Tradeoff: I'll keep the skinny traces for lower capacitance, but put the ground plane back. It felt wrong removing it anyway.
I've remembered wires love to pick up noise, so I've also added footprints for output filter cap. In case this ends up inside a "shielded" cookie box, also a 50R resistor, and removed a bit of soldermask on the ground plane, to solder a coax.
I've remembered wires love to pick up noise, so I've also added footprints for output filter cap. In case this ends up inside a "shielded" cookie box, also a 50R resistor, and removed a bit of soldermask on the ground plane, to solder a coax.
I could also put the ground plane on bottom layer, and all the traces on top layer, so the ground plane would be a bit further away down from the resistors and the chips: less capacitance, but also less shielding.
Perhaps some unscreened pads on both top and bottom of pcb that are connected directly to ground fill to allow someone to easily solder a custom copper 'cover' over the sensitive areas, as a way of augmenting local shielding.
OK, next step: order the parts and check they fit in the holes (especially the switches, pot, 9V battery tabs, and bananas).
However, I'm a cheap bastard so I like free shipping, and DABoy is too cheap! So, that'll have to wait until my mouser shopping backet goes over 60€ lol. Unless someone else volunteers, of course!
However, I'm a cheap bastard so I like free shipping, and DABoy is too cheap! So, that'll have to wait until my mouser shopping backet goes over 60€ lol. Unless someone else volunteers, of course!
I need to order some components anyway, and I'll happily play the beta-tester's role (that's the least I can do)
OK. Here's a zip with BOM and PDF for schematic and pcb. If there are no more modifications, I'll post gerber files.
Elvee, would there be any benefit from putting the two 4066 switches in series rather than parallel ?
I have slightly revised the gain structure to avoid saturation near the max reading:
I have adapted the BOM.
The adjustable capacitor is going to pose unexpected problems: it has become extremely difficult to find low-cost models.
Provisionally, I have placed an example in the BOM, but it costs 4.7€, yet it is the least expensive I could find. It would be impossible to order anyway, as the minimum qty is 1.5k.
It will probably be necessary to use the old HAM trick of a small bit of coaxial pruned until it has the right capacitance.
Series connection might reduce the charge injection, but that is far from certain; the opposite could be true
I have adapted the BOM.
The adjustable capacitor is going to pose unexpected problems: it has become extremely difficult to find low-cost models.
Provisionally, I have placed an example in the BOM, but it costs 4.7€, yet it is the least expensive I could find. It would be impossible to order anyway, as the minimum qty is 1.5k.
It will probably be necessary to use the old HAM trick of a small bit of coaxial pruned until it has the right capacitance.
They are paralleled to reduce their resistance. A higher resistance would mean a lower maximum measurable capacitance value (The CUT needs to be totally discharged before being left open).
Series connection might reduce the charge injection, but that is far from certain; the opposite could be true
Attachments
Just found adjustable caps on Aliexpress (free shipping) and Reichelt...
I'll probably use 2 wires twisted together or a bit of coax.
I'll probably use 2 wires twisted together or a bit of coax.
The ones from Chinese origin are probably better suited, as they go down to 3pF max value.
The one from Reichelt has a minimum value of 1.5pF, which might be excessive (it will depend on the PCB).
In addition, they are 1/10th of the price, and shipping is free.
Found them on banggood too: 10pcs 6mm trimmer variable ceramic capacitor 3pf 5pf 10pf 20pf 30pf 40pf 50pf 60pf 70pf 120pf adjustable capacitors Sale - Banggood.com
The one from Reichelt has a minimum value of 1.5pF, which might be excessive (it will depend on the PCB).
In addition, they are 1/10th of the price, and shipping is free.
Found them on banggood too: 10pcs 6mm trimmer variable ceramic capacitor 3pf 5pf 10pf 20pf 30pf 40pf 50pf 60pf 70pf 120pf adjustable capacitors Sale - Banggood.com
I use CAT5 and snip until I get the right capacitance. In the US such a trick capacitor is called a "gimmick"
I used about 5 cm of 1mm insulated single core wire twisted together to create a 6pF cap. By trimming the end, you can pretty much get any value you want below that.