circuit design aid

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#1
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Hi everybody,

 

I have a circuit which contains Xmega128a1U micro controller. I have ground and +3.3 V DC as supply.

how can I make a -3.3 V from this circuit ?

There is a op-amp which I want to buff a ac signal through it but as I connect it to 0 and +3.3 V , I lost negative part of my input AC signal.

I think if I connect op-amp to -3.3 V and +3.3 V this issue will be answered.

 

Thank you for your help

Best Regards

 

P.S. Op-amp is AD826 and it has BW covers my wave freq

In the end it doesn't even matter ...

Last Edited: Wed. Oct 4, 2017 - 06:49 AM
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This has nothing to do with the XMega - this is a plain old general electronics question.

 

In fact, I'm pretty sure it was covered in the General Electronics forum quite recently ...

 

Please change the title to some directly descriptive of your requirement!

 

EDIT

 

typo

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I did not know there is such a forum 

 

thank you

In the end it doesn't even matter ...

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So how long, exactly, did you spend looking at the list of available forums?

 

It's right there in the list of forums:

 

 

and, on the 2nd page of topics:

 

 

Note how having a properly descriptive title makes it immediately obvious what the thread is about!

 

http://www.avrfreaks.net/forum/how-get-5v-operational-amplifier

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thanks for your kind answer, I did not check it , I saw xmega and placed it there

In the end it doesn't even matter ...

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The important thing about any design is to think carefully about the problem - not just leap straight in with some quick fix!

 

Image result for ibm think

 

 

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Would you PLEASE stop insulting newbies with condescending posts like that?

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tzanti wrote:
how can I make a -3.3 V from this circuit ?
Some SMPS are bi-polar capable.

tzanti wrote:
I think if I connect op-amp to -3.3 V and +3.3 V this issue will be answered.
or negative gain with op amp's + connected to a reference.

 


CUI

P7805-S Series
500 mA Output Current, 4.75~32 Vdc Input, 3 Pin SIP, Non-Isolated, Dc-Dc Converter

http://www.cui.com/product/power/dc-dc-converters/non-isolated/0.5-a-output-current/p7805-s-series

...

  • Suited for negative output applications

...

 

"Dare to be naïve." - Buckminster Fuller

Last Edited: Wed. Oct 4, 2017 - 01:53 PM
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Or you could bias your a/c input so it is with in the dc rails of your opamp, by feeding your signal in via a capacitor to an input that is biased to 1/2 the supply rail.

 

Jim

 

edit: added picture from Forrest Mim's OPAMP notebook

 

Last Edited: Wed. Oct 4, 2017 - 02:13 PM
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This has nothing to do with the XMega - this is a plain old general electronics question.

    First, I would explore how and if I could use a toggling pin to build a capacitive inverter. If the current needed is small and that -3.3V could be a bit less, this would be the way to go. More current? Add some inverter in parallel, they are cheap.

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angelu wrote:
First, I would explore how and if I could use a toggling pin to build a capacitive inverter.
Such as the switch signal of a SMPS.

IIRC, the following is about -1V at 500microA :

Linear Technology

LT3088 - 800mA Single Resistor Rugged Linear Regulator

http://www.linear.com/product/LT3088

http://www.linear.com/docs/46566

(PDF, page 26 for High Efficiency Adjustable Supply)

"Dare to be naïve." - Buckminster Fuller

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I think if I connect op-amp to -3.3 V and +3.3 V this issue will be answered.

Be careful, however.

 

When you feed the signal into the ADC of an AVR the signal needs to be "all positive", i.e. it can't have a negative voltage.

 

You can use a bi-polar op-amp to amplify and filter the signal, but at some point you need to level shift it so that the signal is only positive voltage with respect to ground.

 

Jim showed how to AC couple the signal, if you do not need a frequency response down to and including DC, (audio, for example). 

 

If the DC level of the signal is important, then capacitively coupling won't work, and you will need to formally level shift the signal.

 

JC

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Thank you all , you really helped me out. 

 

I have a new problem, I have a dc constant current in my device. it is instrumental 4-20 mA current. I need to super pose ac signal on this DC current.

should i use a buffer ? is there any cheap way to do so ?

 

P.S. I was thinking simple capacitor could do ac coupling but it's not working

In the end it doesn't even matter ...

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You want to change the current in proportion to the AC signal?

 

Trying to add or subtract voltage of a constant current source does not work because the amount of current supplied is controlled by the voltage between the source and the load.  As a simple example, if you add voltage to a constant current line, the constant current controller would just lower its voltage in proportion to what you added in order to keep the current the same.

 

Alan

Last Edited: Wed. Oct 11, 2017 - 12:16 AM
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Sounds like HART protocol. There’s plenty of examples of how to superimpose the carrier ontop of the current signal.
A capacitor should work - you need to understand the impedances involved.

10 seconds of googling found this:

http://www.ti.com/lit/an/slyt491...

Last Edited: Wed. Oct 11, 2017 - 03:02 AM
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Thanks for your answer.

 

It is HART protocol.

I  have red these documents, but the problem is most of these designs are for transmitters which contain DAC but what I'm trying to make is current input device.

We are not producing any current we just use the current comes from control system (measure it + get power from it).

 

In the end it doesn't even matter ...

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The document i linked to was a loop powered slave. So where is the problem?

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Kartman wrote:
The document i linked to was a loop powered slave. So where is the problem?

 

Quote:

ABSTRACT
This application report provides details for designing an inexpensive HART transmitter.

'This forum helps those who help themselves.'

 

pragmatic  adjective dealing with things sensibly and realistically in a way that is based on practical rather than theoretical consideration.

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The problem is that transmitter produces current , but current input device should not change input current value.

in the linked document there is xtr115 which is a current transmitter IC.

 

In the end it doesn't even matter ...

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Ok, i got confused with the reference to a DAC.
Try this:
http://www.embedded-computing.com/embedded-computing-design/design-an-optimized-circuit-for-hart-enabled-4-to-20-ma-inputs

Figure 2. Uses a capacitor. Now we’re full circle.

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Thank you Kartman.

 

This is a good article and it is originally for TI, but the theory looks accurate.

I tried it before, but I will try it again, maybe previous time I had timing  problems.

 

In the end it doesn't even matter ...

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The last link was referring to Analog Devices. You should be able to validate your operation by using the PC sound card to capture the fsk. You'll need a real oscilloscope to validate the actual signal levels. If you have a real hart master, you can capture its data for comparison.