NEW !!! Full differential PI front end.

[Carl-NC]

Sampling into an ADC has some advantages. First, you get the full instantaneous amplitude of the sample instead of a time-integrated amplitude. Second, an integrator limits your LPF to a simple exponential decay, whereas an ADC approach lets you try out radically different digital filter algorithms. Third, the ADC makes it really easy to run multiple sample channels without building a lot of analog integrator channels.

The drawback is you need an ADC that is fast enough and with enough resolution. I've made a workable PI detector with only 10 bits right off the preamp. 16-18 bits is now very reasonable.

No, the S&H approach does not replicate the LPF of the integrator. The extra series R with CH slows down each individual sample which will LPF that particular sample, but there is no sample-to-sample integration filtering, which is where you get noise reduction.

- Carl

[Aziz]

Hi Tinkerer,

I can add some more to Carls post.

The LF398 is just a gated voltage follower (gain=1). Where as a true integrator is like a transconductance amplifier. The integration is made via voltage to current conversion and the current charges a capacitor (makes the real integration). Op-amp circuits in inverting configuration are typical transconductance amplifiers (I-V conversion). So an integrator circuit is typically in such an inverting op-amp configuration.

Therefore the LF398 can not make the integration. In your circuit, the high series resistor to the holding capacitor forms a LPF but it does not integrate. It also increases the settling time of the S&H amplifier.

One can make an integrator, which also has a S&H capability. It should be called integrate & hold in this case of course.

Good integrators should have a gated differential input and a clear (reset) possibility. As the signal should be fed in differential manner, any leakage input currents will chancel each other. And one can select the polarity of the integration (doing signal addition, subtraction) . And if both inputs were disabled, it will hold the integration output until it is cleared.

Integrators in PI's greatly reduce high frequency noise.

Aziz

[Aziz]

Hi Moodz,

have a look into integrating ADC's:
http://www.maxim-ic.com/app-notes/index.mvp/id/1041
http://en.wikipedia.org/wiki/Integrating_ADC
http://www.allaboutcircuits.com/vol_4/chpt_13/8.html

The integrating ADC eliminates most of the noise during the conversion operation.

Aziz

[Tinkerer]

Carl and Aziz,

many thanks for the explanations.

Looking at the Maxim application notes, I find the 100 conversions per second a bit slow.

The integrating ADC looks good for my planned design that uses large coils.

For small coils, 100 conversions per second would make a rather slow response. Do I get that right?
Considering a sweep speed of 100cm/second, it would mean one conversion per 1cm distance traveled by the coil, not considering any additional signal processing time.

For large coils, the sweep speed will be at 50cm/second or less.
With the large coil, a response time of 5cm, or 100ms is acceptable.

Using the TEM method we can run a large coil at 3000 PPS with 10 Amps coil current. So we can do some oversampling. A Sigma/Delta ADC could also be used.

Does this make sense?

Tinkerer

[Carl-NC]

Integrating ADCs only integrate on a per-sample basis, not across multiple samples like a true sampling integrator. So you still don't get sample-to-sample filtering.

Be careful using sigma-delta. Most require oversampling which means you will be sampling continuously along the RX waveform, and the specific sample points you're really interested in will get buried. You can put a S&H in front of a SD-ADC but you will be limited to one sample point per ADC.

If you are looking at 1-3 sample points on the decay, I would recommend using sampling integrators as in a normal PI. If you want to look at 4 or more sample points, or you want to do some exotic filtering, then consider a high-res SAR ADC and a good micro. 16-18 bits with 1us resolution is easy.

- Carl

[Tinkerer]

Integrating ADCs only integrate on a per-sample basis, not across multiple samples like a true sampling integrator. So you still don't get sample-to-sample filtering.

Be careful using sigma-delta. Most require oversampling which means you will be sampling continuously along the RX waveform, and the specific sample points you're really interested in will get buried. You can put a S&H in front of a SD-ADC but you will be limited to one sample point per ADC.

If you are looking at 1-3 sample points on the decay, I would recommend using sampling integrators as in a normal PI. If you want to look at 4 or more sample points, or you want to do some exotic filtering, then consider a high-res SAR ADC and a good micro. 16-18 bits with 1us resolution is easy.

- Carl

Thanks for the explanation, Carl,

I had a mis-conception about the meaning of oversampling.

I want to sample at 4 or 5 different times. Average each one of the samples over 3,000 to 10,000 readings and store the results. (running at 3,000PPS)

This is my ground sample.

Then I want to repeat the same operation, but with averaging only 50 to 100 readings for target search.
The difference between the two results, processed, should give me the target information.

I am trying to decide how much of the front end to keep in analog before digitizing. Less seems to be better, but I am still trying to wrap my mind around the dynamic range.

So much still to learn............

Tinkerer

[Carl-NC]

As I mentioned, I did a 10-bit sampler off the preamp, just to see what it would do. I did a boxcar average on 16 points for the target, then took that average and averaged it again 16x (effective 256x) for the "tracking" average. It worked surprisingly better that I expected. I was limited by the low-memory PIC I was using.

