Hi All
I'm modeling an example photodiode (transimpedance) amplifier in LTspice using the ADA4505 from Analog Devices. The theory suggests that the location of the first pole (actually a double pole) will be located at frequency \(f_{\rm fp}\) where
\(f_{\rm fp} = \sqrt{\frac{f_{\rm c}}{2\pi R_{\rm f} C_{\rm i}}}\)
where \(f_{\rm c}\) is the crossover frequency of the opamp, \(R_{\rm f}\) is the feedback resistance of the transimpedance amplifier, and
\(C_{\rm i} = C_{\rm j} + C_{\rm id} + C_{\rm icm}\)
where \(C_{\rm j}\) is the photodiode's junction capacitance, \(C_{\rm id}\) is the differential input capacitance of the opamp, and \(C_{\rm icm}\) is the common-mode input capacitnce of the opamp.
From the ADA4505 datasheet; GBP = 50kHz, \(C_{\rm id}\) = 2.5pF, \(C_{\rm icm}\) = 4.7pF. From the photodiode datasheet \(C_{\rm j}\) = 5pF.
Using a feedback resistance \(R_{\rm f}\) = 100M, and substituting these values into the above equation yields a first pole frequency \(f_{\rm fp}\) at 2,554Hz. However, the equivalent simualtion in LTspice appears to show a first pole frequency (peak) at 320Hz (see attached) - an order of magnitude less.
I'm modeling an example photodiode (transimpedance) amplifier in LTspice using the ADA4505 from Analog Devices. The theory suggests that the location of the first pole (actually a double pole) will be located at frequency \(f_{\rm fp}\) where
\(f_{\rm fp} = \sqrt{\frac{f_{\rm c}}{2\pi R_{\rm f} C_{\rm i}}}\)
where \(f_{\rm c}\) is the crossover frequency of the opamp, \(R_{\rm f}\) is the feedback resistance of the transimpedance amplifier, and
\(C_{\rm i} = C_{\rm j} + C_{\rm id} + C_{\rm icm}\)
where \(C_{\rm j}\) is the photodiode's junction capacitance, \(C_{\rm id}\) is the differential input capacitance of the opamp, and \(C_{\rm icm}\) is the common-mode input capacitnce of the opamp.
- Is the crossover frequency \(f_{\rm c}\) of an opamp the same as its gain bandwidth product (GBP)?
From the ADA4505 datasheet; GBP = 50kHz, \(C_{\rm id}\) = 2.5pF, \(C_{\rm icm}\) = 4.7pF. From the photodiode datasheet \(C_{\rm j}\) = 5pF.
Using a feedback resistance \(R_{\rm f}\) = 100M, and substituting these values into the above equation yields a first pole frequency \(f_{\rm fp}\) at 2,554Hz. However, the equivalent simualtion in LTspice appears to show a first pole frequency (peak) at 320Hz (see attached) - an order of magnitude less.
- Any ideas what I'm missing or doing wrong?
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