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The Power VIs are custom designed by RI to provide up to 3 Amps into a capacitive load of less than .1uF. This gives a 2us rise and fall and an output impedance of 5 ohms or less.



Some customers have requested a lower output impedance to prevent oscillations in their devices. This is usually arrived at by adding lots of capacitance to the DUT board. This has the additional side effect of slowing down the measurements as well as causing the VI to become unstable.

A better solution is to use the circuit below to get the lower impedance while maintaining the speed and preventing the VI from oscillating. R1 is a large wattage resistor and is used to isolate the capacitor from the VI. C1 is a ceramic ( not a Tantalum). Tantalum's have resistances of 1 -2 ohms so it would defeat the goal of low impedance. The Dut is across C1.




This photo shows the impedance the DUT sees from DC to 500KHz. The dim trace is this circuit while the other is the capacitor alone. the scale is .5ohms /div. As you can see below 100KHz the VI takes over actually giving better performance than any size of Tantalum cap.





The next photo shows the pulse performance of the circuit. The rise and fall now requires 15 us to settle



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