Output Impedance

NI signal generators have an output impedance of 50 Ω and an optional 75 Ω on select devices.

If the load impedance matches the output impedance, the voltage at the signal output connector is at the needed level. The voltage at the signal output connector varies with load output impedance, up to doubling the voltage for a high impedance load, as shown in the following figure.

In the previous figure, the required output voltage is ±5 V. The NI signal generator internal voltage source generates twice this voltage, ±10 V, expecting the voltage to be halved due to the voltage divider formed by R_O and R_L. The relationship between the output voltage and the load can be calculated as follows:

V_out = [R_L/(R_L + R_O)] × [V_S]

where
	Vout = the voltage level delivered to RL
	RL = the load impedance in ohms
	RO = the output impedance on the NI signal generator
	VS = the voltage level generated by the source previous to RO

By default, R_O = 50 Ω, but you can use NI-FGEN to set it to 75 Ω on select devices. Also, with some devices you can enter the value of R_L, and the device changes V_S accordingly to deliver the requested V_out.