QuantAsylum QA401 User
’s
Manual
Page
36
A common measurement is to measure the noise floor of an output circuit. If the output circuit has a
voltage in the range of +/-5V, then the measurement can be made directly on the QA401 by applying the
voltage to the BNC input terminals without regard for attenuator setting. Most line-level opamp
amplifiers and battery powered headphone amps fall into this category. Even high-power class D
amplifiers will do a very good job of ensuring high-voltage transients are not present on the outputs.
Another common measurement that might be made is measuring the output noise of a power supply.
This requires careful consideration, because if the voltage exceeds 6 or 7V and the attenuator is
disengaged, then very high transient currents can flow inside the QA401 and potentially damage the
device performance.
The correct way to think about this is by the peak currents involved. If the attenuator is engaged, then
there is no concern about the peak currents and the +50V/-7.5V cap rating applies. If the attenuator is
NOT engaged, then a momentary current will flow through the input capacitor, through the 100 ohm
current limiting resistor through a protection diode clamp, and then it will be clamped to the +/- 6.5V
supply rails, limited by the 100 ohm resistor.
The max clamp current should be limited to 10 mA. If an external DC voltage of +/-7.5V is applied, this is
roughly a 10 mA transient current. In other words, measuring a power supply’s noise with an o
utput
greater than 7.5V requires special consideration.
The easiest way to solve this is to use the built-in attenuator. Even at +50V DC (the limit of the input
capacitor), the attenuator will limit peak transient currents. However, if you are concerned about
forgetting to engage the attenuator OR you need a noise floor that is better than the noise floor offered
with the attenuator engaged, then an external current limit resistor can solve the problem.
As an example, to measure a 15V power supply, we want to use a (15-6.5) / 0.01 = 850 ohm resistor in
series with the input. Rounding up to 1K is fine, because the thermal noise from a 1K resistor in a 20K
bandwidth will be about 0.6 uVrms, and a really good power supply output noise value in the same
bandwidth will be 15 uVrms.
FOR MEASURING THE NOISE OR SPECTRUM OF EXTERNAL HIGHER VOLTAGE OUTPUTS (GREATER THAN
5V IN MAGNITUDE OF DC BIAS), MAKE SURE A CURRENT LIMITING RESISTOR IS USED IN IN SERIES WITH
THE INPUT TO LIMIT THE MOMENTARY CURRENTS TO NO MORE THAN 10 mA. MISTAKES HERE ARE NOT
COVERED UNDER WARRANTY.
THD
The THD measurement is enabled by pressing the THD button.
The THD measurement will measure the ratio of all the harmonics of the fundamental (up to 100
harmonics maximum). Note that there are 3 ways to specify the fundamental frequency. You can use
the settings on the first signal generator, or the highest detected peak, or a specific frequency. These are
covered in the context settings below.
