Needing more than a spark test?

[graham-xrf]

@graham-xrf think you are making a fuss out of these component sizes. I have used DFN-8 and MSOP-8 parts on adapter boards. Using a stereo microscope so you can see what you are doing, put some tiny dabs of solder paste on the pads. Position the part on top of the paste. It doesn't have to be perfect to work, since the solder surface tension will align the part. A little hot air, and you can breadboard the assembly as if it were a DIP. It really isn't that hard, even with my wobbly hands. The hot air need not be very forceful, in fact it is better if it is gentle. It is surprisingly easy to do. The hot air guns are rather inexpensive and great for this or repair work.

I, like you, would rather work with slightly bigger parts, but alas, they are hard to find. Attempted to source some parts from all over the world, but they were unavailable. Or, they said they had them and later cancelled my order after a month. Adapter boards were my solution. Not elegant, but functional. Like you, I've spent a bit of time on the floor looking for parts that have flown off to who knows where. Maybe 1/2 the time I find them. I continue to look if they are one of a kind.

Sadly, you are right about the availability in bigger packages. Al lot of the best stuff is only available in those DFN packages that don'y have pins as such. OK then, if you think tiny dabs of solder paste and apply heat works OK, then we try that.

 

[WobblyHand]

OK then, if you think tiny dabs of solder paste and apply heat works OK, then we try that.

All I can say it works for me. Having good hands free magnification is necessary. Sometimes I need both hands to place the part just so. I use a wood toothpick and tweezers, or two tweezers to move the part sometimes. Why two? I use one as a pivot point, and the other to tap in place.

I've never soldered DFN with an iron. Just with hot air. Here's a slightly misfocused pic of an LT6237 DFN-8 soldered on a DIP adapter using the method I described. Was hard to keep the phone still enough! There's not enough room on the package to put the part number on it, the manufacturer has to use a shorthand code.

 

[homebrewed]

Regarding the open-source question, I can't speak for Graham, but my intent is to make all of my contributions open-source. Given the fact that so far the development has been on a publically-accessible forum, this just makes sense to me. Especially since there are lots of members who have (by helping to fund the forum) indirectly supported this.

 

[graham-xrf]

Is this the basic circuit that you (two) have started from?

View attachment 392587

Op-Amp circuits always manage to look similar, even with subtleties.
Yes - we do have a trans-impedance amplifier. The one you posted is exactly What it says. It is a "detector". It counts pulses it makes, hence the differentiator, which basically signals a waveform has changed at some rate faster than usual. My TIA amplifier is different.

Back in the thread, we have had discussions and many simulations. None of it is "private", though it has occurred to me that if we make something ragingly successful, some oriental git will produce a version in a plastic box with nickel paint on the inside. I am open source.

Post #403 on page 41 shows us noise modeling.
Post #421 on page 43 shows Mark's first explorations of signal on the Pocket-Geiger board.
Post #425 was me simulating the front-end circuit gragment with an artificial photon current. Note the new gain distribution, and bandwidth sufficient to preserve the pulse shape, and hence its area energy analogue.
Here is the image..


I went for complete recovery of all the photon energy the diode would give up, it's amplitude, and it's duration, with an integrated area under it's curve either fully calculated, or (better), maybe high-speed peak-detect approximated, and it's amplitude captured. The intent is to be able to tell what mix of energies were in there, and apply some discrimination logic to assigning counts.

I went for as low noise as possible, with enough gain in the first stage to put the (amplified) noise beyond being increased by subsequent gain stages, so locking in the signal-to-noise ratio. The subsequent stages provide differential gain to drive the ADC, which is 16-bit, sampling fast enough to get the pulse captured. They can also incorporate a 50/60 Hz notch filter. In my circuit, I use every trick I can to shunt the noise, and avoid overshoots at zero. I will post the whole thing, when I have done making mistakes with it.

 

[WobblyHand]

Similar, but not exactly. The starting point is the PocketGeiger, sold by Sparkfun. It uses a 10x10 silicon PIN diode for the detector. The bias voltage is higher, about 25V, generated by an on-board switcher. For about $70 you get the detector and electronics. If bought on its own from Digikey you will pay more than that for just the detector!

Good tip on getting the PocketGeiger from Sparkfun. Looking at the Sparkfun schematic, it appears nearly the same as the schematic I posted, with the first two amplifiers. (Same part number! Same basic idea.) It is nice that they showed the resistors in series for the first stage, so it would reduce the capacitance. The schematics differ in the output drive, being a comparator. Actually there are two comparators, each with a different threshold. One is filtered (the signal) with a fat cap across the comparator output (which isn't great practice) and the other without a cap (noise detections?).

What are you doing after the first two amplifiers?

 

[homebrewed]

The detector P/N: X100-7. I didn't find it on Digikey but Mouser carries it. $110.47USD. Interestingly, you can get it with a CsI(Tl) scintillator crystal attached to it for about $70 more. However, if I was to take the scintillator route I probably would spring for a SiPM-based system. Lots of gain with pretty low noise.

