Datasheet error?

[Terry Pinnell]

Would someone take a quick look at the data sheet here please
http://www.national.com/ds.cgi/LM/LM2907.pdf

I've never used an LM2907, but was reading the above a couple of days
ago (in connection with the thread '12v relay on 3v ??' in seb).

The +Vout figure of 67 Hz/V for the 'Minimum Component Tachometer'
circuit shown at the top of page 8 seems wrong to me. Using the
formula given on page 7, I make it 670 Hz/V. That would also be a lot
more practical, at least for a car engine!

I'm curious to know whether it's the datasheet or me that's
wrong.

 

[John Popelish]

Would someone take a quick look at the data sheet here please
http://www.national.com/ds.cgi/LM/LM2907.pdf

I've never used an LM2907, but was reading the above a couple of days
ago (in connection with the thread '12v relay on 3v ??' in seb).

The +Vout figure of 67 Hz/V for the 'Minimum Component Tachometer'
circuit shown at the top of page 8 seems wrong to me. Using the
formula given on page 7, I make it 670 Hz/V. That would also be a lot
more practical, at least for a car engine!

I'm curious to know whether it's the datasheet or me that's
wrong.

From the previous page: "VO = VCC x fIN x C1 x R1 x K Where K is the
gain constant—typically 1.0."

So 15V * 67Hz * .01e-6F * 100,000ohms = 1.005V

That is pretty close to 67 Hz per volt.

 

[Terry Pinnell]

From the previous page: "VO = VCC x fIN x C1 x R1 x K Where K is the
gain constant—typically 1.0."

So 15V * 67Hz * .01e-6F * 100,000ohms = 1.005V

That is pretty close to 67 Hz per volt.

Thanks, plainly my slip. Maybe I used 10k instead of 100k for R1. (I
see manufacturer failed to include component designations, although
should be obvious here.)

Also, assuming this was meant to be a typical car tachometer circuit,
I thought 67 Hz seemed too low by about a factor of 10, reinforcing my
mistake!

 

[[email protected]]

Thanks, plainly my slip. Maybe I used 10k instead of 100k for R1. (I
see manufacturer failed to include component designations, although
should be obvious here.)

Also, assuming this was meant to be a typical car tachometer circuit,
I thought 67 Hz seemed too low by about a factor of 10, reinforcing my
mistake!

I didn't consider whether or not it is a good design. My first problem
is an automotive design that has a regulated 15 volt supply and bills
itself as a minimum compont count design.

But if we assume that this represents a tach for an 8 cylinder car and
the pickup is the distributor pickup (that turns half as fast as the
crank shaft does, then 5000 RPM would produce a frequency of
5000/60/2*8=333 Hz for a voltage aoutput at that speed of about 5
volts. That isn't so bad. If you designed it for 670 Hz per volt, the
output would be about a half volt/ full scale.

 

[Terry Pinnell]

I didn't consider whether or not it is a good design. My first problem
is an automotive design that has a regulated 15 volt supply and bills
itself as a minimum compont count design.

But if we assume that this represents a tach for an 8 cylinder car and
the pickup is the distributor pickup (that turns half as fast as the
crank shaft does, then 5000 RPM would produce a frequency of
5000/60/2*8=333 Hz for a voltage aoutput at that speed of about 5
volts. That isn't so bad. If you designed it for 670 Hz per volt, the
output would be about a half volt/ full scale.

Thanks. My calculation was badly flawed!