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Old 8th June 2011, 06:45 AM   #11
gpapag is offline gpapag  Greece
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You may build a log converter with common components ,which you may already have

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

Regards
George
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Old 8th June 2011, 11:44 AM   #12
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What Frequency range?
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Old 8th June 2011, 08:20 PM   #13
gpapag is offline gpapag  Greece
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I havenít built it.

From the pdf (page 2, left column):

Quote:
Both of the transistors are used in the grounded base connection, rather than the diode connection, to eliminate errors due to base current. Unfortunately, the grounded base connection increases the loop gain. More frequency compensation is necessary to prevent oscillation, and the log converter is necessarily slow. It may take 1 to 5 ms for the output to settle to 1% of its final value. This is especially true at low currents. The circuit shown in Figure 2 is two orders of magnitude faster than the previous circuit and has a dynamic range of 80 dB. Operation is the same as the circuit in Figure 1, except the configuration optimizes speed rather than dynamic range.
If settling speed has anything to do with highest frequency, then, for Fig.2 circuit, using f =1/T, we have f= 1/0.00005sec, that is f=20kHz worst case and f=1/0.00001sec=100kHz best case
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Old 9th June 2011, 04:40 AM   #14
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I know that building your own stuff from scratch is fun and fulfilling but sometimes you have to draw the line somewhere.

Re: NS AN-30, where can you find the thermistor used in the example? Is there a reference board that you can copy to save time? Is the person that will be building this log amplifier have enough equipment and resources to validate the finished unit's operating performance?

Personally, I'm not willing to spend days, weeks and even months designing something that already existed, tried, tested and the best part of all cheap!

Not here to argue, just pointing out the obvious
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Old 9th June 2011, 10:35 AM   #15
gpapag is offline gpapag  Greece
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Quote:
Not here to argue
Hitachi_nut

No problem from my part.
I just hope you didn’t take my intervening as an attempt to argue with you either.

It was only that with my metrics, 12 UKP (from RS Components) isn’t cheap.
The penalty I have to pay (and gains I may get)for choosing the cheap components route though, is well described in your response.

I have nothing to confront your solid logic.
On the contrary, to add to it, from the AD8307 data sheet
http://www.analog.com/static/importe...ets/AD8307.pdf
Fig 19, I can concur that the response time has nothing to do with freq.response.
Got me a null score here too.

Therefore, the question

Quote:
What Frequency range?
Has to be answered by a knowledgeable member. Sorry geraldfryjr

Regards
George
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Last edited by gpapag; 9th June 2011 at 10:57 AM.
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Old 9th June 2011, 01:45 PM   #16
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What Frequency range?

Audio Band
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Sameer Dhiman
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Old 9th June 2011, 02:44 PM   #17
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AD8307 frequency range as specified on the datasheet -> DC-500MHz, not enough

Dynamic range is better than 88d B, Settling time < 500nsec

Almost everything you need to know is on the AD8307 datasheet. Analog Devices even offer samples for crying out loud. That's how I got mine before

AD8307 | Low Cost, DC to 500 MHz, 92 dB Logarithmic Amplifier | Log Amps/Detectors | Specialty Amplifiers | Analog Devices

There's also a guy on eBay selling them for <$10 shipped. There several ways to get the chip "cheaply" if look hard for it.




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Originally Posted by sameerdhiman View Post
What Frequency range?

Audio Band
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Old 9th June 2011, 02:51 PM   #18
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George,

I have no desire getting a score with this thread. No offense.

I just have first hand knowledge in the topic and I'm glad to lend a helping hand to DIY out there

Cheers!

Quote:
Originally Posted by gpapag View Post
Hitachi_nut

No problem from my part.
I just hope you didnít take my intervening as an attempt to argue with you either.

Fig 19, I can concur that the response time has nothing to do with freq.response.
Got me a null score here too.

Regards
George
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Old 9th June 2011, 08:19 PM   #19
gpapag is offline gpapag  Greece
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hitachi_nut

It was my pleasure reading your posts here.



George
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