Application Question Regarding the ONET1141L

Michael,

A typical schematic with part numbers for the ferrite beads and inductors is attached. This also includes some typical results using a 1625 EML from Avago.

ONET1141L Schematic.pdf

Regards,

Alex.

Alex,
Thank you very much. I have some questions about the schematic. I am new to the EML style TOSA's.

1) I see that there is an EA BIAS output. What is that output used for? I also see it in the schematics provided in the datasheet.

2) I see that you are using a current mirror in the PD circuit. For my own understanding, what benefit does using the current mirror provide in the PD circuit? I would like to understand it's function here.

Thank you for your help.

Mike Nycz

Mike,

1. The EA bias sets the operating point for the modulator. The modulator requires a reverse bias voltage so a negative DC voltage is applied to set the correct operating point. The range for this voltage is specified in the TOSA datasheet but it is typically in the range of -0.5 to -1.5V. The bias point changes the behavior of the modulator and is typically more negative for an 80km application compared to a 40km application. The optimal bias point depends on a number of variables but the transmitter dispersion penalty is one main driving factor. The dispersion penalty is the shift in the receiver bit error rate for a back-to-back transmission (no fiber) compared to when 40km or 80km of fiber is used. The dispersion penalty typically should be less than 2dB.

2. The pinout for most EML TOSAs follows a multisource agreement called the XMD MSA. This requires that the photodiode cathode be grounded and the photodiode anode be available. However, in order to operate correctly, a photodiode has to be reverse biased which means the potential on the anode must be less than the potential on the cathode. Since the cathode is grounded the anode requires a negative voltage. Therefore, a current mirror is used to apply the negative voltage to the PD so that the mirrored PD current can be feed into the PD pin of the ONET1141L. This is only required if the laser bias current is being controlled in a closed loop manor. That is, the laser bias current is automatically adjusted to keep the photodiode current constant. The ONET1141L can be operated in open loop or closed loop mode.

Regards,

Alex.

Alex,
1) So for the EA BIAS, I would choose the negative voltage to be applied and then fine tune based upon measured performance results?

2) To run the ONET1141L in open loop, is it simply not connecting the PD pin on the ONET1141L to the PD pin on the TOSA via the current mirror? Or does it require I2C setup as well?

Thank you,
Mike

Mike,

1. That is correct. The Vea voltage is typically fine-tuned for each TOSA.

2. Closed loop is the default operation for the ONET1141L so you have to enable open loop operation by setting bit 4 of register 0 to 1 through the I2C interface. You then do not need to apply a signal to the PD pin.

Regards,

Alex.

Alex,
2) And so the PD pin of the TOSA can be grounded? Or should it be tied to something else?

Mike

Alex,

Also, you used the inverter TPS60400 to create the negative voltage for the current mirror. Can an inverter be used to create the EA BIAS voltage or does it require more current capability?

Mike

Mike,

You can leave the PD pin on the TOSA as a no connect if you are not using it.

Alex.

Mike,

I would not use the TPS60400 since it is unregulated so the output voltage is dependent upon the load current. Depending on the TOSA, the load current could be up to 30mA. You could use the LMR70503 or TPS63700. You will have to modify the circuit to make the output voltage adjustable, usually through MCU control. You may want to speak to someone in the power forum about the best way to do this.

Regards,
Alex.

Hello Mike.

I'm sorry but I do not support the power products at TI. If you have specific questions about these parts then please use the Power Management support forum. Also note that I have never used the LMR70503 in our application circuits. I only suggested this as a possible part to use so you may want to clarify this with the Power Management group. You should clarify if the part can support a negative voltage in the range of -0.5V to 1.5V.

Regards,
Alex.