Meter with a log scale

Regular old ohmmeters have a nonlinear scale... I think its a reciprocal.... there is a bridge in the meter.

Reply to
BobG
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A silicon solar cell already has a log voltage output.

John

Reply to
John Larkin

A forward biased (voltaic mode) photo diode produces a voltage proportional to the light level, so you can use a linear meter to read the self generated voltage. You can add a linear DC voltage amplifier if you need more voltage to match the meter.

Reply to
John Popelish

Hello Dave,

If you want an analog solution check National's AN30 "Log Converters":

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Regards, Joerg

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Reply to
Joerg

Is there a simple way of getting an analogue meter (50uA or so) to give a deflection proportional to the log of the input? It need not be precise at all.

I want to have a 'light level' meter, which will handle a large range of light levels.

I was thinking that perhaps a forward biased diode in parallel with the meter might do this, as for increasing input levels, more current will go through the diode and less through the meter.

I expect there is a chip that will do this sort of thing, but I just want a simple solution. The instrument has a A/D converter and can collect the exact data into a file. I just want a simple analogue meter to show things are not too bad, without having a computer program running.

If it can be done with a diode as I suspect, has anyone tried optimising the layout of resistors that might be needed to give the most accurate log response?

Reply to
Dave

Does the reciprocal scale not result from simply the fact that the current through a resistor is proportial to 1/R. I don't think you need anything clever for that.

Reply to
Dave

This is a well-known solution. My Ph.D. thesis includes a reference to J.F.Gibbons and H.S. Horn's paper on that circuit in the IEEE Transactions on Circuit Theory CT-11, from page 378 (1964). I didn't use the circuit in my Ph.D. work, but I did put together a logging and anti-logging circuits on the same basis at EMI around 1977. They worked fine, but didn't do anything useful for our ultrasound images.

At Cambridge Instruments, the electron microscopes vacuum monitors used much the same circuit to compress some five orders of magnitude onto one analog meter.

------------- Bill Sloman, Nijmegen

Reply to
bill.sloman

I can't take that approach.

I have (and must use, as light levels are very low) a silicon avalache photodiode module from Hamamatsu. The output voltage is 7500 V / Watt of light on the device. With further amplifiation, the voltage level should be 1V or a little less, before feeding into an A/D.

The source is a 50mW laser at 780 nm, but this is attenuated a lot, hence light levels are very low, and I need the sensitivity of the APD. (I also need a bandwidth of DC-100MHz, which I'm not sure if solar cell would provide anyway).

This part is just one part of a more complex project. I just wanted an indication that the light level was about right, and was not too low, or too high.

Reply to
Dave

Wow, I couldn't *afford* and APD module from Hamamatsu!

OK, dump the diode current into a good silicon diode and measure the diode voltage drop with a high-impedance opamp, or put the diode in the opamp feedback. The b-e junction of a small-signal transistor is an excellent diode down to very low currents.

John

Reply to
John Larkin

And if you drive the current into the collector, while maintaining the collector at much the same voltage as the emitter, it works even better. See the Gibbons and Horn paper I cited earlier in the thread.

--------- Bill Sloman, Nijmegen

Reply to
bill.sloman
[...]

Heh - as long as we are getting into that price range, there's maybe good reason to look further. Analog methods drift, aren't too accurate, and get slooooow at low signal levels.

A perfect log converter (cost be dammed) would be a precision A/D, PROM lookup table to convert to log, followed by a suitable D/A.

This would have no trim adjustments, very tiny tempco, a constant conversion rate independant of signal level, and no long-term drift. Perfect for the photographer who has everything.

Mike Monett

Reply to
Mike Monett

Hello Bill,

In medical ultrasound you really need log detection though. There are some nice log amps but their prices have gone up quite a bit so often it becomes necessary to "roll your own". That also solves potential single source issues.

