Best OpAmp for Rx signal...?

[Esteban]

AD8055, single and AD8056, dual.

http://www.analog.com/static/importe...D8055_8056.pdf

[porkluvr]

AD8055A single quantity $2.20 at Digikey. Looks sweet and it won't break the bank.

It's missing null pins but that can be worked around.

AD8056A is $3.71. (At this time.) Good find, Esteban.

[Carl-NC]

AD797 for best noise, MAX410 is also good.

[porkluvr]

mo amps

Intersil EL5130, EL5131

300Mhz, 350v/us and low noise. Vos=0.9mv (max)
+/4ma supply current. Optimized for low voltage supplies.

EL5130, SO08 package, has shutdown pin
EL5131, SOT23-5, no shutdown
(Neither is available in DIP-08.

Not the best supply chain in the world so I'm trying to fanagle samples from Intersil. Parts can be special ordered
through Digikey (or from Intersil.)

http://www.intersil.com/data/fn/fn7381.pdf

Of course a simple circuit could be used but I have posted an overly elaborate circuit that includes DC servo offset nulling,
and complementary outputs. Use part or all of it, or none of it. This circuit was originally designed for use with SSM2210 or
LM394 where I am showing 2N5210. But two or three parallel discrete BJTs (2N4400, 2N5210(?), etc.) can supposedly
give very low noise, inexpensively. 2N5210 might not be the best choice in either the current source or the two-legged pair. Experiment.
Sorry, but I can't offer a good mathematical noise analysis. (Aww, who needs low noise, anyway?)
Included is an exerpt from the SSM2210 datasheet.

[Unregistered]

Best op amp

Hi,
lt1028. use 2 in parallel. Very hard to set up, but good results. very low noise. same pinout as ne5534.
My 2 bobs worth

[porkluvr]

Hi unregistered, if you would register and post your circuit it would be a great help.
*****************
My previous circuit looks good with a small signal input but stability falls apart with a 5V (or 50V, or 200V) input. Minelab uses a NE5534 in a similar circuit in the SD2000 (but without the servo null). 1k is used in the feedback (instead of 10k) and the compensation RC across the opamp inputs has 150 instead of 560. The circuit is stable but overload recovery is not so great.

SD2000 uses a blanking gate so it's hard to say what the highest voltage the preamp input would be subjected to. Being as how the primary sample occurs at about 15us I wondered why a blanking gate was necessary. Well, it very likely because of poor overload recovery characteristics.
Sorry about the bogus circuit, and getting off-topic.

[Unregistered]

Best op amp

Hi Porkluvr,
I do not have the circuit diagram for this set up. Minelab use dual ad797 in their later model detectors. These can be swapped out for the lt1028 for better performance. A 10pf cap put on pins 2 and 6 will lower the bandwidth, and make it more stable and less prone to emi. The gain can be controlled with pin 5.

I think that the reason Minelab use fets is to stop the front end being damaged by the high fly-back voltage, and also then the settling time of the amp is greatly reduced. Also avoid using resistors in the input path as they create noise.

As for your circuit, it is very much the same as the sd2000! To get lower noise from it try parallel ssm2210, or even some mat02 devices. What does the DC servo offset nulling do? The highest voltage that the sd2000 preamp is subjected to is bugger all! Maybe .6v.

[porkluvr]

Hello unregistered. The purpose of a DC servo loop is to reduce DC offset voltage to (sub)millivolt levels even without using a potentiometer across the opamp's null pins. So, theoretically you could plug a viriety of opamps into the same circuit with total disregard to the null-pins configuration (whether it be 1&5, or 1&8, or whatever) and still have almost zero volts offset at the preamp output.
There are some problems, mainly that the DC servo loop components add about 1.5us to the settling time of the circuit - even after hand selecting compensation components. Another problem I'm finding is that with these composite amplifiers there is a need to custom select the compensation components. So, they're tricky to set up. But I think it could be worth the trouble because I forsee low noise, fast response, and fast recovery from overload. I just need to work out the BUGS.

One issue that hangs me up is that a some manufacturers have spice models for their opamps but for some reason they don't model the compensation pin(s) if they exist. That's part of my problem now - one opamp that I thought has great promise (and a half dozen of) has a compensation pin that the spice model completely ignores. Well, there are other fish in the sea. Of course, I could always just plug one in and see what happens. It's like having a shed full of broken garden tools. (Or no tools.)

