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Whites PI3000 Modification Release

Hi Luca,
That is one amplifier I have not tried. There are a lot of other considerations when going to a faster amplifier and trying to sample earlier. I doubt if the coil on the PI1000 would be any good for delays less than about 20uS. Coil design becomes more critical and a good shield is essential. I have tried fast amplifiers such as the AD8055, but one problem is that the low frequency noise is often considerably worse. One thing about the 5534 is that it is nice and quiet.
Eric.
 
Hi Luca,
Pin 2 is a virtual ground and with an ideal opamp, remains at ground potential. The source drives current through the 1K resistor and the output of the opamp goes negative sufficiently to give the same current in the 1M feedback resistor. Hence the voltage gain. Just like a see-saw where pin 2 is the pivot, which is much nearer one end than the other. Connecting a 10K resistor from a virtual earth to ground does nothing. Protection diodes are necessary at this point because the coil back emf is much larger than the amplifier can handle and still look like an opamp. Some devices have the diodes built in, but I always fit two external ones in case I change to a type that doesn't.
Eric.
 
Hello,
I did get a picture of the waveforms on my Tek oscilloscope.
In the picture you see two waveforms:
Upper) taken at Pin2 of 5534 (Vin-)
Lower) taken at Pin6 of 5534 (Vin+)
reference is GND
Upper is 1V division, Lower is 5V division, DeltaT (12uS) is measured from X to X (see picture).
My idea is that my head is 12uS fast and that the 5534 is overloaded until 30uS (measuring from TX end), otherwise I do not understand why Lower) curve takes other 10uS and more to smooth down.
Any other explanation?
HH <img src="/metal/html/smile.gif" border=0 width=15 height=15 alt=":)">
 
Hi Luca, I like your stubborn toilling in harness, while others wait for a copy. Throw the petrified 5534 to thrash, take some opamp with high slew rate and low noise, my favorite are OP37 and better LT1028. With the better speed you will choose new damping resistor. Also check grounding on the DPS - voltage supply, ground end of coil and the reference input of preamplifier should not be distant from each other, the best place of grounding is by the ground end of limitting diodes. Good luck. Sid
 
Thank you very much! I was looking for the AD767 (sounds like a jet plane!) that seems fast enough to handle 10uS signals at 1000 times gain and has a very (very!) low noise figure.
It appears to me that the real problem should be the overdrive recovery time of these op amps.
Even if they should be related to eachother, overdrive recovery time should be one thing and bandwidth another one I guess.
I'm building a small test circuit (I have a function generator) to evaluate some of them.
It will take a while.
Again I'll post my results here.
One question: How do you null OP Amp output offset? I have the example connections (a trimmer) . Should I ground Vin- and Vin+ and trim for zero output at Vout?
Thanks for the help <img src="/metal/html/smile.gif" border=0 width=15 height=15 alt=":)">
HH
 
Hello,
The ad767 is a d/a converter. What is your opinion of the ne5539?
 
Hi Luca,
Good picture. I now see that it is inverted to what I am used to, as the TX is the other polarity. Both waveforms are quite normal. What looks like a sharp knee and straight line on the input, is not so when amplified to a high degree at the output. There is often a long low current tail to the TX switch off, which indicates that the coil may be over damped. This was often the case in earlier PI's where no attempt was made for ultra short sample delays. I remember when 75uS seemed short.
The longest delay between the initiation of the TX switch off and the start of sampling, is dissipating the energy stored in the coil field. This is my first line of approach, which affects both the design of the TX itself and also the coil. Having got this right, then you can use a faster amplifier if required. Beware though, that fast wide band am-lifiers, although having good noise performance at high frequencies, are often poor at the frequency components we are interested in. This will result in the detector having a rough audio threshold.
The 5534 can be used, with suitable coil and transmitter, for sample delays as short as 5uS at a gain of 500x, so I only use a faster amplifier if I need to go shorter than this. So far, this has only been for special industrial uses.
Eric.
 
Hi Sid,
Data on LT1028 looks good. Low noise at low frequencies too. I'll get one and try it.
Eric.
 
