Guard Ring - would it be necessary here - Page 1
Forum section: electronics / projects

author: anishkgt
author_rank: Frequent Contributor
timestamp: February 24, 2018, 09:27:30 pm
content: Hi All,As i was reading through the datasheet of MC6281(http://ww1.microchip.com/downloads/en/DeviceDoc/21811e.pdf), I came across Ring guard being used to preven leakage current.Could somebody please explain this. I do understand the need for it but it wasn't clear how it should be done. From where to where does the current leak to ? Would the attached way be correct ? If i understand how a ring Guard should be placed for a non-inverting configuration. The inverting should go around the non-inverting and connected back to the inverting pin, correct ? The role of the amp is basically level shifting an AC voltage range from 1-1.76 under 100Hz

author: coppice
author_rank: Super Contributor
timestamp: February 24, 2018, 09:35:59 pm
content: Only you can say if you really need a ring to minimise the effects of surface leakage. It will depend a lot on your application, and how much there is to leak across the surface of your board. However, you broke the ring. You are trying to stop leakage across the layer the pins are bonded to. You need a continuous ring on that layer to be effective. Keep the ring continuous on the red layer and use vias to route the other signal through the blue layer.

author: anishkgt
author_rank: Frequent Contributor
timestamp: February 24, 2018, 10:24:24 pm
content: Thank you.is it a correct practice to have it on both layers and should the trace be open as in no solder mask ?

author: anishkgt
author_rank: Frequent Contributor
timestamp: February 24, 2018, 10:26:52 pm
content: Fixed it but is it ok have the trace near to other pins ? i guess there is no other way or is there a better way to do it ?

author: danadak
author_rank: Super Contributor
timestamp: February 24, 2018, 11:23:44 pm
content: Ref material guard rings -http://www.ti.com/lit/an/snoa664/snoa664.pdfAnd attached.Regards, Dana.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 25, 2018, 12:39:20 am
content: Why? You have big fat 100k resistors in there. Guard rings are for gigs!If you know you're having conductive leakage problems on the board, you're screwed anyway: fix the assembly process, improve the washing step, and add conformal coat if necessary.Tim

author: anishkgt
author_rank: Frequent Contributor
timestamp: February 25, 2018, 07:18:00 am
content: Ah thank you T3sl4co1l. The notes linked here had nothing mentioned about only having Guard Ring for resistors in the giga range.You've made my learning a bit easier. I will just clear the Guard Ring and should i leave the GND plane cut-out ?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 25, 2018, 07:46:03 am
content: I don't see a need to remove ground around this device. Doing so, at what appears to be the edge of the board, only invites induced noise.Tim

author: anishkgt
author_rank: Frequent Contributor
timestamp: February 25, 2018, 03:58:41 pm
content: Thank you i've removed those.When would these be actually required ? i have another project in mind where i would need to use some Ph sensors.

author: coppice
author_rank: Super Contributor
timestamp: February 25, 2018, 06:35:00 pm
content: Quote from: T3sl4co1l on February 25, 2018, 12:39:20 amYou are making a big assumption about the source of the leakage there. Many people need guard rings with quite modest impedances, because there are powerful signal sources on the board, or the board's surface is rather untrustworthy.

author: ogden
author_rank: Super Contributor
timestamp: February 25, 2018, 06:53:03 pm
content: Im afraid that R47 trimpot is implemented incorrectly. In one extreme end of it's range opamp output is shorted to GND. Usually you shall avoid such kind of possibilities - that someone trying to tune your circuit, blows it up. Well, ok, most likely that amp have output protection, but anyway it is advised to not design such way.

author: anishkgt
author_rank: Frequent Contributor
timestamp: February 25, 2018, 07:11:32 pm
content: i always get lost with the trim pot connection. so should that pin be left floating ? or should it be connected to the other end ?

author: anishkgt
author_rank: Frequent Contributor
timestamp: February 25, 2018, 07:16:13 pm
content: The source comes from the secondary of a MOT, that is by taping at to points on the secondary 40cm apart. Basically to read the voltage and with ohms law calculate the current. The opamp acts as level shifter and 7.5K resistor connector in series from R42 to the output to shift the voltage. I latter thought the trim-pot would be a good option if i need to shift the voltage up or down.

author: ogden
author_rank: Super Contributor
timestamp: February 25, 2018, 08:10:04 pm
content: Quote from: anishkgt on February 25, 2018, 07:11:32 pmUnusedpin of trim pot must be left floating.

author: John Heath
author_rank: Frequent Contributor
timestamp: February 25, 2018, 08:10:35 pm
content: My rule of thumb. Number of pins divided by the number of components outside said pins to the power of 2 , ^2 , is equal the degree of incompetence of the author of said design. 14 components minus 3 pins is 11^2 or 121 to 1 chance that you are not in strong hands. The 100 K feed back resistor tied to 16 volts with no means of finding ground to have any meaning was the writing on the wall. I would start from scratch with your own FET input op-amp in the interest of putting it behind you in order to move forward with confidence.

