This is just draft.. I have still some open issues with choosing right opamp. I will write about that on some other post.
In case of selection problem, I usually take look on bigger picture.. What are really constraints and issues and requirements.
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| Il gallo...planning |
So here is "Il gallo" amplifier plan.. Il gallo is italian and it means "rooster". It is also famous italian tobacco brand. Its a super quiet cicarette, so this amplifier is going to to be very silent also..
I am not smoker, but smoking SUPER QUIET Il gallo is chool. The reason why I am not smoking Il gallo is that I am vegan. (it comes from chiken? So it is an animal product???)
Anyways, the "Italian spiderman" is my favourite superhero and role-model after "Iron Man".
Bill of materials (just getting in right decade)
For getting big picture, here is bill of material and approximated counts, costs and "unknows"I just realized that somekind of database system tracking prices..links and alternatives would be chool...or must have thing :) Professional tools can read component prices from farnell and calculate BOM prize, but they have their limitations (like posting costs etc..)... as we see. And with ebay, search keywords are more important
| Name | approx count | price aprox. | price | info |
|---|---|---|---|---|
| 100n | 6 | 0.93e/100unit =0.0093e/unit | 0.0558 | http://www.ebay.com/itm/100PCS-0603-1608-100n-100nF-104K-10-16V-X7R-SMD-capacitor-MLCC-/391284797144?hash=item5b1a642ed8:g:SdcAAOSwax5YoIa9 |
| 10u (volt. tol!!) | 2 | 2.9e/50 + 1.5e posting.. 0.088e/unit | 0.0176e | http://www.ebay.com/itm/50x-Stk-10-F-10-SMD-0805-6-3V-X7R-Keramikkondensator-CL21B106KQQNNNG-/191891760171?hash=item2cada3e42b:g:W1kAAOSwWBJXBBga |
| CAPAPOL 22uF (avoid?) | 3 | 0.71e/20 = 0.0355e/unit | 0.10665 | http://www.ebay.com/itm/20PCS-NEW-22uF-25V-22MFD-25Volt-Electrolytic-Capacitor-4mm-7mm-Radial-/232024912184?hash=item3605c33138:g:l4IAAOSw5ZBWN4ID |
| DIODE (not needed?) | 2 | TODO | TODO | TODO |
| CONN_2 | 2 | 5.98e/50= 0.11960e/unit | 0.2392 | http://cgi.ebay.com/ws/eBayISAPI.dllViewItem&item=252044068639 |
| CONN_4 | 3 | 5.98e/50= 0.11960e/unit | 0.3588 | TODO |
| 1k | 11 | 0.71e/100=0.0071 | 0.0781 | http://www.ebay.com/itm/361556501219 |
| 100k | 1 | TODO | TODO | TODO |
| 10M (100M better?) | 4 | TODO | TODO | TODO |
| LM358 (if 2nd needed? alternative) | 0 | 0.71e/10= 0.071e | 0.071 | http://www.ebay.com/itm/10PCS-LM358-LM358D-LM358DR-Ti-SOP-8-3-9mm-Width-NEW-/361879234514?hash=item5441aee7d2:g:4EMAAOSwUKxYeOcU |
| LM2904 (2nd stage alternative) | 0 | 0.84e/10 = 0.084 | 0.084 | http://www.ebay.com/itm/10-Pcs-Smd-Lm2904-Sop-8-Dual-Operational-Amplifiers-Lm2904dr-Ic-Diy-New-Develo-D-/282282528020?hash=item41b9598d14:g:~WAAAOSwnbZYHWTl |
| AMS1117-5 | 1 | 0.97e/20 =0.0485e/unit | 0.0485 | TODO many |
| AMS1117-2.5 | 1 | 0.97e/20 =0.0485e/unit | 0.0485 | TODO many |
| AD7705 module | 1 | 2.19e | 2.19 | TODO many |
| LTC6269 MSOP8 | 1 | 10.84euro/unit+3.45euro for shipping ... 14.29e for one unit | 14.29 | http://www.ebay.com/itm/LINEAR-TECHNOLOGY-LTC6269IMS8E-PBF-Operational-Amplifier-Dual-350-MHz-2-/172196154994?hash=item2817b0ce72:g:uDQAAOSwhRxXLKmS |
| Enclosure 50x25x25mm | 1 | 1.74e | TODO | |
| 50x20 pcb 2 layers enig | 1 | 14.6e/6pieces - 28e/5 pieces | 2.43e-2.82e |
Ok... summing and rounding up numbers.. for one opamp solution
- Normal passives are under 30cents (required solder wire should be included also...)
