MIC3001_11 datasheet(22/76 Pages) MICREL

22 / 76 page

Micrel, Inc.

MIC3001

August 2004

M9999-082404-A

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

C/ ADC Result Registers Reading

In the present revision of MIC3001, 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 registers at the MSB address. For example,

TX power should be read under internal calibration as 16

bits at address A2h: 66–67 and under external calibration

as 8 bits at address A2h: 66h.

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, 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 3. RXPOT Block Diagram

Laser Diode Bias Control

The MIC3001 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, see Figure 4. 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

BIASREF 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.

Furthermore, the reference level for the APC circuit is

selectable to accommodate electrical, i.e., current feedback,

or optical feedback via a monitor photodiode. Finally, any

one of seven different internal feedback resistors can be

selected. This internal resistor can be used alone or in

parallel with an external resistor. This wide range of

adjustability (50:1) accommodates a wide range of

photodiode current, i.e, wide range of transmitter output

power. The APC operating point can be kept near the mid-

scale value of the APC DAC, insuring maximum SNR,

maximum effective resolution for digital diagnostics, and the

widest possible DAC adjustment range for temperature

compensation, etc. See Figure 5.

The APCCAL bit in OEMCAL0 is used to turn the APC

function on and off. It will be turned off in the MIC3001’s

default state as shipped from the factory. When APC is on,

the value in the selected APCSETx register is added to the

signed value taken from the APC look-up table and loaded

into the VBIAS DAC. When APC is off, the VBIAS DAC may

be written directly via the VBIAS register, bypassing the look-

up table entirely. This provides direct control of the laser

diode bias during setup and calibration. In either case, the

VBIAS DAC setting is reported in the APCDAC register. The

APCCFG bits determine the DACs response to higher or

lower numeric values.

Figure 4. MIC3001 APC and Modulation Control

Block Diagram

Figure 5. Programmable Feedback Resistor

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