MIC3000BML datasheet(16/68 Pages) MICREL

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MIC3000

Micrel

M9999-101204

October 2004

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, IBIAS, is simply VVILD/RSENSE. The binary value in

IBIASh (IBIASl is always zero) is related to bias current by:

0 300V

IBIASh

255

BIAS

SENSE

⎛

⎝⎜

⎞

⎠⎟

)

(1)

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

given by:

LSB IBIASh

0 300V

255 R

Amps

300mV

255 R

1191 4

SENSE

()

=µ

(2)

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

conversion factor, “slope”, needed is therefore:

Slope

1191 4 A

2A R

595 7 R

SENSE

µ×

=÷

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 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:

VREF

TXOPh

255

1220mV

TXOPh

255

4 75656

TXOPh

SENSE

()

⎛

⎝⎜

⎞

⎠⎟

()⎛

⎝⎜

⎞

⎠⎟

= ××

(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.

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:

K VREF

RXOPh

255

K 1220mV

RXOPh

255

RX () =

= ×

(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.

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