Log amps? for dB metering.

range.

controller.

about doing it

converter in the PIC.

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.

schrieb im Newsbeitrag news:XZSdnUXttKeiWdvYnZ2dnUVZ snipped-for-privacy@comcast.com... 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.

--
ciao Ban
Apricale, Italy

"Ban" = Lazy Wog Prick

** See:

&

...... Phil

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.

you

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!

--
ciao Ban
Apricale, Italy

"Ban" = Lazy Wog Cunt "

** 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: 2nd hit

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

--
ciao Ban
Apricale, Italy

"Ban" = Lying Wog Cunt "

** 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.

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

........ Phil

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

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Many thanks,

Don Lancaster                          voice phone: (928)428-4073
Synergetics   3860 West First Street   Box 809 Thatcher, AZ 85552
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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

That is certainly a valid design choice, but 20-bit 1-MSPS ADCs are definitely not jellybean parts, nor could the PIC eat the data, let alone process it fast enough.

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 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
  --Schiller