I'm using LTspice XVII (newest version), the simulation model of ADA4625-1 I downloaded from AD's official site.
I'm going to design an inverting voltage amplify circuit with a gain of -10.
The simulation circuits are as below:
- Open-loop frequency response test, the result is consistent with the datasheet, with a 0dB cutoff frequency greater than 10MHz. And it's similar to a single-pole ideal opamp with -20dB/dec over a wide frequency range.
- A model I built for the feedback loop.
ADA4625-1's input capacitance is 20pF (11pF of CM, 9pF of DM), and I added a 10pF stray capacitance of PCB.
To compensate for the pole generated by Cin and Ri, I added a zero by feedback Capacitance, Cf = 0.1 * Cin.
The AC result of Noise gain is plotted below marked as red lines.
And the frequency of the junction is about 1.25MHz, which I think is corresponded to the -3dB cut-off frequency of the closed-loop.
But the closed-loop simulation shows a much lower -3dB(20-3=17dB) cut-off frequency of 50kHz.
Which can't satisfy my need for closed-loop bandwidth.
Please tell me what's the problem, is my simulation model not accurate, or is there some bug in ADA4625-1's SPICE model?
If my simulation circuit is wrong, please tell me how to fix it.
Thanks a lot!