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Clamping Split Rail Node to GND using Transistor(s)

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T3sl4co1l:
Is it supposed to be reset to zero during the event, or held still?

In either case you most definitely do not want to yank on the virtual ground.  Consider what that equivalent circuit actually is: you're connecting a very low resistance to that pin, along with some nonzero offset voltage (which may be the amp's Vos itself, or built-in offsets in the switch or surrounding circuit).  In other words, you've added a second input, with a series resistance MASSIVELY smaller than the normal input.  It'll go bonkers.

What you need to do, is shunt the output to the input, discharging the capacitors; or, for a hold function, disconnecting the normal input so it's an amp with a capacitor wrapped around it, just sitting there.

Both are easily done with normal CMOS switches (I would recommend adding an external resistor to limit the power dissipation from all that capacitance). :)

Tim

Mechatrommer:

--- Quote from: T3sl4co1l on August 02, 2019, 01:02:19 am ---Is it supposed to be reset to zero during the event, or held still?

--- End quote ---
the idea is to discharge capacitors Cintg* whenever reset is required, the reset control is by toggling the RST_Intgr digital control pin (see pink colored attached) to HI. so the ideal condition during reset state is 0V on all U2A's IO pins or very close to that esp on U2A's -ve input, so to keep capacitors charges minimal (to avoid worse Integral windup). this switch must be able to be released for the integrator opamp to work normally again as if its not connected to any reset/clamp circuitry.


--- Quote from: T3sl4co1l on August 02, 2019, 01:02:19 am ---In either case you most definitely do not want to yank on the virtual ground.  Consider what that equivalent circuit actually is: you're connecting a very low resistance to that pin, along with some nonzero offset voltage (which may be the amp's Vos itself, or built-in offsets in the switch or surrounding circuit).  In other words, you've added a second input, with a series resistance MASSIVELY smaller than the normal input.  It'll go bonkers.

--- End quote ---
thats why HI impedance circuit during OFF state is required, during reset (ON) state, the impedance should be lowest to be able to discharge capacitors quickly. and to add insult to the injury, this is dual rail system where current can flow in both direction.


--- Quote from: T3sl4co1l on August 02, 2019, 01:02:19 am ---What you need to do, is shunt the output to the input, discharging the capacitors; or, for a hold function, disconnecting the normal input so it's an amp with a capacitor wrapped around it, just sitting there.
Both are easily done with normal CMOS switches (I would recommend adding an external resistor to limit the power dissipation from all that capacitance). :)

--- End quote ---
that is what Ian suggested and i think i will implement it in the circuit using triac opto that is currently coming my way. thanks for your suggestion on the CMOS switch.

coupled with David's idea of secondary opamp, i think i've made the "active" version of triac to deal with 1.5V forward voltage drop, so i guess with circuit below, the Vdrop should be eliminated and make an ideal reset circuit, no need $4 H11F1M PhotoFET, correct? (R7 is current limiter, i think i can use 0R short if U3B can withstand a short such as the TL072/074, for other opamp that cannot be shorted, R7 will be there)  :-+

Zero999:
Why not use an analogue switch IC?

DG468 single channel
https://www.vishay.com/docs/74413/dg467.pdf

DG445 four channel
https://docs-emea.rs-online.com/webdocs/1300/0900766b813009f9.pdf

Mechatrommer:

--- Quote from: Zero999 on August 02, 2019, 09:46:53 am ---Why not use an analogue switch IC?

DG468 single channel
https://www.vishay.com/docs/74413/dg467.pdf

DG445 four channel
https://docs-emea.rs-online.com/webdocs/1300/0900766b813009f9.pdf

--- End quote ---
thanks for the suggestion Hero, it helps me on part searching and selection time several order of magnitude. i've put few DG468 into my digikey cart...undoubtedly this part is the simplest i can get. my only concern now is maximum current and how quickly they can discharge the ~220uF capacitance, including the MOC3041 (30mA cont, 1A surge capability). btw, the MOC3041 arrived today i can proceed repairing my 852AD station and ready to be used on circuit... but the pcb i will send to fab house soon, so it will take another weeks to arrive. thinking about it, before i send the pcb, i will implement analog switch as well on the pcb along with the triac opto solution, so i can switch between them to see which works better. analog switch has size and component count advantage, so if triac opto does not show significant improvement, maybe analog switch is the solution in the next revision. this can be part of my learning process.

Zero999:
In theory the capacitor never fully discharges, irrespective of which method you choose.

Analogue switches have an on resistance, forming an RC circuit with the capacitor. The DG468 has a on resistance of 10R, so the time constant will be 2.2ms, with a 220µF capacitor.

A TRIAC will stop conducting once the current falls below a certain level, so will always leave some charge left in the capacitor. The MOC3041 is a bad choice, because it has a zero crossing unit which prevents it from triggering at low voltages, although it might work in this case because the voltage across the capacitor will always be below 12V.

Lastly your circuit doesn't close the loop at DC. There's no negative feedback between the non-inverting input and output. In practise you might get away with it with the TL072, which has such tiny bias currents and a high input impedance, the leakage in the capacitor and switch might be enough.

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