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:
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.