NI 5421/5422/5441
Waveform Amplitude Control

The NI 5421/5422/5441 uses both amplifiers and attenuators to achieve needed amplitude settings.

Output Paths and Amplifiers

The following figure shows three different gain paths: the Direct path, the High-Gain Amplifier path, and the Low-Gain Amplifier 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 of 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 Direct path is 0.527. Gain is a unitless value.

The Low-Gain Amplifier 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. The High-Gain Amplifier path has a 12 V_pk-pk amplifier and is used for waveforms that have output voltages greater than 2.0 V_pk-pk into matched load impedance. The gains of the amplifiers are constant. NI-FGEN automatically selects by default between the high-gain and low-gain amplifiers, depending on the NI-FGEN gain setting. 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.

You can configure the High-Gain or the Low-Gain Amplifiers to remain in the Analog Output path regardless of gain setting. Refer to the Analog Path property or the NIFGEN_ATTR_ANALOG_PATH attribute for more information. Configuring the Low-Gain Amplifier path to remain constant regardless of the gain setting affects the maximum output value allowable for that particular gain setting. The maximum gain setting for an Analog Output path configured to Low-Gain Amplifier path is 2.0. The maximum allowable gain setting with NI-FGEN automatically selecting the Gain Amplifier path is 12.0. Gain is a unitless value.

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

NI 5421/5422/5441 Attenuation

The Analog Output signal path has two passive attenuation sections. Pre-Amp Attenuation is prior to the High-Gain and Low-Gain Amplifier paths, and Post-Amp Attenuation is after the High-Gain and Low-Gain Amplifier paths. In addition, the main DAC provides 0 dB 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 still maintain 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-Amp and Post-Amp Attenuation is not available if the Direct path is selected.

For the Low-Gain Amplifier and the High-Gain Amplifier paths, maximum attenuation is 51 dB. For 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 an Analog Output path configured to Direct path is 0.354. The minimum gain setting for an Analog Output path configured to High-Gain Amplifier path is .01691. The minimum allowable gain setting with NI-FGEN automatically selecting the Low-Gain Amplifier 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 Pre-Amp and Post-Amp Attenuation sections 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-Amp Attenuation

The Pre-Amp Attenuation section is before the Low-Gain and High-Gain amplifiers. The attenuators provide a range of attenuation from 0 dB to a maximum of 12 dB in steps of 3 dB. NI-FGEN automatically controls the value of attenuation set in the Pre-Amp Attenuation section depending on the set gain.

Pre-Amp Attenuation improves the distortion of a signal 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 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 signal-to-noise ratio. This is a good reason to implement some of the AWG overall attenuation as Pre-Amp attenuation.

Post-Amp Attenuation

The Post-Amp Attenuation section is after the High-Gain and Low-Gain Amplifiers. 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 Post-Amp Attenuation section dependent on the set gain.

DAC Attenuation

The main DAC output can be fine-tuned for attenuation, which provides 0 dB 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 four NI-FGEN Analog path options. The default path is automatic. NI-FGEN automatically sets the path and correct amount of attenuation depending on the configured gain setting. These values assume a matched load impedance. Refer to the NI 5421, NI 5422, or the NI 5441 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.