1. Home
  2. Electronics Design
  3. F to V converter problem

F to V converter problem

The frequency to voltage converter makes use of a LM324, the circuit schematic can be found in (Page-4)

formatting link

The LM324 is fed off a single supply and the decoupling is based on the design found in (Figure 3, Page-2)

formatting link

The circuit parameters chosen for the frequency to voltage converter (corresponding to the circuit design parameter names used in the above mentioned urls) were chosen to achieve the following performance:

  1. Maximum frequency for max output with Vcc=9v is 1kHz 2. Response time is approximately 100ms 3. Circuit output approaches Low Level output of LM324 (approximately 5mV) as input frequency approaches zero

My question is as follows:

In my current circuit design, the circuit output is not behaving correctly. As the input frequency approaches zero, the frequency to voltage converter output is no where near the Low Level output of the LM324. Further more, the output has unwanted ripples which i am trying to eliminate. I am seeking help on the possible changes and/or corrections required in order to achieve the desired circuit performance as listed above. Thank you.

George.

Reply to
guanziyi
Loading thread data ...

formatting link

What are the component values?

How much is "nowhere near zero"?

How much ripple are you seeing, under what conditions?

John

Reply to
John Larkin

formatting link

Actually, that circuit has a subtle bug.

John

Reply to
John Larkin

formatting link

Two, actually.

John

Reply to
John Larkin

formatting link

Offhand,

o The LM324 input impedances aren't matched, creating an offset error, which is input to the integrator.

o The op-amp's inverting input can get dragged below its common-mode range.

o The R1-C1 called out won't charge or discharge fully or symmetrically for higher frequency inputs, producing non-linearities.

o (d Vf/ dT) of D1-2 not compensated.

o Vout drifts between input pulses, due to dielectric absorption in C2.

Anything else?

Cheers, James Arthur

Reply to
James Arthur

40.pdf.

"As the input frequency approaches zero, the frequency to voltage converter output is no where near the Low Level output of the LM324."

By running this from a single supply you have changed your "zero" to be at 1/2 the supply voltage. I would predict a zero frequency output near 4.5 volts (Vcc =3D 9V) You could add another op-amp to do some level shifting perhaps.

"Further more, the output has unwanted ripples which i am trying to eliminate"

Do the ripples have a 100ms time constant? If so that is just the nature of a F-V converter. You can reduce the ripple by increaseing the time constant (averaging more of the input pulses) but this increases your response time.

George Herold

Reply to
ggherold

formatting link

What I had in mind was your point about the diodes being able to pull the opamp input below ground. Most LM324's go nuts if this happens, and the mess could wind up in any sort of weird state.

The other issue is that if any of the other three LM324 sections are allowed to run open-loop, they can mess up the internal current sources and trash the section that you can see on the schematic.

Depending on who makes it, an LM324 is a bad to very, very bad opamp.

Your other issues depend a lot on the component values, which we don't know.

One tach circuiy I like is a dual one-shot, sections firing on rising/falling edges of the input, with both outputs summing into a lowpass filter. Ripple is lower than a single-edge system; at one magical, selectable frequency, ripple is zero. Makes a nice wideband FM discriminator, too. I invented that when I was just a sprout.

John

Reply to
John Larkin

(Page-4)

formatting link

Page-2)

formatting link

No. Where would the 4.5 come from?

This circuit should settle to millivolts off ground at zero RPM, if it's done right.

The OP should state values.

John

Reply to
John Larkin

N-140.pdf.

"No. Where would the 4.5 come from?"

Oh, He is running this single supply and his second link seemed to imply that he had biasied the non-inverting input up to one half the supply voltage. But this is just an assumption on my part. A real schematic would be helpful.

BTW it is fairly simple to configure the ADVFC32 from analog devices into a F-V converter.

George Herold

Reply to
ggherold

It must be mid-terms. I wonder if we'll ever see any of the latest batch of googlies come back to check for answers, or do they expect them to just show up on thier desk?

Thanks, Rich

Reply to
Rich Grise

formatting link

Equally bad, when C1 pulses below ground its charge parcel dumps partly through the op-amp's input diode rather than getting integrated onto C2.

That can happen at every single pulse, since the LM324 isn't fast enough to prevent it.

A resistor to GND would harden up the LM324's output and make that less troublesome.

James Arthur

Reply to
James Arthur

Most of them post once and never come back. I assume they switch majors, to journalism or something easy like that.

John

Reply to
John Larkin

st

Please be nice to this guy. He's a cadet engineer that I'm supervising here at an electrical untility company and I told him that newsgroups is a wonderful learning resource that he should take advantage of, learn from....and eventually contribute to. I'll suggest he posts values etc...many thanks on his behalf for the help so far.

Daniel

Reply to
Daniel

Gentlemen,

My apologies for not replying sooner, this is the first time i am using this News Group and certainly did not expect such quick replies. So thank you so much for your input. I have sent out an email to all who posted comments to my question, which contains the latest circuit schematic and component values. Any further advice and instructions would be greatly appreciated.

George.

Reply to
guanziyi

DONT take it to email unless you need confidentiality or to transfer a binary file. You asked the question here and for every poster who replied, there are typically between 10 and 100 users of this group who have looked at this thread who might have some comment to make. Can you put the schematic on a web page somewhere and post a link to it here?

Also many of the posters here DO NOT post with a valid email address to avoid vast quantities of spam and will never get your message.

Usenet rule of thumb:

ONLY take it to email if invited to or you are already a friend of the recipient.

--
Ian Malcolm.   London, ENGLAND.  (NEWSGROUP REPLY PREFERRED)
ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk
[at]=@, [dash]=- & [dot]=. *Warning* HTML & >32K emails --> NUL:
Reply to
Ian Malcolm

I'm free here; they probably can't afford me as a private consultant.

John

Reply to
John Larkin

Teach him to lurk, and to use the other side of google.

Good Luck! Rich

Reply to
Rich Grise

u

Thanks for the 'Heads Up' Ian, point taken.

As i do not have a web-site of my own at the moment, shall look into getting one up to make things easier.

George.

Reply to
guanziyi

We often post schematic in ASCII text, right here in s.e.d.

You can use a dedicated editor, like Andy's Ascii-Circuit V1.28

formatting link

or just use plain old Notepad.

Here's your schematic, in ASCII: (view in Courier font)

| || + .-. .---||----. | | | || C2 | R | | | ___ | '-' C1 +--|___|--+ C | 74hc14 R1 | R2 | || | |\\ ___ || D2 | |\\ | o--||----+---| >O--|___|--||-+---|---+--> output | | .----|+/ .-. v | |/ R | | D1 - | LM324 | | | | '-' === === | GND GND === GND

(created by AACircuit v1.28.4 beta 13/12/04

formatting link

Cheers, James Arthur

Reply to
James Arthur

See...

Newsgroups: alt.binaries.schematics.electronic Subject: V-to-F / Tachometer (from S.E.D) Date: Thu, 13 Nov 2008 18:00:58 -0700 Message-ID:

Dual-supply, with _my_ C1 artificially small to speed simulation.

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 I love to cook with wine     Sometimes I even put it in the food
Reply to
Jim Thompson

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.