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Logarithmic compression for op-amp

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JackN

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Hello folks, I'm a new learner of electronics so all help appreciated.
I've been using the TI LMC6482 op-amp for signal conditioning a shock sensor. Since the output range is limited to 0-3V, it's difficult to measure the range of force of the shock I want to measure.
So a solution is to compress the signal using a log amp. This is beyond my ability to find the right chip. So a simple route would be to use a bipolar junction transistor on the feedback of the LMC6482.
Is this a good solution, or is there a better way using integrated log amp chip running at 3V? Or any other automatic signal compression methods? Need the 0-3V out for going into a microcontroller.
Cheers.
 

audioguru

Well-Known Member
Most Helpful Member
Yes, it looks like simply adding a transistor to an opamp to make a log compressor also makes a thermometer.
 

OBW0549

Active Member
So a solution is to compress the signal using a log amp. This is beyond my ability to find the right chip. So a simple route would be to use a bipolar junction transistor on the feedback of the LMC6482.
I think that's the right approach to take, and is described well in the appnote cowboybob linked to. What might work well would be a hybrid of what's shown in Figs. 1(a) and 1(b) in that note: for feedback, use two transistors, one NPN and the other PNP, connected collector-to-collector, base-to-base and emitter-to-emitter. If you're running the opamp off a single 3.0V supply, bias the transistor bases and the opamp's (+) input at the supply midpoint (1.5V) instead of ground. Keep in mind that since Vbe of a BJT varies with temperature, the scale factor of this circuit will be temperature-dependent.

Is this a good solution, or is there a better way using integrated log amp chip running at 3V? Or any other automatic signal compression methods? Need the 0-3V out for going into a microcontroller.
I'm not aware of any integrated log amp chips that will run off that low a supply voltage. There might be some, somewhere, but I haven't seen them.
 

JimB

Super Moderator
Most Helpful Member
You may also like to consider the AD8307.
This log amplifier is often used for RF applications, but does have a response down to DC and the datasheet does show a DC signal application.

Having said that, I just checked the Analog Devices website and the latest revision of the datasheet (Rev E) does not show the DC application.
However, my own local copy (Rev C) does have it, and I am attaching it here.

JimB
 

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OBW0549

Active Member
MAX4206 datasheet: https://www.maximintegrated.com/en/products/analog/amplifiers/MAX4206.html#popuppdf

It'll run with as little as a 2.7 VCC. You can still order a sample.

I should say that I have never used this chip.
Yes, I noticed that chip and read the data sheet; it would do just fine, if JackN's input current were unipolar. However, if I understand his application correctly, he's monitoring the output of a piezoelectric (I assume) shock sensor which produces an AC output. If that's the case, I don't think the MAX4206 would be usable.
 

JackN

New Member
Thank you all for your kind responses.
Yeah the sensor is piezoelectric. But for measuring the impulse magnitude of a shock, rather than the vibration, can we just take the first large positive curve and ignore the rest of the oscillations?
With the TI LMC6482, it kinda rectified the output a bit. It just saturated to 3V too easily.
I've ordered some samples of the MAX4206, so will report on how I get on.
Can I try the set up shown on Figure 4a on the data sheet which was for a photodiode measurement?
Should I change any of the resistors or capacitors values?

Apologies for so many questions. I'm totally new to log amp, and the idea of utilising 5-decade dynamic range. Does that mean the impact force F is proportional to the logarithm ln(Vout)?
Or rather, proportional to the logarithm of ln(charge), which means ln(integral(Vout))?
Finally, is there a simple way to carryout the integral of Vout?

Yeah I'll be running it on a 3V button battery with an Arduino, so hope the MAX4206 would be alright.
Cheers.
 

audioguru

Well-Known Member
Most Helpful Member
A photodiode has a very low output voltage and low impedance. It produces a current output. It feeds the transimpedance amplifier circuit shown in the datasheet of the MAX4206.
A piezo device has a very low output current and is high impedance. It produces a voltage output so the transimpedance amplifier circuit is not suitable.
 

JackN

New Member
That's right audioguru. So my first attempt was to use the LMC6482 op-amp as a charge amplifier, converting the voltage into charge from the piezo. But since I need to run the op-amp at 3V (for battery powered), it saturated really easily with high impact force on the piezo.
So the MAX4206 does not work in the same way I take it?
Would there be other solutions to this problem?
 

audioguru

Well-Known Member
Most Helpful Member
I do not know the spec's for your shock sensor but a piezo produces either a positive or a negative voltage spike that might be hundreds of volts.
The extremely low power supply voltage for your opamp reduces its dynamic range a lot and the log amp works only with a positive input voltage, not negative and not AC.

I think you must rectify the voltage from the shock sensor and use it to feed an opamp with a reasonably high power supply voltage and it can feed a log converter IC if you want.
 

JackN

New Member
So please see if the following works:
3V button battery supply --> Voltage step up converter to 9V --> LMC6482 (Charge amplifier, with 9V dynamic range)
--> Max4206 Log Amp running at 3V.
Piezo sensor --> LMC6482 (9V would give rectified nice long positive curves for each incoming shock) --> Max4206 (3V) --> Arduino (3.3V input)

Would this be a reasonable first prototype?
Cheers.
 
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