MIC3002GML datasheet(22/65 Pages) MICREL

22 / 65 page

Micrel, Inc.

MIC3002

July 2007

M9999-073107-B

hbwhelp@micrel.com or (408) 955-1690

Calibration Modes

The default mode of calibration in the MIC3002 is

external calibration, for which INTCAL bit (bit 0 in

OEMCF3 register) is set to 0. The internal calibration

mode is selected by setting INTCAL bit to 1.

A/ External Calibration

The voltage and temperature values returned by the

MIC3002’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.

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 MIC3002

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

constants and use them to process the measured data.

Voltage

The voltage values returned by the MIC3002’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

one and the offset to zero.

Temperature

The temperature values returned by the MIC3002’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.

Bias Current

Bias current is sensed via an external sense resistor as

a voltage appearing between VILD+ and VILD-. The

value returned by the A/D is therefore a voltage

analogous to bias current. Bias current, IBIAS, is simply

VVILD/RSENSE. The binary value in IBIASh (IBIASl is

always zero) is related to bias current by:

(1)

The value of the least significant bit (LSB) of IBIASh is

given by:

(2)

Per SFF-8472, the value of the bias current LSB is 2µA.

The conversion factor, “slope”, needed is therefore:

The tolerance of the sense resistor directly impacts the

accuracy of the bias current measurement. It is

recommended that the sense resistor chosen be 1%

accurate or better. The offset correction, if needed, can

be determined 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 a resistor carrying the

monitor photodiode current. In most applications, the

signal at VMPD will be feedback voltage on FB. The

VMPD voltage may be measured relative to GND or

VDDA 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:

(3)

For a given implementation, the value of RSENSE 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.

MIC3002GML Datasheet(PDF) 22 Page - Micrel Semiconductor

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