Chapter 5
Signal Generation Fundamentals
NI PXIe-5450 User Manual
5-10
ni.com
High-frequency components and layout techniques should be used
throughout to minimize parasitic effects.
Output Attenuation
Output attenuation is a method of controlling the output voltage level of the
signal being generated. NI signal generators typically generate signals with
a digital-to-analog converter (DAC) that has an output voltage range of
–5.0 V to +5.0 V with a number of bits of resolution. This signal is applied
to an attenuator that controls the output voltage of the signal source.
By attenuating the DAC output signal, you keep the dynamic range of the
DAC; that is, you do not lose any bits from the digital representation of the
signal because the attenuation is done after the DAC and not before it.
For example, if a DAC with a range of –5.0 V to +5.0 V and a resolution of
12 bits with each bit corresponding to 2.44 mV [(+5.0 – (–5.0)) / 2^12]
does not use output attenuation, and the desired signal is 2.0 V
pk-pk
(–1.0 V
to +1.0 V), waveform values can be generated with the DAC that only use
1
/
5
of the DAC range. The resolution of each digital bit is still 2.44 mV.
However, if the same DAC uses the output attenuation, the full range of the
DAC generates the signal, creating the signal at the full 10.0 V
pk-pk
. The
value of each digital bit is still the original 2.44 mV. The signal is applied
to an attenuator, which reduces the voltage level by a factor of 5 to
2.0 V
pk-pk
. The attenuator also reduces the value of each bit, which results
in an effective bit value of 0.488 mV at the analog output connector. The
attenuator allows the use of the full range of the DAC, and reduces the
effective value of each bit corresponding to the degree of attenuation.
