Maximizing resolution when generating standard function waveforms with a PCI-5421

[TEP]

My application requires high resolution generation of DC voltages, sine waves, and sine waves with offset.  I understand that the output range of the 5421 is a function of both the load impedance and the output impedance, and that the 5421 boards implement standard functions in software.  Let's say that my load impedance is relatively high so that the output range is +/- 10V. 

1) If I generate a sine wave with an amplitude of 2V pk-pk, will the wave be produced with less than full DAC resolution or does the board adjust the various gain/attenuation stages to produce the wave with full DAC resolution?

2) If the output range for my load is +/- 10V, for example, but I will only be using amplitudes of +/- 2V or less, what is the best way to limit the output range (maximize resolution) or is this not necessary to achieve full DAC resolution?  Is it recommended to control the output range by how I set the load impedance value?

3) How can I optimize the resolution of the DC offset value? 

[Drew Creel]

Hello,

1)  On the 5421, the entire 16-bit DAC range will always be used in standard function mode.  The amplitude and offset are acheived by the amplifiers and attenuators in the analog path of the device (after the DAC).  If you ask for a 4Vpk-pk sine wave, the driver will download a sine wave to the board with full DAC range (values ranging from -32768 to 32767) and will set the gain of the analog path to 2.0.  Similarly, if you want to add .1V of offset to that, the offset will be added by the offset circuitry in the analog path, after the main DAC, so the data downloaded by the driver would be the same.

2)  I'm not sure what you mean by "output range of the load" - I'm guessing you're referring to the maximum voltage limits that load can accept?  At any rate, they do not affect the resolution or DAC range in any way.  I'd recommed setting the load impedance to something large in NI-FGEN so the driver will correctly compensate when you ask it to generate 2Vpk-pk. 

3)  The offset resolution is tied to the amplitude (the offset circuitry actually comes *before* the amplifier in the analog path).  The offset is limited to +/- 1/4 of the amplitude (pk-pk), so the resolution will scale with the amplitude.  Your highest resolution will be when MaxOffset/Amplitude = 1/4.

Hope that helps,

Drew Creel

NI Software Engineer

Signal Generators Group

[TEP]

Drew:

Thank you for the detailed response.   This is very helpful.

I'm very glad to hear "On the 5421, the entire 16-bit DAC range will always be used in standard function mode."  So, if I understand this correctly, the gain of the analog path will be set based on the amplitude I request and what I have set for the output impedance and load impedance.  Is this correct?

As to the "output range of the load", I should have stated "the maximum output voltage range for the load".  But, this question is moot if the board always uses full DAC resolution for standard waveforms.

I also should have stated my offset question better.  This is based on my need to generate DC voltages as well as sine waves.  When I request a DC voltage (Offset) in Labview, the Amplitude and Frequency values are not used.  So, what is the amplitude value in "Your highest resolution will be when MaxOffset/Amplitude = 1/4."?  Is the amplitude the maximum output voltage for the given output impedance and load impedance?

Thanks for your help on this.  I have been through the help files and the specs but couldn't find this clearly stated.

[Drew Creel]

Yes, you are correct about the analog path - the amplifier gain is set based on your amplitude and impedance settings, while the data is always the same (full range).

Ok, I understand your offset question.  When generating a DC waveform, the driver sets the amplitude to 12Vpk-pk (or 24V in high impedance) and uses the analog offset circuitry for the DC value - the data fed to the DAC is all zeroes.  Due to our 1/4 rule, this means the maximum offset you can attain is 3V (6V in high impedance).  If you need a higher DC voltage, you can use the User-Defined waveform type, download data consisting of all "1"s, and set the amplitude and offset accordingly to get the appropriate value.  You can use this method for higher resolution offset values as well.

For example, if you want to sweep the offset voltage from 8V to 10V into a high impedance load, you would:

1)  Download a user-defined waveform of all "1"s

2)  Set the amplitude to 8V

3)  Sweep the offset from 0V to 2V.

Drew Creel

NI Software Engineer

Signal Generators Group

[TEP]

Drew - Thank you.  You have been very helpful.