THP210: THP210 Open-Loop Output Impedance

Hao Jie Chan,

I agree that the open-loop output impedance in the data sheet and simulation models have some inconstancies.  I will summarize below.  

Summary of the open-loop output impedance on THP210

1. The data sheet curve 6-19 is the differential output impedance.  This is the most accurate source of information in the data sheet.  This curve is typical performance from the IC designers.
2. At 100kHz curve 6-19 is approximately 8 ohms.
3. The data sheet table indicates that the differential open-loop output impedance is 16 ohms.  This needs to be updated to match the graph.
4. The simulated open loop output impedance is 8.5 ohms at 100kHz.  This matches the curve.
5. The simulation model does not perfectly match the graph.  This isn't a major issue as the low frequency Zo generally does not matter for circuit stability.  The frequency where loop gain goes to zero is really what matters for stability.  For this device that would be for frequencies above 10kHz.  For this frequency range the model matches the curve well.
6. Thus, for your stability simulations the THP210 model should work fine.  I will let the systems team know that the data sheet table should be updated.
7. Below is the simulation to check differential output impedance for an FDA.  I think you are doing this simulation correctly as you correctly identified the open-loop output impedance to be 8 ohms at 100kHz.  Sorry for the confusion.

differential open loop zo on FDA.TSC

Best regards, Art