And so, I thought I had another "candidate" opamp along with a composite amplifier circuit to post, and it worked fine - banging it with a 400V jolt, but when I reduced the input pulse to millivolts the amplifier's stability dissapeared and I have a high frequency sine wave riding on what should be a flat peak. Sort of the opposite of what I had the other day - working great at low signal levels and falling apart with a HV input. There is that unmodelled compensation pin that might save the day, or might not. Buh.

Yup, ML's SD2000 has essentially no resistance between the preamp and the coil which means that the blanking gate FETS need to timed reliably. It gives gray hair thinking about it.

I'm still looking for that magic bullet so I'll look into the LT1028 and AD797.

[Tinkerer]

Hello unregistered. The purpose of a DC servo loop is to reduce DC offset voltage to (sub)millivolt levels even without using a potentiometer across the opamp's null pins. So, theoretically you could plug a viriety of opamps into the same circuit with total disregard to the null-pins configuration (whether it be 1&5, or 1&8, or whatever) and still have almost zero volts offset at the preamp output.
There are some problems, mainly that the DC servo loop components add about 1.5us to the settling time of the circuit - even after hand selecting compensation components. Another problem I'm finding is that with these composite amplifiers there is a need to custom select the compensation components. So, they're tricky to set up. But I think it could be worth the trouble because I forsee low noise, fast response, and fast recovery from overload. I just need to work out the BUGS.

One issue that hangs me up is that a some manufacturers have spice models for their opamps but for some reason they don't model the compensation pin(s) if they exist. That's part of my problem now - one opamp that I thought has great promise (and a half dozen of) has a compensation pin that the spice model completely ignores. Well, there are other fish in the sea. Of course, I could always just plug one in and see what happens. It's like having a shed full of broken garden tools. (Or no tools.)

And so, I thought I had another "candidate" opamp along with a composite amplifier circuit to post, and it worked fine - banging it with a 400V jolt, but when I reduced the input pulse to millivolts the amplifier's stability dissapeared and I have a high frequency sine wave riding on what should be a flat peak. Sort of the opposite of what I had the other day - working great at low signal levels and falling apart with a HV input. There is that unmodelled compensation pin that might save the day, or might not. Buh.

Yup, ML's SD2000 has essentially no resistance between the preamp and the coil which means that the blanking gate FETS need to timed reliably. It gives gray hair thinking about it.

I'm still looking for that magic bullet so I'll look into the LT1028 and AD797.

I just interchanged the AD797, the OP37 and the LT1037 and don't find any difference in the output. All work fine but the AD797 is much more expensive.
I use the non-inverting mode.

Tinkerer

[porkluvr]

Hello Tinkerer, everybody.

Are we skirting an important issue?

For a long time I blithely accepted that something like NE5534, or OP37 (LT1037), etc. should work fine as a preamp, running at gain of 500 or 1000, and sampling at, oh about 7us from the end of the tx pulse. I have found out that they are really not up to the task by themselves. The best explanation I can give for this is to quote Carl Moreland from something he wrote in response to some questions in the Hammerhead PI forum.

As I just a moment ago tried to figure out how to set a hyperlink directly to the post of interest I see that you were part of the discussion, so this should be nothing new to you.
http://www.geotech1.com/forums/showt...7835#post87835
(Read the whole page because I don't know how to point to the single pane I find most illuminating).

"... Some opamps that otherwise look great have poor overvoltage recovery.
... the BW spec determines how much gain you can run. This spec is actually a gain-bandwidth product, so an NE5534 (GBW=11MHz) with a gain=1000 (HH) will have a BW=11kHz. This results in a response time constant (tau) of 14.5us. If you want to sample at 15us, then you really need a faster response, so either a higher GBW or a lower gain (for some reason I thought the NE5534 had a higher GBW).
Ferinstance, let's say that at 15us we want to be at 3*tau settling on the opamp, so that means tau = 5us, and BW = 1/(2*PI*tau) = 31.8kHz. For GBW = 11MHz, this limits the gain to 345. You can run a higher gain (as with HH) but the opamp settling won't be out of the way and can degrade sensitivity. So HH really could use a reduction in preamp gain, which Reg Sniff has pointed out previously. Or a better opamp."

(Carl, please accept my appology for hacking up your explanation. Your input here would be greatly appreciated.)