Hello,
I ordered today a couple of AD797 from RS Components Italy.
I decided to try the AD797 after some specs and literature reading.
The problem I'm trying to solve is the rather strange instability of the output of my 5534 in the 20uS-30uS area.
You suggested to check 5534 bandwidth at a factor of 1000 gain as the case is in my PI3000.
I did it. In fact,on the 5534 specs, at a gain of 1000 times, bandwidth seems to be gone for signals faster than 30uS.
Since I do want to respect original PI3000 design, I need to amplify coil signal at least 1000 times.
So my first goal is to try replace the 5534 with something faster.
You also mentioned noise, so I tried to find something that could satisfy both constraints.
The AD797 is a very low noise op amp (less than half of the 5534) and it's bandwidth curve is just above 100000 Khz (10uS) at G=1000 . This should warrantee a stable signal at 15uS delay. Is all that I am saying orribly WRONG?
As for the 500 times gain suggestion, I took it seriously and , in fact , I got a second Ad797 to eventually cascade it to the first one in a Inverting->Voltage follower chain to still obtain the required 1000 times gain (I choose gain 30 for the inverting op amp and 35 for the voltage follower).This configuration will have a badwidth in the Mhzs...
The LT1028 has a good noise figure but is a little slower than the AD797.
Look at the charts for a comparison...
I hope I'm not wrong, I'm new to this kind of electronics..
HH <img src="/metal/html/smile.gif" border=0 width=15 height=15 alt=":)">
 
The ad797 seems to be a good one.
(Anyway i'm not an expert only I interested in the usefulness the high freq OPAs in PI detector. )
 
Hi Luca, you are right, but theory is grey and practise is green. Both these op amps are designed to other purpose and are tested in different conditions,eg. input resistance. The 797 gets his low noise from high biased input currentt mirror, so expect high energy consuption, high offset, low temperature stability, the more, it is about ten years old. The LTs have more progressed technology and wide spectra of properties, I could not get the 1222 and 1226, also promissing ones. Only experiments will solve these conditons of high saturation and its recovery. Good luck, Sid.
 
Thanks , in fact I did try to drop the 797 in the 5534 socket, having checked first to avoid connecting pins 8 and 1 . What I got is a very odd output curve.
I suspect this is due to the low input impedance requirements of the 797.... I'll try the LT1028 if I can find one.
In the meanwhile I replaced the original 5534N of my PI3000 with a 5534AN that has warranteed noise figures.
HH <img src="/metal/html/smile.gif" border=0 width=15 height=15 alt=":)">
 
Hello,
I tried the 797 incircuit with some combinations of resistors. Lowering gain ( I tested it down to G=10) or lowering the surce resistor ( I tried 10Ohm instead of 1K) led to nothing. It looks to me that the 797 is not able to recover fast from overdrive.
It remains in ovedrive condition for about 30uS before reacting violently.
This is the only explanation my poor experience can give.
I hope I can get an LT1028 and try it... <img src="/metal/html/smile.gif" border=0 width=15 height=15 alt=":)">
HH
 
Eric Foster has posted time and again that he has tested many OP AMPs with great specs but don't recover quickly. Looks good on paper but doesn't work out for this application. As for low noise, Eric has said many times also that the noise picked up by the coil far outweighs the small noise generated in the front end op amp and so a less noisy op amp isn't going to make a difference there. It will be interesting to see what the LT does for you.
Good luck,
FJ
 
oi friends! I am in the northeast of BRAZIL and am following all the colloquy of voces I am tecnico in eletronica and I have a PI3000 wanted to know if voces has a solution I to place a discriminator in my PI3000? GOOD LUCK!!!!!
 
oi friends! I am in the northeast of BRAZIL and am following all the colloquy of voces I am tecnico in eletronica and I have a PI3000 wanted to know if voces has a solution I to place a discriminator in my PI3000? GOOD LUCK!!!!!
 
Hello,
I think that you can use my mod to have a sort of discriminator just by increasing the rx sample delay time. I don't see it much useful anyway. If you want discrimination you need a vlf type detector.
cheers
Luca
 
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