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Original question:

ONET8551T AGC

ONET8551T: Expected AGC response to sparse signal pulses

Andrew Huntington
Prodigy 30 points
Part Number: ONET8551T

I am testing a number of photoreceivers assembled from the ONET8551T and a photodiode, using long rectangular optical pulses (e.g. 100 ns duration) with a low duty cycle (e.g. 100 kHz repetition rate).  The measured transimpedance remains below 1 kOhm as input signal current drops from ~100 uA to about 5 uA (as measured during the rectangular pulse -- this is not the time-average current but rather the peak current during signal pulse reception).  Based on Figure 10 of the datasheet, I would expect the transimpedance to increase to a maximum of about 10 kOhm as the signal current drops.  However, I understand that my test signal is quite different from the use case for which the TIA was designed (high-rate telecom bit streams).  Is it expected that the AGC would keep the transimpedance low for this type of test signal?

  • 0
    TI__Mastermind 25775 points

    Hello Andrew,

       Since you are inputting a photodiode current, it should work as expected in the figure. From Figure 10, it seems like even at 100uApp input current, the transimpedance gain should be at around 2kOhm to 3kOhm. Would you be able to describe your setup of the ONET8551T and measure either RSSI_EB or RSSI_IB depending on your setup; since, these pins should mirror the input current.

    Thank you,

    Sima

  • 0 in reply to Sima Jalaleddine
    Prodigy 30 points

    I will attempt to attach a PDF with the connections.  The circuit is assembled on a hybrid ceramic which sits on a TO-8 package header.  We wire-bond from the TIA chip's pads to traces on the hybrid, and to some of the other circuit components (the capacitors are mostly top-bond vertical caps).  Note that the broadband 10 nF capacitors we use to AC-coupled the outputs are located outside the receiver package, so the package pin inductance is between the output pads on the TIA chip and the AC coupling capacitors on the board that mounts the receiver package.  (Note, the slide template is marked "confidential" information, but the information conveyed is not.)Receiver Circuit.pdf

  • 0 in reply to Sima Jalaleddine
    Prodigy 30 points

    Regarding a measurement at RSSI_EB or RSSI_IB, I am looking into how we might accomplish that.  We have not made connections to those pads because we do not use this feature of the chip.

  • 0 in reply to Andrew Huntington
    TI__Mastermind 25775 points

    Hello Andrew,

       Thank you for the diagram. Your connections look good, only part I do want to mention is the bypass capacitor at the power supplies. This is good practice, but the ONET8551T already has an on chip VCC bypass capacitor, so the additional cap is not necessary. However, it wouldn't be the issue in this case or is a disadvantage other than an increased board size. I believe we can use RSSI_EB as a test to check the current. From the datasheet, it does state RSSI_EB will be less accurate than RSSI_IB, but since you are using external bias, we would have to use this pin. I would place a 20k resistor to ground from this pin, and then measure the voltage at the pin. It should be around 0.1V at 5uApp and 2V at 100uApp. As a note, the voltage needs to be set below the max specified voltage in the datasheet of 3.3V - 0.65V. 

    Thank you,

    Sima