OP selection for PMT preamp on scanning electron microscope - Page 1
Forum section: electronics / projects

author: Amper
author_rank: Frequent Contributor
timestamp: January 22, 2021, 08:20:05 pm
content: Hi,for over half a year im now the proud owner of an 90s vintage JSM-6300 Electron microscope.I have retrofitted some things and fixed some others, its working pretty decently now but its still all 90s hardware.Last week i had a little Incident that lead me to swapping the first stage Amplifier (AD507J) of the secondary electron detector. Since i dint have the right replacement (package) i substituted it with the slower AD707 wich is slightly slower. This had a huge impact on the obtained image since its a scanning electron microscope and the SEI Detector is the bottleneck for the signal before its being processed. (The beam is scanning over the sample like in a CRT and the SEI is monitoring the "brightness" of the "reflected" electrons wich is translated into intensity like in a crt, just in reverse)This bandwidth problem is usually fixed by just scanning the sample slower so the bandwidth required is lower but this brings other issues as at great magnification stuff is not that stable and any movement in the mechanics or electronics will result in skewing. At the same time the detector is supposed to have low noise on small signals but im not sure if the amplifier is the limit here or the pmt and scintilator.Since i have an older (2005) sample kit by analog devices i selected the AD8065 since its high bandwidth, runs off of the +-10V supply and i had one at hand. The image quality improved quite a bit over the 507 but im unsure how to determine if it could be better. My knowledge about OPs is more or less limited to comparing slew, noise figure and unity gain frequency so im looking for some mentor who can teach me what to look out for.On the schematics shown there is the Detector itself that basically just contains the PMT and the 2x preamp/ buffer and the second schematic is the amplifiers that are fed by it. The two are connected by 2m of thin coax (type unknown), most likely 50R and 200pF.The scaning is 1024 lines @ roughly 30fps (i think the instrument can produce NTSC directly using the signal without digital processing).I hope someone can aid me a little here so i can learn some more and not just pick random parts just by rough figures ^^EDIT:I totally forgot: would it be worth it also swapping the LM6361 on the second board?

author: Kleinstein
author_rank: Super Contributor
timestamp: January 22, 2021, 09:26:11 pm
content: The AD8065 is very fast and probably already a reasonable good choice. The main weak point may be low frequency noise - this might give some noise in the vertical direct. In this respect the pictures look good, so it does not look like the noise is a problem. The LM6361 is not very low noise, but also not so bad. So I don't hink it would be worth to change them.Without the resistor values it is a bit difficult to judge the real demands on the OP. Chances are one would not even need such a high speed. The usual TV signal has a BW of some 4 MHz. 1024 lines would be a little more.Usually the signal from the PMT is more limited by the PMT internal processes and shot noise. So the amplifier after the PMT should not be so extemely noise critical.It looks like the OP output has a series termination, so it would not directly see the cable capacitance, but more like twice the cable impedance as load.

author: rstofer
author_rank: Super Contributor
timestamp: January 22, 2021, 09:38:34 pm
content: You could Google for AD507J replacement and find that Analog Devices would recommend the AD843 and Digikey has stock.I didn't check the datasheet for specs nor do I know which specs are truly important. This is just Analog Device's recommendation.

author: Amper
author_rank: Frequent Contributor
timestamp: January 27, 2021, 11:28:13 am
content: Well, im not looking for an equivalent one, im trying to improve my machine to get better images at less amplitude (less electron current results in smaller spot size -> more resolution but much smaller signal at the same frequency)@KleinsteinYeah, i guess the 8065 is already enough, if there is no obvious flaw i overlooked i guess its fine. Just i wouldnt mind investing >100$ into new components if it meant better images, i just bought new scintillators for 3 times that amount.There is also the option to replace the pmt, there are hybrid pmt that do picosecond response times but the one i have is limited by its 3mm window diameter, im not sure if its worth a try as thats a fraction of the 10mm light pipe and im afraid ill loose a lot of signal.Thanks for the advice, if i get around to trying other stuff ill keep this updated.