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Cosmic Pi

New main schematic design review

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User Documentation - 2016-07-01 18:29:54.260189

The main schematic (developed in EagleCad) is presented for design review. If you have comments, questions or suggestions please share them with us! Once we've finished on the schematic we'll move to finalise the layout, and we might have another review if necessary. The BOM is also attached.

Thanks to all those who helped out by contributing comments, either beforehand or at the physical review. Below is the completed list of design review comments, along with the actions to be taken if considered necessary. Note that some comments have been deleted due to duplication. We'll be re-working the schematic over the next few days and will archive this list once all comments are actioned.

Arduino source code - not necessarily in the scope of the design review, but might be helpful [https://github.com/CosmicPi/cosmicpi-arduino\]

Design review:
+ Schematics
+ Power Ratings of the components, confirmed as adequate for mA range expected. Total power consumption < 500mA at 5V.
+ Operational Voltages of the ICs. Confirmed 3.3V/5V for all devices with correct connections.
+ It is often seen, that sometimes there are dots missing where a connection between wires should be. This is dangerous because you can get problems with your PCB layout. I guess you used the 'wire' to connect the components. But is better, maybe even necessary, to use 'net' to connect components. Done.
! IC1, IC2 – missing DC bias – operating point of existing design is at 0V where amplifiers tend to oscillate because they cannot output voltages below 0. Rail to Rail doesn’t help much. Just connect R38 to some 1V instead of ground. To be done, RC network impact to be checked.
+ decouple the input of boost converter from + 5V. 1 uF is not sufficient, it will distribute peaks in all + 5V domain. Just use some LC filter and C>1uF at the input. Done.
+ add ferrite + decoupling for GPS. Missing capacitor at the module supply. Done.
+ add C for AND gate decoupling. Done.
+ Small hysteresis to the comparators to reduce glitches to be done based on comparator datasheet. Done, values to be checked.
+ add current limiting resistors (i.e. 330Ohm) at the output of the comparators – they produce 5V while U16 is supplied from 3.3V; Done.
+ Package replacement incomplete, all passives to be changed to 603. Done.
-- Schematic blocks
-- Input Stage
+ Add LED for SiPM feedback. Done.
-- Amplification Stage 1
+ As you might know, the first amplification stage is the most critical, so special care has to be taken. The 10M resistor in your first amplifier stage is really huge. Do you really need that high gain? Because this will introduce quite a big noise in your first stage and this is where you want it the least. Also I strongly recommend to keep the (all) feedbacks as short as possible in your layout. The SIG1A and SIG1B lines should be surrounded by ground planes to provide a shielding for the input. Input stage to be modelled in SPICE.
+ model to quantify the boundary conditions between each stage (e.g. signal amplitude from output, frequency etc.) to be done in spice, starting from SiPM
-- Amplification Stage 2
+ Add ferrite filter from Shaper
-- Shaper
+ remove ferrite filter, raw 5V with decoupling should be sufficient. Done.
+ In your shaper circuit you use the AD8602 opamp. If you experience much noise you can also use the AD8656 if the noise or offset voltage is a problem. The AD8656 is pin compatible with the AD8602 but has much better noise performance and less offset voltage. We’ve decided to stick with the AD8602 for now, but will look at this if we have noise issues after production. No action at this stage.

