datasheet, PDF (16/68 Pages) Micrel Semiconductor – SFP Management IC

MIC3000
External Calibration
The MIC3000 is designed to support the implementation of an
optical transceiver employing external calibration, as de-
scribed by SFF-8472, Digital Monitoring Interface specifica-
tions. The voltage and temperature values returned by the
MIC3000’s A/D converter are internally calibrated. The binary
values of TEMPh:TEMPl and VOLTh:VOLTl are in the format
called for by SFF-8472 under Internal Calibration. However,
since the other parameters are not internally calibrated, an
MIC3000-based transceiver must be labeled as externally
calibrated.
SFF-8472 calls for a set of calibration constants to be stored
by the transceiver OEM at specific non-volatile memory
locations, refer to SFF-8472 specifications for memory map
of calibration coefficient. The MIC3000 provides the non-
volatile memory required for the storage of these constants.
The Digital Diagnostic Monitoring Interface specification
should be consulted for full details. Slopes and offsets are
stored for use with voltage, temperature, bias current, and
transmitted power measurements. Coefficients for a fourth-
order polynomial are provided for use with received power
measurements. The host system can retrieve these con-
stants and use them to process the measured data. Since
voltage and temperature require no calibration, the corre-
sponding slopes should generally be set to unity and the
offsets to zero.
Voltage
The voltage values returned by the MIC3000’s A/D converter
are internally calibrated. The binary values of VOLTh:VOLTl
are in the format called for by SFF-8472 under Internal
Calibration. Since VINh:VINl requires no processing, the
corresponding slope should be set to unity and the offset to
zero.
Temperature
The temperature values returned by the MIC3000’s A/D
converter are internally calibrated. The binary values of
TEMPh:TEMPl are in the format called for by SFF-8472
under Internal Calibration. Since TEMPh:TEMPl requires no
processing, the corresponding slope should be set to unity
and the offset to zero.
Bias Current
Bias current is sensed via an external sense resistor as a
voltage appearing at VILD+ and VILD-. The value returned by
the A/D is therefore a voltage analogous to bias current. Bias
current, I_BIAS, is simply VVILD/R_SENSE. The binary value in
IBIASh (IBIASl is always zero) is related to bias current by:
I_BIAS
=
⎛
(0.300V)_⎝⎜
IBIASh
255
⎞
⎠⎟
R_SENSE
(1)
The value of the least significant bit (LSB) of IBIASh is
given by:
LSB(IBIASh) = ^0.300V Amps = ^300mV mA = ^1191.4 µA
255 × R_SENSE
255 × R_SENSE
R_SENSE
(2)
Micrel
Per SFF-8472, the value of the bias current LSB is 2µA. The
conversion factor, “slope”, needed is therefore:
Slope =
1191.4µA
2µA × R_SENSE
= 595.7 ÷ R_SENSE
The tolerance of the sense resistor directly impacts the
accuracy of the bias current measurement. It is recom-
mended that the sense resistor chosen maintain accuracy of
1% or better. The offset correction, if needed, can be deter-
mined by shutting down the laser, i.e., asserting TXDISABLE,
and measuring the bias current. Any non-zero result gives the
offset required. The offset will be equal and opposite to the
result of the “zero current” measurement.
TX Power
Transmit power is sensed via an external sense resistor as a
voltage appearing at VMPD. It is assumed that this voltage is
generated by a sense resistor carrying the monitor photo-
diode current. In most applications, the signal at VMPD will be
feedback voltage on FB. The VMPD voltage may be mea-
sured relative to GND or V_DDA depending on the setting of the
BIASREF bit in OEMCFG1. The value returned by the A/D is
therefore a voltage analogous to transmit power. The binary
value in TXOPh (TXOPl is always zero) is related to transmit
power by:
P_TX(mW) =
K × VREF^{⎛⎝⎜ TX2O55Ph⎞⎠⎟}
R_SENSE
=
K × (1220mV)^{⎛⎝⎜ TX2O55Ph⎞⎠⎟}
R_SENSE
= ^{K × 4.75656 × TXOPh} mW
(3)
R_SENSE
For a given implementation, the value of R_SENSE is known. It
is either the value of the external resistor or the chosen value
of RFB used in the application. The constant, K, will likely
have to be determined through experimentation or closed-
loop calibration, as it depends on the monitoring photodiode
responsivity and coupling efficiency.
It should be noted that the APC circuit acts to hold the
transmitted power constant. The value of transmit power
reported by the circuit should only vary by a small amount as
long as APC is functioning correctly.
RX Power
Received power is sensed as a voltage appearing at VRX. It
is assumed that this voltage is generated by a sense resistor
carrying the receiver photodiode current. The value returned
by the A/D is therefore a voltage analogous to received
power. The binary value in RXOPh (RXOPl is always zero) is
related to received power by:
P_RX
(mW)
=
K
×
VREF
×
RXOPh
255
=
K
×
1220mV
×
RXOPh
255
mW
(4)
For a given implementation, the constant, K, will likely have
to be determined through experimentation or closed-loop
calibration, as it depends on the gain and efficiencies of the
components upstream. In SFF-8472 implementations, the
external calibration constants can describe up to a fourth-
order polynomial in case K is nonlinear.
M9999-101204
16
October 2004