In a nutshell, the above mentioned opamps are good only so long as you do not try to extract too much gain or sample too soon, otherwise the opamp's output settling is still in a state of flux. If you sample too soon you are sampling the opamp's overload voltage instead of the coil's signal. We all know that, but it is a painful truth that is sometimes ignored. Even the often touted AD844 (60Mhz, 2000v/us current/voltage FB hybrid) is not a magic bullet here.

Super-fast amplifiers may be hard to work with for the average builder. I lean towards a dual stage preamp (splitting up the gain) or else using a composite amplifier, and "common" opamps. My spice tests show that even most common "fast" opamps when operated at gain of 500, are still recovering after 5~6 microseconds, and take even longer at gain of 1000. It is not an instantaneous process. The exact time to recovery depends on the particular opamp and the definition of "zero". What I mean by that, is, as you look on a scope display and zoom in on the recovery event, at what point do we say the voltage is zero? Where, at 1mv, 10mv, or 10uv?? I have my own view on that but I"m lacking practical experience to "prove" it.

I don't have all the answers, but I intend to play it safe because after I go to the expense and trouble of purchasing a suitable length of specialty wire (18ga. stranded, tin plated, teflon insulated will do for me ), and the trouble of constructing a coil (and PCB), I don't want opamp performance to be a limiting factor. I would be very, very mad at myself.

I am not a mathematician nor an engineer so I rely on LTspice to draw pictures for me. I try to input accurate data and what I expect in return is reasonably accurate representation of events that I cannot actually see for myself until I have test equipment and a place to use it.

I've probably pissed everybody off here with my long-winded diatribe so I'd better go. I don't want to monopolize this thread and I have a receiver design to botch, or not.

edit: I may as well say it. I'm looking at the AD825 in a composite amplifier. It's not perfect, but it looks "nice".

[Tinkerer]

High signal amplitude

porkluvr,

you are very much right about the Opamp recovery time from saturation.
When I mentioned the AD797, OP37 and LT1037, I should also have mentioned that I use low gain.
I add the link to my post below, where I show scope pix of sampling at 3uS.
The pre-amp has a gain of 40.

http://www.geotech1.com/forums/showthread.php?t=15383

The idea is to produce a signal that is of high amplitude at the input of the preamp, then use only the amount of gain that does not saturate the pre-amp.

The secret lies in the coil, not the Opamp.

Tinkerer

[nick_f]

Hi Porkluvr,
You have quite some knowledge about Minelab circuits. Do you have any links or details about their circuits? I am interested in their choice of input opams and MOSFET. Do they use one stage amplifier or two amplifiers (like QED project)?

Regards,
Nicolae

[porkluvr]

porkluvr,

... I should also have mentioned that I use low gain.
I add the link to my post below, where I show scope pix of sampling at 3uS.
The pre-amp has a gain of 40.
http://www.geotech1.com/forums/showthread.php?t=15383

The idea is to produce a signal that is of high amplitude at the input of the preamp, then use only the amount of gain that does not saturate the pre-amp.
The secret lies in the coil, not the Opamp.
Tinkerer

Ahhh! Thank you for clarification. I don't know nearly enough and sometimes I get to wondering if the laws of physics work differently in other parts of the world. I assume, incorrectly, that everybody is using a gain stage of 500 ~ 1000 AND sampling in the single-digit microseconds. Your revelation is a relief!!

edit: I need to stop doing so much assuming and believe instead what I know (cough, cough) to be true.

[Tinkerer]

Ahhh! Thank you for clarification. I don't know nearly enough and sometimes I get to wondering if the laws of physics work differently in other parts of the world. I assume, incorrectly, that everybody is using a gain stage of 500 ~ 1000 AND sampling in the single-digit microseconds. Your revelation is a relief!!

edit: I need to stop doing so much assuming and believe instead what I know (cough, cough) to be true.

Yeah, I spent a long, long time trying to combine high gain and short delay. I devised complicated feedback schemes to avoid saturation with up to 3000 gain, but eventually it did click.

As mentioned before, the secret is in the coil, not the Opamp.

I would love to compare some pre-amp output values. It is the only way I know how to confirm that what I am doing is good or useless.

All the best

Tinkerer

[nick_f]

Hi Tinkerer,

I have AD797 and LM318 and I will also look to some similar experiments as yourself. I will let you know of my results with DD Coils and Concentrical Coils, in the next weeks. I checked your waveforms at 3us, they are very interesting. You have some oscillation in circuit, but I think that doesn't matter.

Regards,
Nicolae