-- HV PSU
+ I am really curious about how your 47V generation will do! As I understood so far, the noise/ripple performance of the bias voltage has a huge impact on the SiPM noise performance. So an accurate voltage is really important but hard to realize in this voltage range. Maybe a 5V/5V DC/DC converter can be helpful to isolate your + 5V from the Arduino and the Boost PSU control IC input. This could prevent errors in the supply system. This comment is addressed by the earlier one where we will add an inductor and capacitor to the DC/DC supply. Done.
+ Add feedback mechanism for 47V FSD to 3.3V ADC, via potential divider, include reverse Zener at 3.3V to prevent pin overload!
-- GPS
+ 50 Ohm trace for the antenna, not required due to short trace length – validated with our version 1 prototype; trace length <10mm. Review only if trace >10mm
+ Pulse shaver for the PPS, to be actioned and added.
-- Accelerometer & Altimeter
+ Transistor for turn on/off as hard reset
-- Humidity & Temperature Sensor
+ Transistor for turn on/off as hard reset
-- Discriminator Threshold
! You should be cautious with your Voltage reference MAX6125. Voltage references often cannot drive large currents (< 2mA?). So your have to check the minimum resistances of the potentiometers. If necessary you have to buffer the voltage reference output with an opamp (low noise - > AD8656). Also be aware that the voltage references change its output voltage in dependence of their load. Verified, load is ok to 1mA, resistance is 10kOhm, so load =1mA; 40mV dropout, which we consider acceptable. No action.
-- Trigger
!Trigger block: JP3 should be JP8 in this comment "JP3 to short if NOT isn't mounted, active low.". JP3 is the SPI header while JP8 is for shorting the NOT gate. Correct. To be changed.
!You should also provide a capacitor (10nF-100nF) directly placed at the IN+ inputs of the comparator. Done.
!Maybe they are provided somewhere else, but I do not see any capacitors at the VCC pins of the AND gate and NOT gate! 100 nF should do. At fast switching digital ICs, also a bigger capacitor should be provided additionally. Done.
+ consider 555 timer(s) as an alternative architecture, to give a fixed duration pulse. Maybe next iteration, not this time.
-- DUE Interface
! pin 20 on the DUE Shield I think should be labeled "SDA0" for the data pin of I2C bus 0... currently reads "SDA." Done.
-- Filters
-- Status LEDs
-- BOM
+ Check passive values for rationalisation; e.g. standardise on 100nf if possible etc. same for resistors.

(As yet incomplete)
+ Layout
+ Any need of track impedance for delicate signals. Not required, short traces only.
+ Power Planes and return tracks for delicate signals. No power planes, 2 layer board. No differential signals, all returns to ground plane.
+ all components mounted on the UNDERSIDE of the PCB
-- rest of layout
Here are links to the data sheets for the main components:

[We will be using two Advansid ASD-NUV3S-P SiPM's as the signal source, connected at the input stage, biased to 30V [http://advansid.com/attachment/get/up_54_1432731416.pdf\]\]
[Maxim Application Note AN2236 - Gamma-Photon Radiation Detector [https://www.maximintegrated.com/en/app-notes/index.mvp/id/2236\]\]
[MAX4477 Opamp, Dual version is used [https://www.maximintegrated.com/en/products/analog/amplifiers/MAX4477.html\]\]
[MAX987 Comparator [https://www.maximintegrated.com/en/products/analog/amplifiers/MAX987.html\]\]
[MAX1474 Electronically Trimmable Capacitor [https://www.maximintegrated.com/en/products/analog/sensors-and-sensor-interface/MAX1474.html\]\]
[MAX5387 Dual Tap Digital Potentiometer [https://www.maximintegrated.com/en/products/analog/data-converters/digital-potentiometers/MAX5387.html\]\]
[MAX6102 Voltage Reference, 4.096V used [https://www.maximintegrated.com/en/products/analog/voltage-references/MAX6102.html\]\]
[AD8602ARMZ Opamp [http://www.analog.com/media/en/technical-documentation/data-sheets/AD8601_8602_8604.pdf\]\]
[MAX1932 Boost APD power supply [https://www.maximintegrated.com/en/products/power/switching-regulators/MAX1932.html\]\]
[L76 GPS module, QUECTEL [http://www.quectel.com/UploadImage/Downlad/Quectel_L76_Hardware_Design_V1.1.pdf\]\]
[3D accelerometer and compass [http://www.st.com/content/st_com/en/products/mems-and-sensors/e-compasses/lsm303d.html\]\]
[BMP280 Barometric Pressure Sensor [https://www.bosch-sensortec.com/bst/products/all_products/bmp280\]\]
[HTU21D Humidity & Temperature Sensor [http://www.meas-spec.com/product/humidity/HTU21D.aspx\]\]
[SN74AHC1G08 AND Gate [http://www.ti.com/lit/ds/symlink/sn74ahc1g08.pdf\]\]
[MC74VHC1GT14 NOT gate + Level shifter [http://www.onsemi.com/pub_link/Collateral/MC74VHC1GT14-D.PDF\]\]