NI 5402/5406
Waveform Amplitude Control
The NI 5402/5406 uses both amplifiers and attenuators to achieve needed amplitude settings.
Output Paths and Amplifiers
The following figure shows two gain paths: the High-Gain Amplifier path and the Low-Gain Amplifier path.
The Low-Gain Amplifier path has a 2 Vpk-pk amplifier and is used for waveforms that have output voltages equal to or smaller than 2.00 Vpk-pk (1.67 Vpk-pk for sine waveforms) into matched load impedance. The High-Gain Amplifier path has a 10 Vpk-pk amplifier and is used for waveforms that have output voltages greater than 2.0 Vpk-pk (1.67 Vpk-pk for sine waveforms) into matched load impedance. The gains of the amplifiers are constant. Refer to the Amplitude property or the NIFGEN_ATTR_FUNC_AMPLITUDE attribute for more information.
By default, NI-FGEN automatically selects the High-Gain and Low-Gain Amplifiers based on the amplitude setting. You can override NI-FGEN and configure the High-Gain or the Low-Gain Amplifier to remain in the Analog Output path regardless of amplitude by setting the Analog Path property or the NIFGEN_ATTR_ANALOG_PATH attribute. Configuring the Low-Gain Amplifier path to remain constant regardless of the amplitude setting affects the maximum output value allowable for that particular amplitude setting. The maximum amplitude setting for an Analog Output path configured to Low-Gain Amplifier path is 2.0 (1.67 Vpk-pk for sine waveforms). The maximum allowable amplitude setting with NI-FGEN automatically selecting the Gain Amplifier path is 10.0 V with a high load impedance.
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.
Square Wave Path
The NI 5402/5406 uses dedicated hardware to generate low-jitter square wave functions. A comparator is fed two signals: a sine tone from the main DAC (using the Digital Filter and the Analog Filter to obtain a 400 MS/s signal) and a DC signal using the Level DAC.
The output of the comparator is switched back into the Main path in order to obtain the desired amplitude and offset. The fine gain control of the signal is controlled through the Square Wave Gain DAC.
When the signal generator is configured to generate a square waveform, NI-FGEN automatically selects the Square Wave path. When the device is idle, it generates the low state of the square waveform.
When using the Square Wave path, the DC Offset can be ±50% of the current Amplitude.
The SYNC OUT is generated through the same comparator and is routed by default to the SYNC OUT/PFI 0 connector. You can unroute this route or change the output terminal of SYNC OUT using the niFgen Export Signal VI or the niFgen_ExportSignal function.
NI 5402/5406 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.
For the Low-Gain Amplifier and the High-Gain Amplifier paths, maximum attenuation is 51 dB. NI-FGEN automatically determines the correct value of attenuation in dB, and configures the attenuation based on the set amplitude. The minimum amplitude setting for an Analog Output path configured to High-Gain Amplifier path with a 50 Ω load impedance is .01691 V. The minimum allowable amplitude setting with NI-FGEN automatically selecting the Low-Gain Amplifier path with a 50 Ω load impedance is .005635 V.
NI-FGEN calculates and sets the correct amount of attenuation required that corresponds to your NI-FGEN amplitude 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 amplitude with the Amplitude property or the NIFGEN_ATTR_FUNC_AMPLITUDE 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 amplitude. You can read the value NI-FGEN has selected for Pre-Amp Attenuation using the Pre-Amplifier Attenuation property or the NIFGEN_ATTR_PRE_AMPLIFIER_ATTENUATION attribute.
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 signal generator overall attenuation as Post-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 amplitude. You can read the value NI-FGEN has selected for Post-Amp Attenuation using the Post-Amplifier Attenuation property or the NIFGEN_ATTR_POST_AMPLIFIER_ATTENUATION attribute.
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. You can adjust the Gain DAC using the Gain DAC Value property or the NIFGEN_ATTR_GAIN_DAC_VALUE attribute. The main DAC also provides the fine resolution for the attenuation settings.
The fine gain control of square wave signals is controlled through the Square Wave Gain DAC. You can adjust the Square Wave Gain DAC by setting the Gain DAC Value property or the NIFGEN_ATTR_GAIN_DAC_VALUE attribute.