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Design >> Analog Design >> Bipolar to Unipolar
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Message started by vasu on Sep 20^th, 2006, 12:52pm

Title: Bipolar to Unipolar
Post by vasu on Sep 20^th, 2006, 12:52pm

Hello,

I am looking for an op-amp circuit to convert a bipolar signal say(+/-10v, sine-wave) to unipolar(half-sinewave of 0-10v). I don't want to build a half-wave rectifier, neither I want to use any diodes.

Can you point me to web-site or circuit diagram?

TIA,
Vasu

Title: Re: Bipolar to Unipolar
Post by loose-electron on Sep 20^th, 2006, 1:43pm

If the frequency is low enough, you can use some MOS switches, and some comparators to control when they are on/off.

Title: Re: Bipolar to Unipolar
Post by jbdavid on Sep 21^st, 2006, 4:26am

national semi has a good appnote on Opamp Applications
about 20-50 different circuits for different uses

(ok I haven't even looked at it for 7 years but If I wanted an opamp circuit like that, thats where I'd look..
I'm assuming you want the circuit that uses a 741 type opamp + some resistors for this.. right?

to be clear what you really want is
V(out) <+ V(in)/2 + 5.0;

NO rectification.. just a little attenuation + shift the midpoint from 0v (midpoint between +10 and -10 to +5v (mid point between +10 and 0v)
right?

Title: Re: Bipolar to Unipolar
Post by vasu on Sep 21^st, 2006, 5:21am

Hello,

thanks for the reply. What I am looking for is exactly like half-wave rectifier, where the input if it is sine wave of -/+10v, the output will be only +ve cycle and -ve cycle will not appear in the output but this I wanted to do without using diodes.

It is easy to design a bipolar +/-10 to unipolar 0-10v with attenuation but the output will again be a full sine-wave.

Attached is the document, which will describe about what I am looking for.

Thanks for your help.

Title: Re: Bipolar to Unipolar
Post by loose-electron on Sep 21^st, 2006, 12:20pm

OK, looked at the word.doc attachment --

some questions:

integrated or board level design?
(the voltages are big, but there are 60V tolerant CMOS processes out there.)

what frequency is the sinusoid?
(the approach is different at 60Hz and 20GHz)

Some compartors and MOS switches are viable. Also as suggested before, both TI, National and others have lots of application notes on function circuits.

Title: Re: Bipolar to Unipolar
Post by jbdavid on Sep 22^nd, 2006, 12:39am

So instead of one diode you want to use 20 transistors?
On chip i just connect drain to gate of mos, and I have a diode.. (ie input side of current mirror...)

But - OK.. lets say your working at chip level and have an extra amp, (you're only using 3 of the 4 in the dip)
and you'd rather add two resistors instead of a resistor and diode.. ??

why not use a single supply (0-10v) amp.. that can handle inputs below the lower rail.. ??
because when the output clamps the output transistors saturate and recovery from that takes longer than a normal small circuit transient excursion..
but if you can live with that - ok..
Other wise you'll want to put a diode in the input side rather than using the output of the amp as a diode..

hm
on chip you could design a unity gain buffer.. (diffpair into current source with resistive load.. tie one gate to 0v,
let the other one get the +/-10v input - hmm.. probably not linear enough...

I'd be surprise if you didn't find one somewhere though.. jbd

Title: Re: Bipolar to Unipolar
Post by jbdavid on Sep 22^nd, 2006, 12:53am

by now you should have already found this..
www.national.com
application note AN-20
jbd

Title: Re: Bipolar to Unipolar
Post by jbdavid on Sep 22^nd, 2006, 1:01am

also look at AN-4 (from 1968)

Title: Re: Bipolar to Unipolar
Post by vasu on Sep 22^nd, 2006, 5:42am

Dear David and Jdb,

The reason which prompted me to do this design is this, I was playing with a function generator and I found that it generates +/-10v sine-wave and using a potentio-meter, it changes that wave-form to unipolar 0-10v with same amplitude range.

One way to do the same is: using a switch, which will allow to change the -VEE of op-amp(make it single supply) but I don't know whether this is normal practice implemented in a function generator and other circuits.

Any pointers will be helpful.

Thanks,

Title: Re: Bipolar to Unipolar
Post by vasu on Sep 22^nd, 2006, 5:43am

Sorry, I forgot to mention it, it is board-level design and mainly used for data-acquisition boards in the front-end circuit for selecting bipolar voltage ranges (-/+10v) or unipolar(0-10v).

Title: Re: Bipolar to Unipolar
Post by jbdavid on Sep 22^nd, 2006, 3:46pm

Once upon a time I worked on a project with 128 analog measuring points, and we used a signal conditioning circuit per measuring point to convert the parameter we were measuring (temperature, voltage, current .. etc) into the voltage range of our ADC circuit.
To give you an idea of how long ago this was, we used several chips to build the ADC itself.. (comparator, R2R chip, and SARDAC ) so we only had 1 input range to use.. and that did not accomodate negative voltages, so while there were a couple of voltages we could measure directly (ie no conditioning) many needed inversion and or scaling..
Today I'd look at using a GP Data Aquisition System (on chip) and only have special conditioning circuits..

But ONE idea you might consider would be to do the clipping in post processing..
accept the +/-10 input signal, convert to 2's complement out put data, and then do a simple latch that reads the input data if the sign is NOT set, and uses 0's if the sign bit IS set..
(that could even be in SOFTWARE or firmware!!)

Otherwise I'd have a separate amplification /rectification circuit before the mux - and I think I would probably use the diode unless I had to build 2 million units.. and then I don't know what I'd do, but who ever wanted to build 2 million units would be willing to pay me good $$ to figure out how to reduce the parts count.. and I would.
and I'd spice it, and breadboard it 20 time, until I figured out how to get rid of the diode..

accuracy, speed, cost, development time, power -
pick any three

Title: Re: Bipolar to Unipolar
Post by vasu on Sep 25^th, 2006, 7:04am

Hello,

Please see the attached pictures. These are the output of a function generator. There is a potentio-meter which is named as OFFSET and if I turn that to either end, I get a half-wave when the input is full-sine wave.

I would like to do the same in hardware or using software. Any idea, how this is implemented in function generator?

Thanks,

Title: Re: Bipolar to Unipolar
Post by vasu on Sep 25^th, 2006, 1:48pm

Hello,

Please explain me the role of R15 in the attached function generator diagram. Can that part could be used for generating a half-cycle.

Thanks,