 

[WobblyHand]

Op-Amp circuits always manage to look similar, even with subtleties.
Yes - we do have a trans-impedance amplifier. The one you posted is exactly What it says. It is a "detector". It counts pulses it makes, hence the differentiator, which basically signals a waveform has changed at some rate faster than usual. My TIA amplifier is different.

Back in the thread, we have had discussions and many simulations. None of it is "private", though it has occurred to me that if we make something ragingly successful, some oriental git will produce a version in a plastic box with nickel paint on the inside. I am open source.

Post #403 on page 41 shows us noise modeling.
Post #421 on page 43 shows Mark's first explorations of signal on the Pocket-Geiger board.
Post #425 was me simulating the front-end circuit gragment with an artificial photon current. Note the new gain distribution, and bandwidth sufficient to preserve the pulse shape, and hence its area energy analogue.
Here is the image..
View attachment 392590

I went for complete recovery of all the photon energy the diode would give up, it's amplitude, and it's duration, with an integrated area under it's curve either fully calculated, or (better), maybe high-speed peak-detect approximated, and it's amplitude captured. The intent is to be able to tell what mix of energies were in there, and apply some discrimination logic to assigning counts.

I went for as low noise as possible, with enough gain in the first stage to put the (amplified) noise beyond being increased by subsequent gain stages, so locking in the signal-to-noise ratio. The subsequent stages provide differential gain to drive the ADC, which is 16-bit, sampling fast enough to get the pulse captured. They can also incorporate a 50/60 Hz notch filter. In my circuit, I use every trick I can to shunt the noise, and avoid overshoots at zero. I will post the whole thing, when I have done making mistakes with it.

Ah, thanks. Was replying to @homebrewed while you were posting. I'll take a look. Hopefully I can get an updated version of LTSpice to work on my linux PC. The work that I did on my 7 pole filter was on an earlier PC whose motherboard failed.

 

[graham-xrf]

Good tip on getting the PocketGeiger from Sparkfun. Looking at the Sparkfun schematic, it appears nearly the same as the schematic I posted, with the first two amplifiers. (Same part number! Same basic idea.) It is nice that they showed the resistors in series for the first stage, so it would reduce the capacitance. The schematics differ in the output drive, being a comparator. Actually there are two comparators, each with a different threshold. One is filtered (the signal) with a fat cap across the comparator output (which isn't great practice) and the other without a cap (noise detections?).

What are you doing after the first two amplifiers?

I be mulling over what to do up the front end, and how to get a noise-less bias, and how not to contaminate instrumentation capable of detecting a single photon with racket from downstream circuits, including clocked computers.
Have a look at articles from Analog Devices expert ex-Trappist Monk (I kid you not)!

The need for a very high gain-bandwidth product comes from the need to pack as much gain as possible in the first stage. The higher the gain, the slower it goes. The pulses we need are past the peak in about 6uS, and descend back to zero by about 13uS - unless a new incoming pre-empts that.

My final circuit draws from this, and other, transimpedance photo-amplifiers.

 

[homebrewed]

Good tip on getting the PocketGeiger from Sparkfun. Looking at the Sparkfun schematic, it appears nearly the same as the schematic I posted, with the first two amplifiers. (Same part number! Same basic idea.) It is nice that they showed the resistors in series for the first stage, so it would reduce the capacitance. The schematics differ in the output drive, being a comparator. Actually there are two comparators, each with a different threshold. One is filtered (the signal) with a fat cap across the comparator output (which isn't great practice) and the other without a cap (noise detections?).

What are you doing after the first two amplifiers?

The second amplifier's output is routed to one of a set of on-board test points so it's relatively easy to tap into the analog signal. We need that to pick off the peak height of the pulse. For XRF purposes, we aren't using the comparator outputs.

At present I'm attempting an approach based on the Theremino scheme, which performs some pretty severe filtering -- increasing the pulse width to 80-100uS. Among other things, this permits the use of sound-card ADCs to acquire the data (although I'm first going to try using my Teensy4.0's ADC). To improve the effective SNR of each pulse I'm going to perform a least-squares fit to the data around the peak -- using the Teensy to do that. It has its own FPU and runs at 600MHz so it's no slouch.

 

[WobblyHand]

I be mulling over what to do up the front end, and how to get a noise-less bias, and how not to contaminate instrumentation capable of detecting a single photon with racket from downstream circuits, including clocked computers.
Have a look ate articles from Analog Devices expert ex-Trappist Monk (I kid you not)!

The need for a very high gain-bandwidth product comes from the need to pack as much gain as possible in the first stage. The higher the gain, the slower it goes. The pulses we need are past the peak in about 6uS, and descend back to zero by about 13uS - unless a new incoming pre-empts that.

My final circuit draws from this, and other, transimpedance photo-amplifiers.

Haven't seen Mr. Brisebois But, well aware of the challenges of analog pulse processing. Had to do some work with an impulse like radar. I've used GHz GBW OP amps for challenging designs. Thanks for updating your schematics. It has been illuminating.