author: anishkgt
author_rank: Frequent Contributor
timestamp: February 25, 2018, 08:18:14 pm
content: Quote from: John Heath on February 25, 2018, 08:10:35 pmAppreciate your reply but i did not seem to get what you meant.

author: John Heath
author_rank: Frequent Contributor
timestamp: February 25, 2018, 08:40:42 pm
content: Quote from: anishkgt on February 25, 2018, 08:18:14 pmI understand why the meaning was not clear. Had to correct the original post to clarify. The 100 K resistor is tied to 16 volts going directly to the op-amp input with no ground reference resistor? You see the problem. It would depend on the leakage gain of the op-amp that makes for a tricky balancing act of the op-amp gain transistor vs 100 K resistor. No one worth their spit would entertain such a solution. My thoughts are forget these guys and do it your self. This way you can proceed forward with confidence.

author: anishkgt
author_rank: Frequent Contributor
timestamp: February 25, 2018, 09:43:21 pm
content: Thank you john.If i really knew what i was doing or to design this part of the circuit i wouldn't have brought it here to the community. In fact some of "these guys" had helped me here and there on the project and others on other topics.The design was originally provided to me by a member at element14 community. Since he did not respond thought i'd get some help from here. About the design to how much i understand, the 100K is part of the divider that shifts the voltage to level it is safe for the Arduino. Simulating it, i did not find a problem. I seem to get a reasonable variation when varying the AC voltage 1 and 1.76

author: John Heath
author_rank: Frequent Contributor
timestamp: February 25, 2018, 10:28:55 pm
content: This is not the same diagram. It has a 100 K ground reference and R1 39 K is not the same. Why all these changes ? And secondly why are you relying on a member of element14 community for an elementary op-amp design? Go look in the mirror and tell yourself you are the best. After that finish this minor op-amp deal to move on to the next step. Max you are looking at a few hours googling op-amps and another hour or two with spice to confirm. Go for it and remember you are the best.

author: anishkgt
author_rank: Frequent Contributor
timestamp: February 25, 2018, 10:44:33 pm
content: I was following the LTspice circuit but for turned to draw it wrong way in the CAD. Sorry about the confusion.I am just another hobbyist and i am just learning to use opamps the practical way theory is good for writing it down in an exam perspective. I learn much faster with practical examples. So far learned voltage followers and Gains using opamps. i guess that is just of the iceberg.

author: ogden
author_rank: Super Contributor
timestamp: February 25, 2018, 10:50:38 pm
content: Quote from: anishkgt on February 25, 2018, 09:43:21 pmChange impedance of signal source and you will see your problem. Let R8 to be 30k for example.QuoteOh... BTW this is first time I see ridicule in form of schematics

author: anishkgt
author_rank: Frequent Contributor
timestamp: February 25, 2018, 10:57:29 pm
content: why should that the resistance be increased ? is that assuming for some reason 'if' it does ? what are the chances it would ?

author: ogden
author_rank: Super Contributor
timestamp: February 25, 2018, 11:53:14 pm
content: Quote from: anishkgt on February 25, 2018, 10:57:29 pmFor educational purposes - to see that such "level shifter" design is not the best around and it could have a problem contrary to what you said "i did not find a problem" . It does heavily depend on signal source impedance which could not always be 1k as in your simultion.QuoteWhy not?QuoteSuch as: You use other source with different impedance or output of the source have decoupling capacitor - who knows what YOU will do with your circuit and how it will change compared to your simulation. Only you know.

author: anishkgt
author_rank: Frequent Contributor
timestamp: February 26, 2018, 12:07:58 am
content: Thank you it was quite an info.As for the source, it is going to be the voltage between two taps of a MOT at the secondary and it would take the voltage during a Spot weld and that is at 1-2vac below 100Hz.

author: ogden
author_rank: Super Contributor
timestamp: February 26, 2018, 12:18:19 am
content: Quote from: anishkgt on February 26, 2018, 12:07:58 amFor welder applications you better don't use current shunt made out of unknown length of unknown wire but current transformer instead.

author: anishkgt
author_rank: Frequent Contributor
timestamp: February 26, 2018, 12:22:54 am
content: This is a spot welder commonly seen in the hobbyist field for spot welding 18650 cells with nickel strips. I had measured the resistance of the whole cable to be 0.00003Ohms and i can get the voltage from this circuit after sampling it several times with an arduino. With voltage and resistance current would be calculated using ohms law.Initially i did think of using a CT but the core are small and larger cores are a fortune hence this way.