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Amplifier board debugging notes (view source)
Revision as of 15:31, 13 September 2011
, 15:31, 13 September 2011no edit summary
This page contains a list of all currently known problems with the amplifier board and the solutions implemented
This page contains a list of all currently known problems with the amplifier board and the solutions implemented
= Issue List =
* check that the summing circuit is up to date
* finalize DC plane "fingers"
* settle on switch circuit design
* check voltage references
* check impedance matching, where relevant (possible on internal layers?)
* ensure sufficient bypass
= Bug List =
== Improper DC levels from voltage regulators ==
== Improper DC levels from voltage regulators ==
[[Image:AmpBoard_Vref.png|frame|Pinout of voltage regulators ([http://www.powerdi.com/datasheets/ZR431L.pdf ZR431L]) used on ampplifier board.]]
=== Description ===
=== Description ===
=== Cause ===
=== Cause ===
The pads for pins Gnd and Vz of the voltage regulators were reversed. The proper assignment of pins is shown in the adjacent figure.
The pads for pins Gnd and Vz of the voltage regulators were reversed. The proper assignment of pins is shown in the adjacent figure.
* '''modifications of the PCB layout is pending'''
* '''modifications of the PCB layout is pending'''
== Change of reference for summing circuit input stage DC level ==
[[Image:VrefSum.png|thumb|136px|Labels in diagram parallel those in the NCP100SNT1G unit manual.]]
The new PNP input stage of the summing circuit, in order to be placed far from cutoff and saturation, is very sensitive to its emitter current, which, with RE=154Ω requires transistor base control on the order of 10mV. Basically, we need the set the base for the desired emitter voltage and accounting for the diode drop ''referencing Vcc, not ground, as it was with NPN''. The shunt voltage regulators used so far can be employed for this, but the ~0.9V offset from Vcc cannot be done with most regulators as they use a double diode drop - 1.24V as internal reference and allow settings above this value. A reference NCP100 SNT 1G (On semiconductors) has been chosen because of it's lower possible settings, making reference to just one diode drop: 0.698V
The adjacent diagram shows the necessary circuit. The component values are set as follows.
* Recommended values for V<sub>cc</sub> - V<sub>B4</sub> = 0.9V - {R1,R2} = {30kΩ,100kΩ}
* Good value for I<sub>k</sub> is 2mA. Current through R1, R2 is negligible, as is the current going to the load (6 summers x 10uA base current) Thus I<sub>k</sub> ~ I, so R ~ 2kΩ (but anywhere 1-5kΩ works)
* 154Ω worked well for the emitter resistor (R<sub>f</sub>) on the summer input stage.
== Wrong voltage on R<sub>C</sub>/R<sub>D</sub> junction ==
== Wrong voltage on R<sub>C</sub>/R<sub>D</sub> junction ==
=== Description ===
=== Description ===
A voltage of 2.75 V was measured on the junction between R<sub>C</sub> and R<sub>D</sub> resistors, instead of 2.25
A voltage of 2.75 V was measured on the junction between R<sub>C</sub> and R<sub>D</sub> resistors, instead of 2.25
== FET switch compatibility ==
== FET switch compatibility ==
[[Image:AmpBoard_FETorient.png|frame|Quick fix for the package-footprint mismatch of the FET swtich]]
=== Description ===
=== Description ===
The FET switch BF1108 was not available in our market. Only its mirror package variant BF1108R could be procured.
The FET switch BF1108 was not available in our market. Only its mirror package variant BF1108R could be procured.
* ''Temporary solution'': The adaptation of the board for the BF1108R is shown in the adjacent figure. The IC is positioned in the shown orientation and its pins (pads) linked as shown.
* ''Temporary solution'': The adaptation of the board for the BF1108R is shown in the adjacent figure. The IC is positioned in the shown orientation and its pins (pads) linked as shown.
* '''modifications of the PCB layout is pending'''
* '''modifications of the PCB layout is pending'''
== LEMO receptacle does not fit through holes ==
=== Description ===
The through whole LEMO receptacle does not fit its designated holes.
=== Cause ===
The pin spacing looks correct, but there does not appear to be sufficient slack in the hole: both pin and hole are specified to be 30mil diameter. Additionally, for the hole size - that specification may only refer to the drill size, not the after-plating diameter.
=== Solution ===
* The purchases LEMO receptacle pins were filed to fit the holes of the prototype board.
* '''modifications of the PCB layout is pending'''
== Thermistor/resistor tolerance ==
=== Description ===
The 100k resistor and thermistor picked for the voltage divider (used by control board/software to determine thermistor's reading) is of 5% tolerance
=== Solution ===
* Non implemented on prototype board. Suggestion: review whether this tolerance is sufficient at operating temperatures.
* '''consideration of the revision and modifications of the PCB layout are pending'''
=== Update ===
Readout and interpretation of the temperature from this circuit results in about 5degC error! Interpretation:
* 5% tolerance may contribute a few degrees (though replacing with 1% 100k resistor didn't move the accuracy significantly)
* There is an ambiguity in the B parameter for this NTC thermistor: the table given in the manual does not seem to agree with the stated value of B of this device. However, this effects the reading on the scale of a 0.1deg
* few mV drop in the voltage may be occurring between the thermistor pin and ADC chip. Path resistance can be calculated and taken into account. (Significance to be verified)