sb0 changed the topic of #m-labs to: ARTIQ, Migen, MiSoC, Mixxeo & other M-Labs projects :: fka #milkymist :: Logs http://irclog.whitequark.org/m-labs
<GitHub>
[artiq] sbourdeauducq closed issue #694: ImportError: cannot import name 'Keep' when installing artiq 2.2 from source https://github.com/m-labs/artiq/issues/694
<GitHub>
[artiq] sbourdeauducq commented on issue #695: No, because this creates additional maintainance at every major release, and will be forgotten. But we can add a note telling developers that they should be careful which branch they are on. https://github.com/m-labs/artiq/issues/695#issuecomment-289947817
<bb-m-labs>
build #1424 of artiq is complete: Failure [failed python_unittest_2] Build details are at http://buildbot.m-labs.hk/builders/artiq/builds/1424 blamelist: Chris Ballance <chris.ballance@physics.ox.ac.uk>
<sb0>
hmm... should I buy a low-bias current sense amplifier IC, or mess around with capacitors in parallel with the current sense resistor and low duty cycle operation of the amplifier?
<rjo>
what do you want do do? measure small currents?
<sb0>
measure the emission current of a bayard-alpert gauge through the cathode
<sb0>
the bias/leakage of the current sense amplifier would substract from the actual current
<sb0>
I could avoid this problem by measuring the anode current instead, but then I need a current sense amplifier that can take much higher common-mode voltage
<rjo>
that's mA usually.
<sb0>
it's down to 100's of uA, in low vacuum
<rjo>
you can download the SRS RGA schematics to get an idea how to do that.
<sb0>
and eg a TSC1031 can leak 15uA
<rjo>
take a FET amp if bias current is your problem.
<sb0>
well, might be good enough, the gauge isn't very precise to begin with
<rjo>
and drive a virtual ground with a transimpedance amp.
<rjo>
assuming your collector is a reasonable voltage (0V usually).
<sb0>
yes, I'm putting the collector at 0V to reduce leakage via the cable/feedthrough
<sb0>
then the cathode is at ~ +30V
<sb0>
what virtual ground?
<sb0>
for having negative voltages?
<rjo>
or "virtual 30V"
<sb0>
well, I want to keep the ground at 0V, since this might go to earth/cable shields/vacuum chamber
<sb0>
and the collector at 0V to reduce leakage
<sb0>
negative voltages are quite easy to do with a charge pump, just needs a small IC and 2 capacitors
<rjo>
sure. with corresponding ripple.
<rjo>
what's your 30V supply. if it is isolated just place it above the TIA.
<rjo>
or even check the SRS gauge controllers' schematics. the circuits are not terribly complicated.
<sb0>
I have a +250V supply connected to the anode, an isolated ~2V supply on the filament (cathode), and the cathode is connected to ground via a (controlled) "resistor" that develops the +30V
<sb0>
the plan is to 1) control the resistor so the cathode stays at +30V 2) control the filament heating so the emission current stays at some fixed value
<sb0>
whitequark, so, SRS are using a diode with 0.8V drop for filament power! no fancy synchronous rectification stuff...
<sb0>
ah, right, they use a TIA instead of a current sense amp for measuring emission current