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

[douread]

Hello

 I am working on a project where I have to apply some voltage up to 40 Vrms using labview to a hardware.

LabView >> daq card >> SCB-68 box >> HardWare.

The problem I have is that the voltage applied by labview is to small  i.e.  .2mV, I need 0 - 40Vrms. 

I am thinking about using DAC with external voltage source to amplify the small voltage. 

Do you know any DAC chip that will match the SCB-68 box and 6024E DAQ card ? 

Thank you 

[Coq_Rouge]

Could you please fill in some more details. This is not enough to help you. But you are correct you need a amplifier. Your daq card can handle signals down to +-50mV. So you need to amplify your signal 25 times or so.

[douread]

Thanks for the reply,

Any DAC chip would work long as it is able to amplify the mV coming from PC and works as the following.

Labview is used to apply voltage to the circuit then sense how much voltage is applied to the circuit, thereby controlling voltage application in a feedback manner. 

So basically I need this DAC chip to be compatible with the SCB-68 box and the DAQ (6024E).

What I mean by compatible is driving current, higher output voltage (up to 50Vrms) and so forth so the DAC chip functions with the listed components.

Please let me know what others details you would like, if it is still not clear.

 Thank you. 

[Coq_Rouge]

I se now that my first answer was too quick. If I understand you correct, you need an output voltage up to 40 Vrms. Your daq card has an output voltage in the range +/- 10 volt. The easiest way is to use an analog amplifier.  You do not need a new daq chip. If you only have 0.2mV as your output signal I guess something is wrong with your setup. You can use some simple Labview example to check your analog output. Also read the documentation thorough so you use the right connectors on the connector block. IF everything works OK, you have done something wrong in your software

[douread]

I have tried analog amplifier and upon simulation I only got up to 20 volts. If I have to cascade op-amps then issue of linearity will come up.

That is why based on a professor advice I want to try out a DAC. All I want is a DAC that is compatable with the scb-68 box and the DAC should have external voltage supply.

This external voltage supply should be as high as 40Vrms.

Thanks 

[Dennis_Knutson]

A DAC does not amplify. You also have no input to the DAC since the NI board is outputing an analog voltage. If you only got 20 volts from a 10 volt input, then you've designed the amplifier incorrectly.

[johnsold]

Your questions seem to indicate that you do not really understand your problem very well.

If you simulated an amplifier with the power supplies set to 100 V, you would have no difficulty getting an output or 40 - 50 V.  Simulators do not care.

Cascading multiple op-amps may have a small effect on linearity, but with proper design it could be minimized.  DACs also have non-linearities and they may be larger than those of the op-amps.

The SCB-68 is simply a connector box.  Anything you can hook  suitable cables to is compatible.

Do you want to connect your external DAC to the digital outputs of your 6024E? Or to the DAC output?

Although someone may make one, you will not find very many DACs with a 40 V supply rating.  Most people use low voltage DACs and an amplifier.  You can buy off the shelf amplifiers with voltage ratings well above the 40-50 V outputs you have mentioned.  Some of them have large gains and good linearity.

Please tell us what you are actually trying to do.  How much voltage and current does your  load require?  What frequency range will your signals cover? How often do you change the signal frequency, amplitude, or phase? Do you need protections against overvoltage or overcurrent?  What power sources do you have available?  Are you trying to buy a packaged device with connectors and  a power cord or part which you will assemble into your own circuit?

Lynn 

[AnalogKid2DigitalMan]

Mr. Lynn

You took (most of) the keystrokes right out of my fingers 🙂

-AK2DM

[douread]

Hello Lynn,

   A DAC does not amplify. You also have no input to the DAC since the NI board is outputing an analog voltage. If you only got 20 volts from a 10 volt input, then you've designed the amplifier          

   incorrectly.

** A DAC can amplify given small signals as input to activate transistors inside DAC, there by outputting lower voltage than the high voltage supply of DAC. I did not invent this it is 

    what I learned in school. Please correct me, if I am wrong then I will review it.  

** The input to the DAC should be the digital value of voltage amplitude, from the PC via SCB 68 box.

** I did not get 20 volts from a 10 volt input, but I did get 20 volts from .2mV in B2 Spice simulation.

Your questions seem to indicate that you do not really understand your problem very well.

 ** No problem, I am here to learn and understand. I appreciate your time, effort and concern.

If you simulated an amplifier with the power supplies set to 100 V, you would have no difficulty getting an output or 40 - 50 V.  Simulators do not care.

** 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

Cascading multiple op-amps may have a small effect on linearity, but with proper design it could be minimized.  DACs also have non-linearities and they may be larger than those of the op-amps.

** Yes I agree effect on linearity could be minimized, I am switching from op-amps to DAC based on my advisor's suggestion.

Do you want to connect your external DAC to the digital outputs of your 6024E? Or to the DAC output?

** Connect external DAC to the SCB 68 box so the connection should be

    PC >> DAQ (6024E) >> SCB 68 >> DAC chip >> Circuit that takes amplified voltage and apply it to hardware. 

Although someone may make one, you will not find very many DACs with a 40 V supply rating.  Most people use low voltage DACs and an amplifier.  You can buy off the shelf amplifiers with voltage ratings well above the 40-50 V outputs you have mentioned.  Some of them have large gains and good linearity.

** Very true. I have not found anything with 40V supply rating. The ones I found which are in higher that 40V supply rating are not inexpensive 

Please tell us what you are actually trying to do.  How much voltage and current does your  load require?  What frequency range will your signals cover? How often do you change the signal frequency, amplitude, or phase? Do you need protections against overvoltage or overcurrent?  What power sources do you have available?  Are you trying to buy a packaged device with connectors and  a power cord or part which you will assemble into your own circuit? 

** 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. 

** I don't need protections against over voltage of over current 

** Power source is PC but for the DAC it should be any external high voltage source

** Part which I will assemble into my own circuit.  

What I am trying to do ?

** To have a DAC chip that takes a small voltage signal from SCB 48 box and output 40-42 Vrms. This DAC should operate with supply voltage source of  120 - 130Vpp so I can have a max output of

   40-42Vrms. I hope I am clear.

[Coq_Rouge]

Hi

A well thing seems a little bit clearer now. We all have a deeper understanding regarding your project. Your high needs for a high voltage output is not a problem. I guess you do not need high current > 25mA . But higher current should not be a problem. Anyway check out this circuit  from TI http://focus.ti.com/docs/prod/folders/print/opa454.html. It should solve your problem. In the datasheet you will find some useful design tips a +/100 volt output should not be a problem. Remember also that you can transfer these design methods to other circuits, in case you need higher current. If you choose to this circuit I will advise you to select HSOP-20 PowerPAD package and a large heat-sinks. This because we all fry circuits, especially students. Also make your design very easy to service. Because you will have to solder in and out components a lot. And keep tracks near components as wide as possible. Thin tracks on PCB have a tendency to unfasten. After all this is a student project, not a marketing product needing a small and neat footprint.

I will also give another useful tip. Then you do your design make absolute sure you have a supplier who can sell you components in small number. It is not funny to make a great design, and then find out your vendor only will sell you a component in 200 Qty.

By the way I also find your advisor's suggestion to use a high voltage dac strange. Are you sure you did not misunderstood him/she