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# LTC6269-10 Supports 100 ohm feedback resistance

I am building light balancing circuit.  For higher bandwidth, I need lower feedback resistance (i.e) 100 ohm.  When I use feedback resistance of 100 ohm and feedback capacitance of 2 pF. I am facing ringing effect. So, how do we select the capacitance according to the resistor. Will the opamp IC LTC6269-10 works with the feedback resistance 100 ohm. I am attaching my circuit.

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• Hi Vilashini,

The resistor has nothing to do with it, look carefully at this condition:

Cf and Cin capacitors set the noise gain at high frequencies. This condition is not met in your circuit

Regards,

Kirill

• CIN = CPD + CCM + CDM

CPD = Photodiode capacitance

CCM = Common Mode Capacitance of Amplifier

CDM = Differential Mode Capacitance of Amplifier

CIN = 0.25 pF + 100 fF + 450 fF

= 0.8 pF

CIN/Cf >= 10

0.8 pF / 0.025 pF = 32 which is greater than 10. But  I am facing same type of error.

• Hi,

"The LTC6269-10 has a “Gain-Bandwidth-Product” of 4GHz and a minmum Gain-of-10 requirement for stability.  That means that in a gain of 10 it will have about 400MHz of bandwidth.  There is no way to get 3GHz of bandwidth in it.

I think the only amplifier we have that could support this design is the LTC6409.  It has 10GHz of GBWP, and is unity gain stable.   However, I tried this in LTspice and the results are not promising.  See attached.

You mentioned the input bandwidth is 3GHz.  But do you really need 3GHz of output bandwidth?  Photodiodes have extremely high reception bandwidth, but you can remove the charge slowly and not suffer losses.  If this is a split photodiode app, for positioning purposes, then any mechanical system will be much slower and does not need such high bandwidth.   So can the customer reduce the output bandwidth requirements?  I once did a control loop for a 1GHz data bandwidth system, where the position control loop bandwidth (on a 4-way PD) was only 2kHz.   It used LTC6081’s for their precision."

3GHz TIA attempt.asc

• Hi,