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Old 5th September 2006, 05:21 AM   #1
xiaonan is offline xiaonan  China
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Default another method to set up a digital controlled voltage supply which can provide 0~30V

This is another method to set up a digital controlled voltage supply.
I want to use a DAC and a Operational Amplifier. I want to use the LTC 2609 and control the output of DAC from 0 to 5V. Operational Amplifier I want to use the OPA541 which can provide -35V to +35V output and the maximal output current is about 2A. This is the circuit pic.

I have few experience on that, So could anyone help me check the circuit up to see whether there are some problems in it. Thanks a lot!!
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Old 5th September 2006, 10:03 AM   #2
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It's a bit rough, isn't it? ;-) Assuming R1 is at the inverting (-) input, this will basically work. Caveats:

* Do check out the OPA541's price tag before you commit to this circuit - there's a price to be paid for "simplicity".

* The LTC2609 is a quad DAC. Obviously, if you intended to use all four channels to drive OPA541s, the outputs must share a common ground. You'd still be well advised to include opto-isolation between the DAC and the computer, to protect the latter.

* Your initial choice of digital pots was going to give 10 bit resolution, now you pick a 16bit DAC; depending on actual needs, there may be cheaper parts than the 2609 also.

* The 2609 needs an external reference; the supply is usually not good enough for that (for sure not if you need anything over 10 bits). While it is specified as rail-to-rail output, Vref must not exceed Vcc; the typical application would run Vcc=5V Vref=4.096V (there are some reference chips for that "odd" value); this yields 1/16 mV resolution (not accuracy), times the OPA541's amplification.

* Remember to compensate for offset values somewhere - since the 2609 is unipolar, doing it digitally may not work and a trimpot could be a better choice. Span can be calibrated digitally.

* Look closely at the OPA541 data sheet for current limiting (it wants an external resistor for that); use output protection diodes; put 100n blocking capacitors close to V+ and V-; and check that you're inside the SOA + cooling requirements. You may also need extra compensation.

* If you're looking for positive outputs only, you can run the OPA on asymmetrical supplies, see datasheet example Fig. 6 (+60/-8V).

* Note voltage drop on OPA = 4.5V@2A, 5.5V@5A per data sheet.
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Old 6th September 2006, 06:39 AM   #3
xiaonan is offline xiaonan  China
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For wine&dine
Thank you again for so detailed reply.

Yes, the price is so expensive!! Why? OPA541AM is $US96.2,but the OPA541AP and APG3 is about the $US11.1. Why the difference is so large!!?? The "lead time" means that I have to wait that time for the products??

Maybe I should use the OPA544 which is much cheaper!!

OPA544 and OPA541 have not pin for offset trim, so how can I do that?

A good news, I just know that I have labview which can provide four DAC line by a card. So now I need not consider the DAC. Four is enough for me now. In the future I will need that.

So if I want use the DAC I have to add a operational amplifier between them? like the OPA602 in the pic I attached?

Your suggestion about the digital pots is right. I think the 10 bits is ok for me.

But I think most the DAC need a external reference, Maybe I should use a voltage regulator.

Now I have not found how to use the SOA. I think tomorrow I will get it.

Yes, I only need the opsitive output, So if I use the OPA544, Can I use the same asymmetrical supplies like OPA541? I did not see the same figure in OPA544's datasheet.

Now I want a large output voltage, I hope can get the 100V output. But I googled and did not find that high output voltage op-amp. Do you know is there some chip or device can provide that voltage? or is there some other solutions? I saw a figure in LM143's datasheet which can provide 130V p-p. But I think I can not use that, because there is no absolute ground.

Thanks a lot!!
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Old 6th September 2006, 09:27 AM   #4
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That extra opamp in the circuit you posted is needed only because the DAC in question has a current output. You were considering voltage-out DACs or D-pots, so you don't need it.

The LM143 has an output voltage swing of +-37 V only. (You'd be using an LM343 anyway, that's the commercial-grade part).

For offset trim, attach the ground side of the feedback resistor to a trimpot between +10/-10 mV.

You CAN buy opamps to do 100V output directly, see e.g. http://eportal.apexmicrotech.com/mai...linvoltage.asp which lists parts up to 1140V p-p output - but again, fasten your seat belt before asking for prices. It's the old make vs. buy tradeoff here.


>most the DAC need a external reference,
>Maybe I should use a voltage regulator.

You should use a voltage REFERENCE, which has been designed for lower tolerances and temperature drift. That's what VR1, a TL431, does in my circuit - with R4=R5=10k, it holds +5V across R6.

> I think I can not use that, because there is no absolute ground.

Since you already have a DAC card, you can also use analog isolation. Set your card to 0-5V out, stick an ISO124 (http://focus.ti.com/docs/prod/folders/print/iso124.html) at each output, and use the isolated output of the ISO124 as the reference in your output stage without having to worry about ground levels.

In my floating circuit, omit R3, R4, R5, VR1 and feed the ISO124 output to the left side of R10. Attach R8 to +12V and use a trimpot or fixed value for R7 to set current limit. Leave out the current limit stuff but don't complain to me when you make it smoke ...
(Again, that circuit will happily handle 100V or more if B1/C1/T1 are up to it.)
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Old 6th September 2006, 09:38 AM   #5
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Oh BTW, you can also look at how the pros do it - Keithley makes several products with a +-100V source and some manuals (DL from keithley.com) have schematics, e.g. 230_901_01H.pdf page 61. They essentially build their own HV OPAMP. Nice, but NOT simple ...
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Old 6th September 2006, 10:23 AM   #6
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This is the floating regulator with ISO124. The reference is no longer needed, it's part of the DAC board (for current limiting, the 78L12 is good enough).
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Old 6th September 2006, 10:14 PM   #7
xiaonan is offline xiaonan  China
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Thank you for your suggestion.

I found the Op-amp PA240CC&CX is $19.65, whose supply voltage is about the 350V, and the output current is about 60mA. So it is enough for us.

What confuse me is that the PA241CE&CEA&CEM's price are $55 $74 and $145 repectively. The function of the PA241 and PA240 are almost the same! But the price is so different. Is that just because the different package?

If I want to use PA240CC or CX, I shoud consider ISO124, offset trim, add some capacitors to the voltage supply of the Op-amp, input voltage protection, SOA, add some heatsinking. Is there any other things or problems I should consider?
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Old 6th September 2006, 10:24 PM   #8
xiaonan is offline xiaonan  China
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I think another choice is PA78DK, the price is not very high,about $25.5.

So if I can use the high voltage Op-amp, the circuit will be more easy for me. And we need at least six voltage supply, so if I have to do a complex circuit, it will be a large project. But we just want to test our device more easily.
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Old 6th September 2006, 10:38 PM   #9
xiaonan is offline xiaonan  China
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I saw the OPA541 can be use the asymmetrical supplies to get 0 to 50V output voltage. Can any other Op-amp do that? How can we make sure the asymmetrical supplies voltage(like 60 and -8V for OPA541)?
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Old 7th September 2006, 02:02 AM   #10
xiaonan is offline xiaonan  China
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Now we think the most suitable device for us is PA240CX. Because firstly the price is cheapest(less than $19), secondly it can be used from 0 to 160V at least and that is nice for us, thirdly its legs looks like more easily used on breadboad.

But now I found that it is a little difficulty to find a DC supply which can provide +175V and -175V at the same time. So is there some products about that? Or I should use the bridge fectifier and capacitor to be a supply?

If we set up the circuit on the breadboad, is that reliable for a long time using?

And do you think that can work?
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