LTC5100EUF datasheet(30/52 Pages) LINER

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LTC5100

sn5100 5100fs

Conversion Sequence

The ADC has a 1ms conversion time and operates in a four-

cycle sequence. Three of these cycles are dedicated to the

needs of the servo controllers for laser bias and modula-

tion current. One cycle is available to the user to convert

any desired quantity. Table 2 shows how the four conver-

sion time slots are allocated. The temperature compensa-

tion and servo loop calculations are done during the User

cycle. The source and modulation DACs are also updated

during this cycle.

Table 2. ADC Conversion Sequence

RESULT STORED IN

CYCLE

APC MODE

CCC MODE

T_INT_ADC

2IM

IM_ ADC

3IMD

IMD_ADC/IS_ADC

User

USER_ADC

User Access to the ADC

The results of each conversion cycle in Table 2 are stored

in user accessible registers. The last die temperature

measurement can be read over the I2C bus at any time by

reading the T_INT_ADC register. Note that the quantity

converted during the third cycle depends on whether the

chip is in APC or CCC mode. The result of the third

conversion cycle is stored in a register that is called

IMD_ADC in APC mode and IS_ADC in CCC mode. There

is only one register, but it is given two names to indicate

the quantity it actually holds.

The fourth cycle, called the user cycle, is available to

digitize any of the six multiplexed signals. The result can

be read out over the I2C serial bus. The signal to be

digitized during the user cycle is selected by setting the

three-bit field USER_ADC.Adc_src_sel (see Table 23).

For example, setting Adc_ src_sel = 2 programs the

multiplexer to select the laser diode voltage, VLD. During

the next user conversion cycle, VLD is converted and

stored to the USER_ADC. Data field. When the conversion

complete,

USER_ADC.Valid

set

and

USER_ADC.Adc_src indicates the signal source whose

converted value is stored in USER_ADC.Data. Reading or

OPERATIO

writing the USER_ADC register clears the Valid bit. The

Valid bit remains cleared until the next user conversion is

complete. USER_ADC.Adc_src always corresponds to

the signal source whose data is stored in USER_ADC.Data,

not the source that was most recently selected by writing

USER_ADC.Adc_src_sel. The Valid bit and ADC_src field

are useful for monitoring when the ADC has updated the

USER_ADC.Data field. Table 3 gives an extended example

of accessing the USER_ADC register.

Note that the content of the USER_ADC register is different

for writing and for reading, even though the I2C command

used to access this register is the same in both cases. See

Table 23 and Table 24 for a detailed definition of the bit

fields in the USER_ADC register. Table 23 also shows how

to convert ADC digital codes to real-world quantities.

DIRECT MICROPROCESSOR CONTROL OF THE LASER

BIAS AND MODULATION CURRENT

Setting Lpc_en to zero turns off the LTC5100’s digital

Laser Power Controller (see Figure 2). The source and

modulation DACs (Is_dac and Im_dac) can then be written

from the I2C serial bus, allowing an external microproces-

sor or test computer to directly control the source and

modulation currents.

DIGITAL CONTROL AND THE I2C SERIAL INTERFACE

The LTC5100 has extensive digital control and monitoring

features. These features can be used during final assembly

of a transceiver module to set up the laser and verify

performance. In normal operation, the LTC5100 can oper-

ate standalone or under microprocessor supervision. Op-

erating standalone, the LTC5100 automatically loads its

configuration and laser operating parameters (bias cur-

rent, modulation current, monitor diode current) from a

small external EEPROM at power up. Operating under

microprocessor supervision, the microprocessor is in

total control of setting up the LTC5100.

I2C Serial Interface Protocol

The digital interface for the LTC5100 is I2C, a 2-wire serial

bus standard that is fully documented in “I2C-Bus and How

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