AD8337 Evaluation Board Fig 82, DC couple, Single Supply Biasing

Hello,

Amplifying a signal from a TIA powered from 5V.  The signal swings from ~0V up to ~3V depending on received light intensity. I am looking to DC couple the TIA to the AD8337 and would like to ask and verify the following:

Regarding the Single Supply AC coupled Design shown in Figure 82:

  1. Why is RFB1 connected to the output of the 2.5V reference?
  2. Why is R1 connected to the 2.5V Reference and not Ground?  
        -I realize the R1 must connect from INPP to INPN for AC coupling, however would like clarification as to why this is so.
  3. If it was desired to DC couple this device in single supply mode is the following correct:
    • Remove C6 and R1
    • Connect RFB1 to Ground 
      • Could connecting RFB1 to ground create a large DC offset at VOUT?

  4. A lead/lag comp is feeding pin 7 GAIN, what us the purpose of CG?

    Thanks for the assistance.
Parents
  • Hi Analord,

    Thanks for the email!

    The single-supply configuration was offered as a way to adapt an application so as to accommodate the AD8337, which is nominally a dual supply part. All ICs are actually single supply, where the supply is connected across a 'battery', however if the design uses a 'third' supply, e.g. a ground, then the ground functions as a reference for the plus and minus power supply connections. Think of  this reference as a common-mode voltage (Vcm).

    So the function of U2, the 2.5V reference and U3, the op-amp, is just to create a 2.5V common-mode voltage to replace the ground-reference. Since the AD8337 was characterized for +/-2.5V, the single supply eval board assumes a +5V supply but he supply could actually be as high as 10V, in which case the reference should be 5V.

    With that background in mind:

    A1. RFB1 is connected to the 2.5V reference so as to match the 0V Vcm condition.

    A2. Same as above.

    A3. R1 is there just to terminate a 50-ohm source reference to Vcm. You can remove C6 but the source Vcm would have to be 2.5V. Yes connecting Rfb to ground would create a problem.

    A4. Cg is just a by-pass to control ambient HF noise. It nominally transparent to the operation of the AD8337.

    Best,

    js

Reply
  • Hi Analord,

    Thanks for the email!

    The single-supply configuration was offered as a way to adapt an application so as to accommodate the AD8337, which is nominally a dual supply part. All ICs are actually single supply, where the supply is connected across a 'battery', however if the design uses a 'third' supply, e.g. a ground, then the ground functions as a reference for the plus and minus power supply connections. Think of  this reference as a common-mode voltage (Vcm).

    So the function of U2, the 2.5V reference and U3, the op-amp, is just to create a 2.5V common-mode voltage to replace the ground-reference. Since the AD8337 was characterized for +/-2.5V, the single supply eval board assumes a +5V supply but he supply could actually be as high as 10V, in which case the reference should be 5V.

    With that background in mind:

    A1. RFB1 is connected to the 2.5V reference so as to match the 0V Vcm condition.

    A2. Same as above.

    A3. R1 is there just to terminate a 50-ohm source reference to Vcm. You can remove C6 but the source Vcm would have to be 2.5V. Yes connecting Rfb to ground would create a problem.

    A4. Cg is just a by-pass to control ambient HF noise. It nominally transparent to the operation of the AD8337.

    Best,

    js

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