This is a 30 year old design which resolves mostly around replacing obsolete parts. The AD759P is the toughest, requiring significant circuit changes to fit in a new device. What might be the most practicable to use?
I unfortunately cannot find a datasheet for AD759, so I do not have a reference. Can you tell me what specifications you're looking for? The application would also help. For now, here is a list of the logarithmic amplifiers that we offer:
Translinear Logarithmic Amplifiers | Analog Devices
Let me know if any of these suits your needs.
It is a very old part, still apparently a few at Intronics (at great expense)
However, I have a job to replace a board with this circuit due to obsolescence of this part and others. I am looking at possibly the ADL5303 but I notice most log amps these days are geared to RF signal strength measurement or photodiode amplification.
This application is pretty static, low frequency if not DC, performing the log function in an analog system. In a modern system, this would be done in S/W but the system is not to be updated, only the component boards.
My concern is that the device used is the AD759P, the -ve version.
Hope you can advise. Thanks for the assistance in advance.
Oh yes, it is configured with a "k" of 2.
Aside from the 10k resistor, which is just a voltage-to-current converter, the 759 is functionally the same as the ADL5303, and many other ADI's trans-linear log amps.
If driving from a voltage source, you could add a properly scaled external resistor, that would map your voltage signal range to the measurement range of the ADL5303. Remember to account for the 0.5V DC bias that is on the INPT pin of the ADL5303.
Well OK then, I'll give it a try. I was thinking along these lines. Thanks for pointing out the 0.5VDC input bias, I wasn't aware of that.
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