NI 5442
Waveform Amplitude Control

The NI 5442 uses both amplifiers and attenuators to achieve needed amplitude settings.

Output Paths and Amplifiers

The following figure shows two different gain paths: the Direct path and the Main path.

The Direct path provides the output of the main DAC to the CH 0 with the fewest electronic components in the path. There are no programmable amplifiers and no method for adding DC offset to the waveform. The Direct path can generate a maximum of 1 V_pk-pk at the CH 0 output into matched load impedance. The maximum gain setting for an Analog Output path configured to the Direct path is 0.527 (gain is a unitless value).

The Main path has a 2 V_pk-pk amplifier and is used for waveforms that have all output voltages equal to or smaller than 2.0 V_pk-pk into matched load impedance. Refer to either the Arbitrary Waveform Gain or Amplitude property or the NIFGEN_ATTR_ARB_GAIN or NIFGEN_ATTR_FUNC_AMPLITUDE attribute for more information about configuring the gain.

In addition, the DC offset amplifier, which adds DC offset to the signal, is located in the High-Gain and Low-Gain Amplifier paths prior to the attenuators and amplifiers. The DC offset amplifier can be fine-tuned to add offset to your signal. This fine-tuning of the main DC offset amplifier is performed by the offset DAC.

NI 5442 Attenuation

The Analog Output path has two passive attenuation sections. Preamplifier attenuation is prior to the amplifier, and postamplifier attenuation is after the amplifier. In addition, the main DAC provides 0 to 3 dB of signal attenuation. The amplitude control is implemented after the DAC. Attenuating the DAC output signal allows you to vary your signal amplitude and while maintaining the dynamic range of the DAC. You do not lose any bits from the digital representation of the signal as does the method of controlling amplitude by using smaller data ranges of the DAC, sacrificing dynamic range. Passive attenuation by pre-amplifier and post-amplifier attenuation is not available if the Direct path is selected.

For the Main path, maximum attenuation is 51 dB. For the Direct path, maximum output attenuation is 3 dB. NI-FGEN automatically determines the correct value of attenuation in dB and configures the attenuation based on the set gain. The minimum gain setting for the Direct path is 0.354. The minimum allowable gain setting with NI-FGEN automatically selecting the Main path is .00282 (gain is a unitless value).

NI-FGEN calculates and sets the correct amount of attenuation required that corresponds to your NI-FGEN gain setting. The correct amount of attenuation is implemented in the preamplifier and postamplifier attenuation blocks to best achieve the desired output signal amplitude. You can set the amount of gain with the Arbitrary Waveform Gain property or the NIFGEN_ATTR_ARB_GAIN attribute.

Pre-Amplifier Attenuation

The preamplifier attenuation section is located before the amplifier in the Main path. The attenuators provide a range of attenuation from 0 to a maximum of 12 dB in steps of 3 dB. NI-FGEN automatically controls the value of attenuation set in the preamplifier attenuation section depending on the set gain.

Preamplifier attenuation improves the signal distortion because amplifiers provide lower distortion performance with smaller signals. However, attenuation lowers the amplitude of both the signal and the noise in a signal as the signal-to-noise ratio (SNR) is unchanged upon attenuation. Amplifiers also have a fixed noise associated with them. The total noise at the amplifier output is obtained by taking the root of the sum of squares of the following factors:

• The input signal noise multiplied by the gain of the amplifier
• The amplifier noise

The total noise is dominated by the larger factor. If the signal is attenuated so that its noise when multiplied by gain at the amplifier input is smaller than the amplifier noise, then the output has a higher SNR. This higher SNR is a good reason to implement some of the AWG overall attenuation as preamplifier attenuation.

Post-Amp Attenuation

The post-amplifier attenuation section is located after the amplifier in the Main path. The attenuators provide a range of attenuation from 0 dB to a maximum of 36 dB in steps of 12 dB. NI-FGEN automatically controls the value of attenuation set in the postamplifier attenuation section dependent on the set gain.

DAC Attenuation

The main DAC output can be fine-tuned for attenuation, which provides 0 to 3 dB of the Analog Output path signal attenuation. This fine-tuning of the main DAC attenuation is performed by the Gain DAC. The main DAC also provides the fine resolution for the attenuation settings.

Summary of Gain Settings

The following table summarizes the maximum and minimum gain setting that you can apply for the NI-FGEN Analog path options. Refer to the NI 5442 specifications for more information about gain resolution.

Note  Digital gain is applied to the digital data before the data is passed to the DAC. Because relays are not used, digital gain allows glitch-free gain control at the expense of dynamic range.