MIC3003GML datasheet(25/75 Pages) MICREL

25 / 75 page

Micrel, Inc.

MIC3003

November 2009

M9999-111209-C

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

Figure 3. Internal Calibration RX Power Linear Approximation

C/ Reading the ADC Result Registers

The ADC result registers should be read as 16-bit

registers under internal calibration while under external

calibration they should be read as 8-bit or 16-bit

registers at the MSB address. For example, TX power

should be read under internal calibration as 16 bits at

address A2h: 66h–67h and under external calibration as

8 bits at address A2h: 66h. 9-bit temperature results and

12-bit receive power results should always be read as

16-bit quantities.

Reading the result registers using two-byte burst reads on

the SMBus guarantees that the two bytes are coherent

with each other—that is, they form a matched result pair.

If the two bytes were read separately, it is possible that

the internal result could be updated between the reads,

leading to an incorrect ADC result.

RXPOT

programmable,

non-volatile

digitally

controlled

potentiometer is provided for adjusting the gain of the

receive power measurement signal chain in the analog

domain. Five bits in the RXPOT register are used to set

and adjust the position of potentiometer. RXPOT

functions as a programmable divider or attenuator. It is

adjustable in steps from 1:1 (no divider action) down to

1/32 in steps of 1/32. If RXPOT is set to zero, then the

divider is bypassed completely. There will be no scaling

of the input signal, and the resistor network will be

disconnected from the VRX pin. At all other settings of

RXPOT, there will be a 32kΩ (typical) load seen on

VRX.

Figure 4. RXPOT Block Diagram

Laser Diode Bias Control

The MIC3003 can be configured to generate a constant

bias current using electrical feedback, or regulate

average transmitted optical power using a feedback

signal from a monitor photodiode, as shown in Figure 5.

An operational amplifier is used to control laser bias

current via the VBIAS output. The VBIAS pin can drive a

maximum of ±10mA. An external bipolar transistor

provides current gain. The polarity of the op amp’s

output is programmable with BIASREF (bit-5 in

OEMCFG1) in order to accommodate either NPN or

PNP transistors that drive common anode and common

cathode laser, respectively. Additionally, the polarity of

the feedback signal is programmable for use with either

common-emitter or emitter-follower transistor circuits.

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