looking for DAC that will amplify voltage coming from PC via SCB -68 box to HW

[johnsold]

A DAC does not amplify.  Internally a DAC is a voltage divider with many taps.  The digital inputs (which you seem to be interpreting as the signal to be amplified) control switches (transistors) which select the divider taps to be connected to the output.  The output of the DAC is usually expressed as Vreference*(Ndigital/Nmaximum) where the N values are integers.  The output voltage is always less than or equal to the reference voltage.  Some DACs output current rather than voltage.  Some have internal references and others require an external reference source.

* Upon power supply of 24 V I got for .2mv input,  20-23 V output, but when i increased power supply to 35 V then output went to 500mv

Something in your simulation must be causing limiting as the power supply increases.  Look at the model of the amplifier  you are simulating to see if it has a source which defines the maximum power supply.

** load requires 0 - 42Vrms,  so I can see the loads behavior,  I will post current load tomorrrow.  

** Frequency is at 1kHz

** Amplitude is due to user input in labview, phase is variable. 

42 Vrms for a sine wave is  120 Vpp.  To get a reasonable sine wave at 1 kHz the DAC sampling rate should be much faster.  The 6024E series is specified at a maximum DAC update rate of 10 kHz using DMA, system dependent, or up to 1 kHz via interrupts.  Most likely you will get a step-like approximation to a sine wave at best.  You may need a faster DAC device if you want a good approximation to a sine wave.

I would recommend that you look at a consumer grade audio amplifier.  They are quite inexpensive and will amplify 1 kHz just fine.  An amplifier rated 220 watts at an 8 ohm load will deliver 42 Vrms.  If your load resistance is higher than 8 ohms a somewhat smaller amplifier may work, but these are typically not designed for high voltage outputs.  I have used audio amplifiers for several projects where the computer generated signals which then needed to be amplified.

Lynn 

[Coq_Rouge]

johnsold  Have some good points here. I also forgett to mention in my last post that a transformer my solve your problem. If you selct to use a transformer an audio amplifier may be the perfect driver circuit. But in such a setting be carefull to avoid DC input. It will fry your driver very fast (I know from learning)

[Dennis_Knutson]

The actual update rate would be much less than 10 kHz if the op insists on designing his own DAC. There are only 8 digital outputs from the 6024E that could be wired to the digital inputs of this mythical Digital to Analog Converter. The digital I/O of the 6024E is software timed and under the best of circumstances, a 1 kHz rate is about the best you can get.

[douread]

Bingo, yes I don't need > 25mA. In fact at at 42.1Vrms the current is 15.8mA. Do you think I can get samples so I don't have to worry about the quantity issue ?

I was approaching my design with op-amps but cascading them to amplfy was a headache. That is why my advisor mentioned the DAC approach.

It is interesting and if I had more time, I would be curious to see how it functions 🙂

Thank you so much

[AnalogKid2DigitalMan]

Your Gain-Bandwidth product is quite low, this could be done with a single high voltage op amp, or a convential op-amp driving a transistor in linear mode as an output stage.

-AK2DM

[johnsold]

Let's look at the basics.

1. Can you get a 1 volt signal at 1 kHz out of your 6024E with your computer?  Hook an oscilloscope to the Analog Output connection on your SCB68.  Use an Analog Out Example VI if you do not have a VI of your own which works.  Can you vary the amplitude and phase as you need for your project, say from 1 V to 0.1 V?

This will determine whether you can generate a signal from LabVIEW using your DAQ device which meets the frequency, phase, and amplitude control requirements for your project.

2.  Is your load resistive or reactive?   Put another way, is the 15 mA in phase with the 42 V?

This will determine the type of amplifier (or high voltage DAC) you will need. 

3.  What is the waveform requirement for your signal?  If it is a continuous signal like a sine wave, how good does the DAC approximation need to be?

This will determine what kind of DAQ board or alternate DAC design will be required if the answers to question 1 indicate that you need something better than what you have now.

Lynn 

[douread]

1. Can you get a 1 volt signal at 1 kHz out of your 6024E with your computer?  Hook an oscilloscope to the Analog Output connection on you rSCB68.  Use an Analog Out Example VI if you do not have a VI of your own which works.  Can you vary the amplitude and phase as you need for your project, say from 1 V to 0.1 V?

**  Yes at 1 KHz  I can get a 1 volt signal in      6424E >> SCB 68 box >> mutimeter

**  Yes I see readings in the multimeter varying as I vary amplitude.  

2.  Is your load resistive or reactive?   Put another way, is the 15 mA in phase with the 42 V?

** Yap the 15 mA is in phase with the 42 V

** The following table shows corresponding mA to Vrms, collected from a hardware I am trying to mimic in labview and my own (opamp / DAC ) circuit.

    Vrms  >>  6.1     9.9    15.2   19.9     25.0      29.9      35.2       40.0       45.1       50.2        55.1

    mA     >>  1.5     2.0      3.2     4.7       6.4        8.6      11.5       14.4       17.9       21.8        26.4

3.  What is the waveform requirement for your signal?  If it is a continuous signal like a sine wave, how good does the DAC approximation need to be?

** Sine wave, the best DAC approximation I can get.

[Coq_Rouge]

As someone has mentioned before your daq unit may not be the best choice for signal generation. If you only need a simple sine wave output with constant amplitude, maybe you should go for an analog oscillator. You can use the daq output to control a “Voltage Controlled Amplifier” based on sampled values. This way you only have to sample two signals and not worry about signal generation. I think your regulator may work more smoothly this way. My suggestion is only doodling, it is not a proven solution. It may work or maybe not. Anyway I have made a simple drawing. You can Google the term “Voltage Controlled Amplifier” (VCA) for mere information on that topic. Both Analog devises and TI make such circuits or you can make your own. Regarding the high voltage amplifier I guess you can call local Texas Instruments Sales Office or Authorized Distributor. Tell them you are a student and a small number of components. I am sure they may help you out some way. And yes both you and your advisor, even fellow students may order free samples (for your use). No problem at all. Rember also the GBW product. The circuit I gave you has a GWB=2.5MHz. But this is at unity gain. At gain=4 you only have a 0.625 MHz bandwidth. Sad but true

Message Edited by t06afre on 12-17-2008 07:24 AM