[Krzysztof]

Hi Tinkerer.
I use this method successfully. Samples of the prescaler counts the 6-12bit and counter 16bit driven by clock 2MHz.
As to what occurred indefinitely unexpected increases (EMI?).
I used repetition - 4x enough, the situation as the perfect fairy tale.
Total applied to the magnetometer - gradiometer with ATMega8 (ATMEL) and SD card through the RS 9600baud. It was enough for two results per second.
Now I'm learning, "PSoC" (Cypres), a similar system, controls the 8bit prescaler counter 16bit driven by clock 4MHz.
For now, no communication is sufficient for 100 results per second.
Best regards Chris

[Tinkerer]

As I mentioned, I did a 10-bit sampler off the preamp, just to see what it would do. I did a boxcar average on 16 points for the target, then took that average and averaged it again 16x (effective 256x) for the "tracking" average. It worked surprisingly better that I expected. I was limited by the low-memory PIC I was using.

Your HH II, is a very good way to show how an analog PI detectors can be changed into a digital one.

At how many PPS are you running it in the example above?
How does the HH II compare with the HH I in sensitivity and depth?
Is the response faster? slower? the same?

I consider the HH I and the HH II the best designs for beginner metal detector builders.

Is there any chance you get time to work on the HH III?

Tinkerer

[Tinkerer]

Hi Tinkerer.
I use this method successfully. Samples of the prescaler counts the 6-12bit and counter 16bit driven by clock 2MHz.
As to what occurred indefinitely unexpected increases (EMI?).
I used repetition - 4x enough, the situation as the perfect fairy tale.
Total applied to the magnetometer - gradiometer with ATMega8 (ATMEL) and SD card through the RS 9600baud. It was enough for two results per second.
Now I'm learning, "PSoC" (Cypres), a similar system, controls the 8bit prescaler counter 16bit driven by clock 4MHz.
For now, no communication is sufficient for 100 results per second.
Best regards Chris

Hi Krzyszof,

Thank you for your comments.
I am not sure I understand all of it. The translator has twisted some meanings.

Is your magnetometer a Proton Precession Magnetometer? Proton
magnetometers run at a much lower pulse repetition rate than metal detectors.

It is also difficult to run a traditional PI-MD at 10A coil current at a high repetition rate.

The fact that it is possible to run a PI-MD at a rate of several thousand pulses per second at high coil currents, is what has me excited about the TEM TX method.

Tinkerer

[Carl-NC]

Your HH II, is a very good way to show how an analog PI detectors can be changed into a digital one.

At how many PPS are you running it in the example above?
How does the HH II compare with the HH I in sensitivity and depth?
Is the response faster? slower? the same?

Actually, my digital experiment wasn't done on HH2, though it can be. But I need MicroChip to release the 12F1822 so I can do that. Supposedly they are sending me samples now.

I think I was running 600Hz. Depth was not good (but still better than I expected), and I'm sure the response speed was not optimized.

Is there any chance you get time to work on the HH III?

I wish. My latest thought on HH3 is to just set up a 2-stage preamp followed by an 18-bit ADC and micro. Very simple circuit. Then everyone can play with pure processing methods to see what works and what doesn't. The Big Question is what micro? PIC may be under-powered.

- Carl

[Krzysztof]

Hi Tinkerer.
My system magnetoinductive fluxgate, the coil current 1 - 2mA.
Repetition frequency of 20 - 100kHz.
Best regards Chris.

[greybeard]

I wish. My latest thought on HH3 is to just set up a 2-stage preamp followed by an 18-bit ADC and micro. Very simple circuit. Then everyone can play with pure processing methods to see what works and what doesn't. The Big Question is what micro? PIC may be under-powered.

- Carl

I'm currently starting to play with the HH2. As part of what I'm implementing is a fancy front panel ( 128x64 ) with a rotary encoder and menuing system. Obviously the 8 pin pic's don't have enough I/O to do this without addons so I'm considering a seperate processor for the 'front panel / HMI' interface. What would be nice is if a suitable processor can be found with a few spare i/o for the base board so it can be easily interfaced into a HMI interface. ie instead of ann 8 pin PIC use an 18 pin which would then make a serial interface between the two pretty simple.
It also give you an option of making the base processor configurable via software rather than requiring a new program.
By seperating the functionality, the incremental cost for an extra pic is only $4-$5, you still have the full power of the pic available to run the metal detector without the code having to cope with real time processing of data and human's.
I know code can be written to do anything, I code for a living, but it could be much simpler for others to play and experiment.
Rod

[Altra]

Carl,

I have been waiting for the Pic12f1822 also, 32mhz internal osc! They say it will be for sale in august. What do you think of increasing the AD resolution using over sampling? See attached article

Regards
Mark

[Carl-NC]

Traditional oversampling and decimation is done on a Nyquist-bandwidth periodic signal. In PI, we are normally sampling a roughly DC point, then averaging it to remove noise. So essentially, we are oversampling and using noise as a dither signal to get a higher filtered SNR.

- Carl