Measuring Dynamic Range in APx500

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Created on 2008-07-11 13:17:00

Question:
How can I measure Dynamic Range with an APx instrument?

Answer:
Dynamic range is the ratio between the full scale output of a device and the spurious noise products created when the device is producing a very low level signal. It is a commonly made measurement for DAC's, ADC's, and other digital audio devices.

Dynamic range is usually measured by stimulating a device with a full scale sine wave and measuring its output level, then stimulating the device with a sine wave that is -60 dB below full scale and measuring the level of the noise and distortion products that remain after the fundamental is removed from the output of the device with a notch filter. The dynamic range is the ratio of the two, usually expressed in dB.

You can make this measurement with an APx as follows:

  1. First, configure the connections to the device under test using Signal Path Setup. The APx515, 525, 526, 585, and 586 can work with any combination of digital or analog, input or output devices.
     
  2. Switch to the Reference Levels view to find the full scale output level for the device:
    1. If the device has a digital input, stimulate the device with a 0 dBFS signal and set the input reference dBrA to the device's output level. If the device has adjustable gain, you would normally adjust the gain until the output is just below clipping. You can find this level using the "Scope" or time domain signal monitor view.
    2. If the device has an analog input, adjust the generator level until the output clips, or (for digital outputs) reaches 0 dBFS. Now set the generator reference dBrG to the generator level and the input reference dBrA to the device's output level.
       
  3. Now switch to the THD+N Level result in the THD+N measurement view:
    1. If the device has a digital input, set the generator to -60 dBFS. If the device has an analog input, set the generator to -60 dBrG.
    2. If desired select a low pass bandwidth limiting filter (20 kHz is the default) and a THD+N weighting or high pass filter.
    3. Set the units in the THD+N level result to dBrA.
    4. Turn the generator on. The absolute value of the measurement is the Dynamic Range of the device. For example, if the measured result is -90 dBrA, you would report a Dynamic Range of 90 dB.