Author Topic: PCB for picoammeter  (Read 1174 times)

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Offline MHDKAMTopic starter

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PCB for picoammeter
« on: April 11, 2024, 09:35:56 pm »
Hello all,

This is kind of a crosspost of this: https://www.eevblog.com/forum/beginners/capturing-picoammeter-output-with-an-adc/msg5444927/#msg5444927

I am designing a PCB for the picoammeter originally designed by Gyro. I already built the circuit on a solder board with air wiring but now i want to see if it can be done on a PCB.

Schematic:




I used guard traces with the solder mask removed from the high impedance areas as suggested by PAUL GROHE in these three articles:

https://www.edn.com/design-femtoampere-circuits-with-low-leakage-part-one/

https://www.edn.com/design-femtoampere-circuits-with-low-leakage-part-2-component-selection/

https://www.edn.com/design-femtoampere-circuits-with-low-leakage-part-3-low-current-design-techniques/

This is the PCB that i have designed so far.











Do you think this could work? Do you see any immediate red flags?

I used a LMC662 as a buffer to drive the guard traces. I added the 100R to the output to deal with the capacitive load. I am thinking about connecting this guard to the triax cable guard which goes to the measurement box, hence the guard header.

Will this 100R be enough?

I thought about doing a guard pour instead of traces around the high impedance input, would that be better? Something like this:



Should i add ground traces/pour around the output (yellow net) in the solder mask stripped box to shield it further?

I am unsure of what type of connectors i should use on the PCB, especially for the input. Do you think the standard ones i chose would work?

For all traces i used 0,25mm width. For the guard traces i used 2mm (1mm around at the end of the loop).

The socket headers are there for an ADS1115 board. The pin headers are for connecting to the Arduino.

Regarding the board material, should i try using FR4 or something better? Paul Grohe suggests that Rogers 5880 gives the best low current and stray capacitance. I couldn't find a PCB supplier that offers that material. And Paul's article was written in 2011 so i guess there might be a new material that is better than the one Paul recommended?
« Last Edit: April 12, 2024, 08:04:35 am by MHDKAM »
 

Offline magic

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Re: PCB for picoammeter
« Reply #1 on: April 13, 2024, 08:37:05 am »
This is a subforum about PCB design software. You would receive more, faster and better response simply posting in the original picoammeter thread or starting a thread in "Projects, Design and Technical Stuff". You can stil move this thread there, there is a button at the bottom of the page.

As for your questions, I don't think you need a dedicated guard driver here at all. The point of guard is to have the same voltage as the protected high impedance node and in this circuit ground could work for this purpose just as well. For maximum effectiveness you would want to trim RV1 to cancel U2 offset voltage exactly, so that U2 IN- is as close to GND as possible.
 
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Offline MHDKAMTopic starter

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Re: PCB for picoammeter
« Reply #2 on: April 14, 2024, 06:39:41 pm »
This is a subforum about PCB design software. You would receive more, faster and better response simply posting in the original picoammeter thread or starting a thread in "Projects, Design and Technical Stuff". You can stil move this thread there, there is a button at the bottom of the page.

Sorry about that! I have moved the thread now!

As for your questions, I don't think you need a dedicated guard driver here at all. The point of guard is to have the same voltage as the protected high impedance node and in this circuit ground could work for this purpose just as well. For maximum effectiveness you would want to trim RV1 to cancel U2 offset voltage exactly, so that U2 IN- is as close to GND as possible.
I didn't think this through enough  :-[ . It's the offset-adjustment circuit that i am having a hard time to understand. In the normal configuration of a TIA (IN+ connected to GND) i can easily see how the IN- is virtual ground, but in this configuration i can't see it as easily. Anyways this makes my design easier! I can probably just do a complete GND pour and just take the solder mask off the rectangular area around the input.
 

Offline magic

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Re: PCB for picoammeter
« Reply #3 on: April 14, 2024, 07:45:20 pm »
The offset pot makes no meaningful difference to TIA operation, it only changes the constant DC voltage regulated by U2 at its IN- pin. It should be ground if IN+ is grounded, but in real world it's not because of U2 offset voltage. By applying an opposite constant  voltage to IN+, you can make IN- equal to ground exactly.

Or at least "equal more exactly", to some degree ;)
 
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Offline Terry Bites

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Re: PCB for picoammeter
« Reply #4 on: April 15, 2024, 03:26:41 pm »
I've had stability problems with guard drivers. They can introduce a peak in the response. I used a simple mod to reduce the gain to about 0.95 and isolate the opamp from the cable capacitance.
« Last Edit: April 15, 2024, 03:28:53 pm by Terry Bites »
 
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Offline MHDKAMTopic starter

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Re: PCB for picoammeter
« Reply #5 on: April 18, 2024, 09:40:00 am »
I've had stability problems with guard drivers. They can introduce a peak in the response. I used a simple mod to reduce the gain to about 0.95 and isolate the opamp from the cable capacitance.
Thanks for the reply and the tip! I have removed the guard driver completely and instead extended the ground pour to cover the whole board. I ordered the PCB and should have it by next week.
 


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