- Regulators under 10cents
- Connectors on pcb 60cents (if jst connectors are enough coaxials are expensive)
- Enclosure 1.74e.. round up 200cents :)
- ADC 2.19e round up 220cents
- Pcb round up 300cents
These do total 8.2euros.
BUT system needs some niche parts
- feedback resistors.. one 100M would do... price range is wide starting from around 4euros
- 10M resistor prices are around same as "normal resistors" and are widely available
- Expensive,but very compact LTC6269 solution 14.29euros
Final price of whole unit would fall under 30euros? Put good rf connector on board and price goes to 50euros?
Feedback resistor price is still the big unknown thing in this calculation... part cost 50euros is max.
Selling price 250euros would not be too bad. Expecially when I assume that first batch would be 5 units...and I am able to sell max one unit to somewhere. and use one by my own purposes and three others end up laying in project boxes..
Connect to raspberry pi SPI or arduino SPI. Expecially if software is also provided (open source?)
Ideas behind circuit (hasseb)
Hasseb solution have 100M or 1G resistor behind switch on first stage..
It is 0.1v/nanoampere. 1v/nanoampere conversion selectable
That is boosted on 2nd stage..with gain
(umm... ... how this switch component works.)
Three positions
- 1
- -160k/1.6k=-100
- -160k/16k=-10
so the highest gain gives 100v/nanoampere.... meaning 0.1v signal on one picoampere.
Bandwidth is also adustable from 6 position switch (only 4 are used)
f=1/(2*PI*R*C)
1 n=10^-9 and 1kilo is 10^3... so this is calculated almost in head... :D
- 0.1n -> 9952Hz ~ 10kHz
- 1n -> 995Hz ~1kHz
- 10n ->99Hz
- 100n -> 9Hz
After this end user must somehow get analog signal level shifted and digitalized with ADC?
Ideas behind circuit (Il gallo)
Using mechanical switches sucks!! It is so 1900's. Gain and bandwidth have to be changeable by computer or microcontroller. Do you have hand adjustable vacuum tube glow curren adjust knob on your cellphone?.. I dont think so... so dont use fucking mechanical switch!!
My solution is to
- Use fixed gain first stage.
- Schematics have 40M feedback (0.025v/nanoampere)
- Plan is to use 100M feedback 0.1v/nA
- Use fixed 2nd stage.
- gain 100
- "no gain" is provided also as option. (ADC have two muxed channels)
- Use AD7705 ADC with internal analog frontend
- gain 1,2,4,8,16,32,64,128
- Two channels "no gain" and 100 gain.
- 16bits
So the maximum gain on ADC is 128000 times :-O
12.8v on one picoampere. I think that somewhere something comes as limitation.
Using widely available 4x10Meg resistors migh be good idea if it drops one expensive niche part away.. I am not going to purchase russian (radioacive) glass coated 1Gigaohm speciality resistors :D
About frequency response. Upper frequency limit comes from ADC sampling frequency But the practical limit for comes from the first stage. What is input capacitance (faraday cup, LED, solar cell, ionization chamber etc..) and how much there is disturbance.
If I got 100M resistors cheaply... dropping 2nd gain stage off would one option :)
Enclosure
Enclosure... thing that is easilly forgotted on PCB design. In case of high gain transimpedance amplifier metal enclosure is needed or amplifier catches... 50Hz.. coupled
by anything.. even person walking at same room...
