lionelgreenstreet@[EMAIL PROTECTED]
wrote:
> If i consider an opamp in corrent-to-voltage configuration with:
> -ideal generetor current in inverting input
> -non inverting input to ground
> -feedback resistance equal to 100Mohm
> -open loop gain egual to A=128dB
> -capacitance common mode plus differential mode equal to 3pf
> i'm looking for pole frequency that shoud be equal to A0/(2*pi*C*Rf)
> 120dB=20LogA0=>A0=2.512e6
> fp=2e6/(2*pi*3e-12*100e6)
> but this frequency is too high for this opamp (http://focus.ti.com/
> docs/prod/folders/print/opa128.html) , so i think that i've made some
> errors...what's wrong?
Your question is obfuscated in details.
Real op-amps are not ideal, and your model is missing im****tant pieces.
Most im****tantly, nearly all op-amps available today are internally
compensated, which means that the dominant pole is determined by an RC
pair thats inside the amp, and is not something that you can change
short of changing the part number of the op amp you're using.
Second most im****tant, I don't know of any op-amps that are going to
operate realistically with a 100Mohm feedback resistor. There may be
new, ultra-low-power ones that could do this, but that seems awfully low.
Finally (I don't know how much im****tance to attach to this), I don't
think your A0/RC calculation means anything much in particular, so I
don't think it's going to be much use to you.
I think you need to understand op-amp design better, or get yourself a
better cookbook.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
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