Regards, Joerg

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Reply to
Joerg

Hello Dave,

If cost isn't critical and here I mean one or two $20 bills look at the Analog Devices log amps. AD606 and others. During layout be careful that the input never "sees" the output but that goes for every fast log amp.

Regards, Joerg

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Reply to
Joerg

Hello Mike,

Not necessarily. I have used an AD log amp in a design a while ago. IIRC it was the AD630 and two of them in series to achieve the necessary dynamic range. Nice wide BW down to low levels and very precise. However, isolating input from output in such a high gain setup is no small feat.

That works nicely but when you need a dynamic range north of 80dB and several MHz of bandwidth it can really become cost prohibitive. The other downside I have experienced here was that top notch AD converters do not nearly have the production life of a typical log amp. Innovation happens so fast that it may be only a few years and you'll have to redesign the AD section which pretty much means the whole board.

Regards, Joerg

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Reply to
Joerg

No there isn't. The basic circuit is extremely complex: from one lead to a battery to a selected resistor, to the meter and to the other lead.

Reply to
Robert Baer

I thought the AD630 was a lock-in amplifier. Maybe it was the AD640?

Yes, high gain and wide bandwidth are a recipe for late nights:)

I thought the OP's app was a light sensor for photography.

That's called a "lifetime buy". The new devices usually have better specs and cost less, and you may end up redesigning anyway to keep up with the competition or the customer's requirements may change. That's called a "sales opportunity".

So you need to keep the soldering iron plugged in, and have Eagle and LTSpice running on the desktop:)

Mike Monett

Reply to
Mike Monett

Hello Joerg,

[...]

OT, but I just had two wisdom teeth pulled. The dentist said he wanted to take X-Rays and asked me if it was OK. I figured he was concerned about possible radiation damage, and said sure.

You know how they used to give you a film plate to hold in your mouth, then bring the X-Ray head next to the tooth? This was completely different.

He told me to get up out of the chair and took me to another room the size of a closet. The nurse put my chin on an adjustable rest and went into another room. When she pushed the button, a mechanism on either side of my head started rotating. When it stopped, she took me back to the other room.

A minute later, the dentist came back with a small viewer. It held a long negative that showed a 360 degree view of my entire mouth and teeth.

I have never seen such a clear X-Ray. It made it easy to see the damage was unrepairable and the teeth had to go. The next time I have to visit a dentist, I will make sure he uses a machine like this.

This is an example of how fresh designs can vastly outperform older technology.

[...]
--

Mike Monett
Reply to
Mike Monett
[...]

I don't know. The whole thing was paid by a medical plan and I never saw the bill. It couldn't be too much - this is a small town in Canada with mostly retired people on a pension.

Even if it costs a bit more, it sure is worth it. The amazing thing is you get the entire picture all at once. I could see the beginning of infection on the roots of some teeth on the other side, so I know I am in for a return visit.

I found a site that shows what it looks like. It's called Panorex and gives a panoramic view to show the general condition of all the teeth. It is usually taken every 5-7 years. Other types of X-Rays are needed for more specific diagnosis. (The panoramic image is much larger and better than shown here:)

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Mike Monett

Reply to
Mike Monett

It was available here in Arizona at least 15 years ago.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

Hello Mike,

You are right, I believe it was the AD640. The 630 is a commutating mod/demod.

Yes but the commercial chips are usually high BW so some care is necessary. Many people who use them in apps where only a few Hz are needed tend to overlook that they are dealing with the temperament of a Ferrari when using commercial log amps.

CFOs and corporate boards hate lifetime buys and the resulting surge in inventory. In the med world many designs just don't need any improvement over years, often over much more than a decade. Just look at the xray machines of many dentists. These can be 20+ years old and work just fine for them. Sometimes it's the same with automotive, for example the Ford F150.

Interesting. That's the same combination I use. I decided to switch from OrCad to Eagle a half year ago. I still treasure the cloth covered manuals of the old PSpice but now prefer LTSpice.

Regards, Joerg

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Reply to
Joerg

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