Log amps? for dB metering.

I'm trying to come up with an LCD dBv meter. Not the bargraph type but a
numeric LCD type. It needs to have a range of +30 down to -70. 100 dB total range.
the reference is .775V = "0" dBv. I want to use one of the PIC's for the controller.
My question is should I use a Log amp to do the log calculations or think about doing it
in software. The smaller pic's I've looked at don't offer much math capability.
To do the calculations in hardware I think I'll need an RMS to DC converter,
and a LOG amp. Then I would read the output of the LOG amp into the A/D converter in the PIC.
Any Ideas?

Thanks

 

Is this for audio or RF signals?

For RF I would look a successive detection chips that convert AC to log
DC. They might be usable for audio too.

Mark

 

Thanks Mark,
This is for audio up to 200Khz

 

You might have trouble doing A2D at this rate with PIC, not to menson
the calculations.

 

I'm trying to come up with an LCD dBv meter. Not the bargraph type but a
numeric LCD type. It needs to have a range of +30 down to -70. 100 dB total
range.
the reference is .775V = "0" dBv. I want to use one of the PIC's for the
controller.
My question is should I use a Log amp to do the log calculations or think
about doing it
in software. The smaller pic's I've looked at don't offer much math
capability.
To do the calculations in hardware I think I'll need an RMS to DC converter,
and a LOG amp. Then I would read the output of the LOG amp into the A/D
converter in the PIC.
Any Ideas?


You can do all of the above with a single IC: THAT4103, Design Note 119
describes a 120dB range dB Meter.
And please turn off that HTML in your posts to NG.

 

"Ban" = Lazy Wog Prick

** See:

http://www.thatcorp.com/4301desc.html

&

http://www.thatcorp.com/datashts/dn119.pdf





....... Phil

 

Thanks all
Sorry about the HTML
RonL

 

I'm trying to come up with an LCD dBv meter. Not the bargraph type but a
numeric LCD type. It needs to have a range of +30 down to -70. 100 dB total
range.
the reference is .775V = "0" dBv. I want to use one of the PIC's for the
controller.
My question is should I use a Log amp to do the log calculations or think
about doing it
in software. The smaller pic's I've looked at don't offer much math
capability.
To do the calculations in hardware I think I'll need an RMS to DC converter,
and a LOG amp. Then I would read the output of the LOG amp into the A/D
converter in the PIC.
Any Ideas?

Thanks

A 100dB range is a tall order. While you can use a PIC for the display, you
can't use a PIC for the A to D function because these are only 10 bit
coverters, which at best would give a range of 60dB. And that uses every
bit! A log amp is also going to have problems with a 100dB range. Therefore
you need to re-evaluate what you are trying to do. You could use a PIC with
its built in A to D function for anything less than a 60dB range. Instead of
a log amp, you simply digitize at the fastest rate the rectified peaks of
the audio waveform then in software use a lookup table to convert the
digital values into their dB equivalent which you display. No math is
required. It's easy to add the necessary delays, storage and averaging so
that the display is readable without flicker. The display drivers can all be
in the PIC as well. LCD displays require AC drive which you need to become
familiar with if you do it in software. A two digit display could easily be
handled by an 8 bit pick. It would be possible to extend the range maybe to
100 dB by using a switchable "front end" that simply amplifies low level
signals by say 100 times (40dB) when the processor says it doesn't have
sufficient signal. Forty dB is then subtracted from the display in software
to make the display show the lower values.

If you are serious about a 100dB range directly without switching, you will
need to process at least 17 bits. You can use a 20bit A to D converter then
a 20 bit or 24 bit or even 32 bit processor or DSP chip to do the math and
display. That's a much bigger deal than the 60dB PIC single chip solution.
Bob

 

That's assuming you do it the hard way; if you feed the signal to an
AGC
amplifier, the gain control pin of the amplifier can easily give you
the
dBv indication with eight bits of digitization. You just have to
remember the gain of a transistor is logarithmic in base voltage,
and you do the amplitude detection/logarithm function before
digitization.

 

Reliably over 10 decades ?

Graham

 

And the noise problems will be far worse than using a 16 bits A2D.

 

So, in your mind that is simpler than a switched gain, op-amp based front
end using a few resistors, a FET switch, an op-amp and an 8 bit processor?
How about cost, temperature stability, drift, offset and all of the other
problems of using p-n junctions as circuit elements? Thanks, but I would
stay away from analog p-n juction based designs and go as digital as
possible if I were doing it.

 

Not only just lazy, but I'm on dial-up and my daughter is blocking the
phone. I have the nice Applications handbook from THAT and I gave detailed
information that even a complete idiot like Phyllis could immediately find
the links. Well done!

 

"Ban" = Lazy Wog **** "

** Are you so full of shit cos your pet goat is blocking the dunny ???

** Just try using "THAT 4301" or "THAT" as inputs on Google - see how
far you get.

You arrogant pile of stinking wog shit !!






......... Phil

 

I gave in "that 4301"
1st hit: http://www.thatcorp.com/4301desc.html
2nd hit http://www.thatcorp.com/datashts/4301demo.pdf

and I also tried without the space, same result.
when clicking the first link, there is the "applications" link on that
product page, which nicely lists AN119.

You are even worse an idiot than I thought

 

"Ban" = Lying Wog **** "

** Shame how your FUCKWIT post gave the WRONG number !!!


Quote:

" You can do all of the above with a single IC: THAT4103, Design Note ...
"


Google responds to " THAT4103 " with ** ZERO ** matches.

http://www.google.com.au/search?hl=en&q=that4103+++&btnG=Google+Search&meta=


YOU PITA ASININE DAGO F U C K H E A D !!!!!!!!!!!





......... Phil

 

Aren't their jellybean RSS circuits that routinely do this?



--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email:

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

Thanks all
Analog devices AD636 will work for this project. I'll use a preamp for the
low level signals
and a PIC for the LCD and switching gains.

Thanks
RonL

 

I'd use 2 x the PIC solution. One with a preamp. The PICs can figure out
which of the 2 has valid data on the output. i.e. the range switching is
digital.


Thomas