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ONET2804T Suitable for APD & PIN photodiodes

Other Parts Discussed in Thread: ONET2804T, ONET8551T

Is the ONET2804T device suitable for use with either APD or PIN photodiodes (similar to the ONET8551T) or is it recommended for PIN photodiodes only?

Is there anything different in the design of devices such as the ONET8551T that cite "PIN and APD preamplifier" in their datasheets vs devices that just mention PIN photodiode applications?

  • Hello Steve,

    The ONET2804T was designed specifically to interface with a PIN diode with a 2.5V reverse bias voltage.  The "Filter" pins self bias to this voltage and the pad spacing on the ONET die match with the pad spacing on several PIN diodes. 

    The input state of the ONET2084T is a trans impedance amplifier and will work with any current source.  The impedance is about 35 Ohms, so you can even drive it with a small signal 50 Ohm voltage source as well. 

    If you do want to use the ONET2804T with an APD you will need to provide an appropriate bias voltage for the diode and I would recommend AC coupling. 

  • Follow-up:

    Is there anything different in the circuit design of the ONET2804 device when compared to say the ONET8551T that specifically cites on the datasheet under the list of applications “PIN and APD preamplifier” with respect to the characteristics of the input and bias pins of the TIA?

    This question is to gain understanding as to whether there is some design optimization done on some TI TIA devices to support interfacing with APD devices that is not found on all TI TIA devices.
  • Hi Steve,

    If you look at Figure 4 and Figure 3 on the 8551 datasheet you will see that the TIA input is not fundamentally different for the different diodes.

    I don't see any real design optimization for the 8551 that are significantly different than those in the 2804.

    It is likely that there were just no APD devices that are fast enough for 25GBps operation when the datasheet was written.

  • The 8551 has a feature not found in the 2804 devices.  Specifically, the 8551 has an external bias RSSI output pin (“RSSI_EB”) that the 2804 does not allowing for an RSSI function even when the biasing pins of the TIA are isolated from the photodiode.  Would that be why the datasheet cites APD applications?