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 Forum index » DIY Hardware and Software
integrated logamps
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zipzap



Joined: Nov 22, 2005
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PostPosted: Sat Oct 21, 2006 4:40 am    Post subject: integrated logamps Reply with quote  Mark this post and the followings unread

Hi
Just came across these ti chips. Log amp, v/decade.
http://focus.ti.com/docs/prod/folders/print/log101.html
http://focus.ti.com/docs/prod/folders/print/log102.html

Is that doing what we want for our v/oct vcos?

Maybe they are even better than those ssm2210?
I´m not that good at reading datasheets, but could it be that these are very temerature stable (without a tempco)?

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StephenGiles



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PostPosted: Sat Oct 21, 2006 7:15 am    Post subject: Reply with quote  Mark this post and the followings unread

Not in that package it's not!!
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zipzap



Joined: Nov 22, 2005
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PostPosted: Sat Oct 21, 2006 7:42 am    Post subject: Reply with quote  Mark this post and the followings unread

well, i´ve got no experiance with that, but someone told me that you just have to glue the smd chip to the pcb, cover the pins with soldering rosin, wipe across the pin with your soldering iron. Then remove what´s to much of solder and you´re done. never tried it, but it must be possible.
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StephenGiles



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PostPosted: Sun Oct 22, 2006 5:05 am    Post subject: Reply with quote  Mark this post and the followings unread

Mmmmmmmm, sounds as simple as giving Iraq a good quick biffing eh!!
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State Machine
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PostPosted: Sun Oct 22, 2006 6:04 pm    Post subject: Reply with quote  Mark this post and the followings unread

The Log amplifier is generally used for receiver signal strength (RSSI) functions in RF receivers as the Log amp will create a linear voltage output in response to some dB input signal level (Non-linear). For example, a typical output transfer function would be 25 mV/dB input. They are generally wideband and are hard to control noise at low input signal levels as the gain of these puppies are very high. Log amps act like signal compressors as the slope of the output voltage decreases with greater signal inputs. Also, Log amps are sometimes put in front of an A/D converters to increase their effective bits, and thus dynamic range, since the Log amp will compress a very wide signal range in a smaller span of output voltage. Software will then expand the signal (a compander) to create a very large dynamic range.

A V/Oct VCO needs an exponential converter so that a current that charges a timing capacitor will double itself for every 1-volt increase at the CV input (An anti-Log function). Doubling the current, halves the time to charge and thus doubles the frequency, thus goes up an Octave.

I would not use a LOG amp for a VCO.

Hope I cleared things up a bit ....

Bill
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zipzap



Joined: Nov 22, 2005
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PostPosted: Thu Nov 02, 2006 6:30 am    Post subject: Reply with quote  Mark this post and the followings unread

yes you did.
Would you use them for a (fx)compressor?

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PostPosted: Thu Nov 02, 2006 11:40 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
yes you did.
Would you use them for a (fx)compressor?


Glad the information was helpful.

I would say that a Log amplifier could be used up front of the audio signal processing chain and used as the voltage controlling element for a VCA in a compressor application. The compression ratio can then be controlled by the "delta V/dB " of the log amplifier. Since the output of a Log amp is usually a current source, this slope can be modified with a simple variable resistor to control the (IxR) drop per dB of input signal.

Bill
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