I'm trying to simulate the distortions of transimpedance amplifiers with TI OPs. I started with Neil Albaughs example of an OPA627 based transimpedance amplifier from the examples that come with TINA.
I'm somehow surprised, that after a simulation time of 1.1s (starting time for the FFT ist 1s) that I only get for an input current of 1uA, feedback resistor of 100k, frequency = 250Hz a decent SNDR - the second order harmonic is only around 70dB below the output signal. From the very low harmonic distortion of he OPA627 and the linear behaviour of the feedback of the transimpedance amplfier structure I would have expected more than 100dB. what s the reason for his poor result ?
I apologize for the delay in addressing your post. The internal structure of the spice model for the OPA627 does not simulate the part's total harmonic distortion characteristics, therefore any numbers given for THD in a simulation would not be accurate. As you said, I would expect the THD of the OPA627 operating as a transimpedance amplifier to be very low, however it would be best to predict the performance in terms of THD+N. The THD+N curve of the OPA627 can be broken into two parts, a noise dominated region and a distortion dominated region. In the noise dominated region, the noise of the circuit is greater than its non-linear distortion and the THD+N performance can be reasonably predicted by the output noise calculations.
While the noise of the transimpedance amplifier will be dependent upon its gain, I would expect the distortion to match the G=+1 curve shown above. The net effect is that the noise dominated region will extend much higher in frequency than it would for a typical voltage amplifier. I should note that the spice model for the part does accurately simulate the noise of the OPA627 so these simulations should be valid. A good primer on THD+N measurements was written by one of our characterization engineers, Jorge Vega, and can be found here: http://www.en-genius.net/site/zones/acquisitionZONE/technical_notes/acqt_013012
Analog Applications Engineer
PA Linear Apps
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