Log voltmeter

[Lou1]

Hello

Many years ago I've seen an application of a converter IC (ICL7106 style) displaying a dB output without any explicit log converter.
The circuit was based on a special connection of the input, reference and integrating pins, but I do'nt quite remember how it worked or where it was published.
Does anybody have details about such a circuit?
Thanks

 

[Olihou]

Is the output you have in mind digital or analog?

If just for analog, there are functional circuits using op amps and diodes, as far as I can recall.

Have you search for it in the cyber?

 

[Lou1]

No, it's digital: the ADC displays dB's instead of mV, and I did obviously search the web as well as specific resources, but without results.

 

[Olihou]

Hello Lou

How about this? See if it helps.

http://www.national.com/an/AN/AN-30.pdf

The log amplifier output gives a DB equivalent of the input voltage.

An ADC would be required should you need the digital form.


Oli

 

[MP1]

A simple op-amp with a NPN transistor and small switching diode in the feedback loop makes a logarithmic amplifier. You also need a small cap there to keep the circuit from going into oscillation.

It is called a transdiode logarithmic amplifier. Try this in your word search and you should find the information you are looking for.

MP

 

[Lou1]

Thanks Oli and MP, but what I want is precisely to avoid a separate log converter; I know that the DVM chip is able to implicitly perform the conversion, but I don't remember the trick.
Lou

 

[MP1]

Lou, I have not seen the project you are mentioning, but when you find it, please post it for others.
Thanks,

MP