- 1k <=> 1V / 1 mA
- 1M <=> 1V/ 1 uA
- 1G <=> 1V / 1 nA
So for 1G There is old finnish saying:
"Sensitive as insulted pissa-liisa after two ciders" :D :D
Pissa-Liisa or "pissis" is translated as pee-lisa.. meaning teenage who drinks too much (ands pee at public places like park..or street), cries makeups off... and pissis is more intrested about having sex with amis boys or refugee seekers than engineering students. (not amish... amis, i will write introduction about amis in english )
Continuing.... the issues with the enclosure.
So it have only two requiments
- Material have to be conductive whole or only coating.. or metal coated with paint
- Size suitable
How enclosure is assembled. It affects
- Slide pcb inside and no access to... cant have panel connectors on front and back or internal wiring is going to be quite bitchy
- Wired connectors on lid and pcb connectors + hole on back side is fine.
- Enclosure is assembled around pcb. Gives more freedom on internal wiring and layout
I was thinking about possibility to 3d-print enclosure and sandwitch aluminium foil inside box.. I got some ideas but I think postpone that to future when I am optimizing mechanics for specific purpose.
I found one quite widely available enclosure model
Search:
Aluminum Electronic Project PCB Instrument Box Enclosure Case DIY 50x25x25mm
Enlosure price is under 2 euros 1.74eur/piece is ok
ADC + analog frontend
Pcb should provide also analog connector for use without ADC...well who does not use ADC? some old fart who measures output voltage with multimeter (or needle voltmeter)???
Or want user external ADC for avoiding noise? Or use very high quality ADC.
Or poor user who uses arduino's internal ADC?
Thing what I need is way to connect amplifier to raspberry. I purchased one board for development..
I even started developing golang library for that
Ok... its crap. I have tested module only with potentiometer divider. Real bugs are spotted when I got my application working and do some research on signal.
Module prices starts from 2.19eur and many sellers are selling it under 3 euros
When making pcb.. it is good idea to avoid dauther-boards, connectors etc... points of failures.
Cheapest ad7705 chips are 1.93eur on ebay..... farnel prices start from 7euro.... depending on model
So purchasing ad7705 chip from ebay... will save 2.19-1.93... 0.26eur... But ad7705 requires xtal 4.915M....
SO ad7705 modules are cheaper choice, are better available today as module than chip.
Analog frontend provides max 128 times gain... 2nd stage opamp might not needed at all :)
PCB
As private person I havent ordered any PCBs from "cheap manufacturer". I used to use "professional grade"/ "high quality" / "reliable" / "with quality certificates" PCB manufacturers previously on my career. Now as hobbyist (I was never professional electrical engineer)..
Lets see... company from finland..
Now I dont have such money.. Money goes for paying loan ;( ;( (ex left..she was unhappy with situation)..
but the http://pcbshopper.com/ will help. :)
Choosing ENIG might be overkill..but..ENIG will help with soldering job... when board have to wait populating. Bare copper (and tin) will oxidize while waiting parts coming from ebay.
Enclosure sets limits under 25x50 (must be smaller)... Ok with batch of 5 board.. it is possible to get boards with long waiting time under 3 euros with ENIG :)
CHOOL!
First stage amplifier
I will write later more about transimpedance amplifier selection
I was looking for low voltage, high performance, low offset bias current, huge gain bandwidth, low noise überamplifier.. And it should be possible to purchase that from ebay. Ordering something from farnell sucks... as private person.
I was looking for low voltage, high performance, low offset bias current, huge gain bandwidth, low noise überamplifier.. And it should be possible to purchase that from ebay. Ordering something from farnell sucks... as private person.
