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Entry  Sun Apr 10 19:14:08 2011, Koji, Update, Green Locking, New Green PD test1 SGD444A_240.pngSGD444A_240_test.png
    Reply  Mon Apr 11 09:49:53 2011, rana, Update, Electronics, New Green PD test1 
       Reply  Mon Apr 11 11:34:05 2011, Koji, Update, Electronics, New Green PD test1 SGD444A_240_test.png
          Reply  Mon Apr 11 21:13:15 2011, Koji, Update, Electronics, New Green PD test1 
Message ID: 4508     Entry time: Mon Apr 11 11:34:05 2011     In reply to: 4507     Reply to this: 4513
Author: Koji 
Type: Update 
Category: Electronics 
Subject: New Green PD test1 

Quote:

 Ooh. Can you explain the purpose of the resistors which are connected to the (+) inputs? It looks like some real electronics ninjitsu.

51 Ohm for CLC409

The datasheet of CLC409 uses 25Ohm there. This is to cancel the input bias current of the two inputs of the opamp.

The source impedance (series) of SGD444 is 50Ohm. So I used 50Ohm for the + input shunting.

However, I could probably use anything between 0-50Ohm as the datasheet itself tells that the bias currents are
not related between the two inputs. In addition, I am not sure how much the real series resistance of the PD is.

1kOhm for OP27

This resister is to ensure the (+) input to have a high impedance at high frequencies.

As far as OP27 is behaving as an ideal opamp, the (+) input has a high impedance.
Also if the inductor behaves as the ideal inductor, no photocurrent comes to the AF path.

However, if both of the op27 and the inductor show similar impedances to the RF transimpedance of 240Ohm,
the AF path absorbs some photocurrent and affects the RF transimpedance of the RF output.

We know that the inductor has a self resonance where the shunt capacitance take over the impedance of the coil.
Above that frequency, the inductor is no longer the inductor. The self resonant freq of this inductor is ~300MHz. It is OK, but not
too far from the freq of interest if we like to see clear cut off at around f>100MHz.
Also OP27 is an AF amplifier and I had no confidence about the input impedance of the OP27 at 100~300MHz.

If I put 1k in the (+) input of the OP27, I can ensure the entire AF path has the impedance of ~1k (at least 500Ohm even when L and OP27 are shorted).
I think the chip resister easily works as a resister up to 1GHz.

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