Found LTC6269 dual opamp
http://www.ebay.com/itm/LINEAR-TECHNOLOGY-LTC6269HMS8E-PBF-Operational-Amplifier-Dual-350-MHz-2-/172196154061?hash=item2817b0cacd:g:zhAAAOSw3mpXLKmP
http://www.ebay.com/itm/LINEAR-TECHNOLOGY-LTC6269IMS8E-PBF-Operational-Amplifier-Dual-350-MHz-2-/172196154994?hash=item2817b0ce72:g:uDQAAOSwhRxXLKmS
MSOP8 is only hand hand solderable (without paste option)
Ebay prices are
12.42euro/unit + 3.45euro shipping
or
10.84euro/unit+3.45euro for shipping ... one unit is 14.29e getting 4 units costs 46.81e... that will hit hard on customs... so not good idea
On farnell LTC6269 is 10.78e MSOP8 PLUS
http://fi.farnell.com/linear-technology/ltc6269ims8e-pbf/op-amp-dual-500mhz-400v-us-msop/dp/2464186
When purchasing 100 units...price will be 5.36eur/unit
SO Ordering from farnell would be better idea if going to mass production. Well.. I am doing just prototypes and release desing as open source hardware.. and let other people do manufacturing. Hardware is bitch.... believe me I KNOW.
LETS MESS UP.... there is als single amplifier version available...LTC6268.. available in easier soic8 package.. it have two spare pins... and smarter. It allows to make guard ring around input.
Its not available on ebay. :(
LTC6268IS8#PBF FARNELL 2464182, 6.59e
LTC6268HS8#PBF FARNELL 2464180,
-6.59eur under 25 unit + farnell delivery price (plus extra from local dealer)
-3.17eur/unit over 100 unit + farnell delivery price(plus extra from local dealer)
FUCK!! IT is so cheap.. except ordering from farnell is hard and expensive
Lesson learned: So looking prices from ebay and making desing decision based on ebay is not always good.
Should learn to use https://octopart.com/
LDO protection and bulk capacitance
Diodes: NOT REALLY NEEDED... Use only if running out space
Microsecond surge currents of 50A to 100A can be handled by the
internal diode between the input and output pins of the device. In
normal operations it is difficult to get those values of surge
currents even with the use of large output capacitances. If high
value output capacitors are used, such as 1000µF to 5000µF and
the input pin is instantaneously shorted to ground, damage can
occur
Ok... no one is not going to short circuit pwr input... And this is reason that desing should be moderate with bulk capacitance. 22uF would be enough...
Old fashioned way would to use electrolytic capacitors... Downside is the ESR and maybe "limited" life span.
15v voltage rating is ok... comes from regulator maximum...so safety margin included.
Use same cap also on 2.5v side, less unique parts..less hassle
20pcs/0.71eur
I was also thinking about option to use "all ceramic" solution where instead electrolytes.. I use ceramics.
Looks at least better and price is not bad.. I take this in as one possible option.
Connectors
I think that JST connectors on pcb and small holes for wires (with insulation tape preventing wear-out and short circuiting) is enough for me... But on option is to use 90 degree versions from connectors. So jst plugs stick out from back and front.
http://www.ebay.com/itm/100-pcs-of-XH-2-5mm-2-Pin-JST-Right-Angle-PCB-Socket-Male-Connector-/171380146389?hash=item27e70d80d5:g:GvwAAOSwirZTuqPj
http://www.ebay.com/itm/100-pcs-of-XH-2-5mm-4-Pin-JST-Right-Angle-PCB-Socket-Male-Connector-/171380150965?hash=item27e70d92b5:g:Em0AAOSwEK9TuqZb
I try figure out solution while drawing layout :)
http://www.ebay.com/itm/100-pcs-of-XH-2-5mm-2-Pin-JST-Right-Angle-PCB-Socket-Male-Connector-/171380146389?hash=item27e70d80d5:g:GvwAAOSwirZTuqPj
http://www.ebay.com/itm/100-pcs-of-XH-2-5mm-4-Pin-JST-Right-Angle-PCB-Socket-Male-Connector-/171380150965?hash=item27e70d92b5:g:Em0AAOSwEK9TuqZb
I try figure out solution while drawing layout :)
Conclusion
Now amplifier design is started and basic ideas are written down. There are still some "degrees of freedom" left. One option for amplifier would be solution where LTC6268 is supported if need for gain is not so much..(AD7705 should still provide gain 128)
Or I could add extra 2nd stage amplifier.
I also have to plan purchases so I wont blow up my budget...
Lot things will change when I start really fitting parts inside 50x25 area (with margins)
I do that at next weekend.
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