# Ultra low leakage capacitor for nano amp current source

I am looking for capacitor type which has very low leakage such as 0.5nA since I am working with a sensor which generates between 0.9nA to 0.55uA.

Based on my research Film or foil Capacitor has the lowest leakage but not even close what I looking for, they all close to micro and not nano.

Note: the cable lenght is 20ft not 200 sorry

• What range of capacitance are you looking for? – Dave Tweed Dec 13 '18 at 21:10
• What is your circuit configuration? – Andy aka Dec 13 '18 at 21:37
• Leakage depends on applied Vdc . What is your spec? – Tony Stewart Sunnyskyguy EE75 Dec 13 '18 at 21:52
• I am looking for 2.2uF the purpose of the capacitor is to reduce cable noise., – Shahreza Dec 13 '18 at 22:05
• Are you using a coax cable? Is it necessary for the shield to be at VGND? If you use an amplifier with good CMRR, the noise shouldn't be an issue. – Colin Marcus Dec 13 '18 at 22:27

In my experience any standard ceramic capacitor will be good enough. For example, I have measured a 100pF NP0 capacitor having <10fA of leakage with a 2V bias.

You probably won't get a good result with electrolytics though.

Film cap or NP0 ceramics are good. For example the film Panasonic ECQ-E series has >9000M$$\\Omega\$$ insulation resistance at 20°C for <= 0.33uF. That's about 0.5nA maximum guaranteed.

A 0.33uF NP0 Kemet C2220C334J1GACTU has 1000M$$\\Omega\$$-uF or 100G$$\\Omega\$$ so it would be 3000M$$\\Omega\$$ worst case at 25°C, so about 3x worse than the film part.

Comparing those two series, the ceramic is better for capacitances less than about 0.1uF, which is where NP0 capacitors start to get expensive anyway.

Both types need some care with mounting to keep the leakage low. Ceramic caps can get micro-cracks which result in excessive leakage and SMT film caps are also prone to damage from the soldering process (through-hole much less so).

If you want to go nuts, there are PTFE (Teflon®) capacitors that are much (maybe 1000x) better, but harder to source and much more expensive.

Edit: Given your 2.2uF requirement, I would suggest the film type. Do consider the time required at 900pA to charge 2.2uF to, say, 1V will be more than 30 minutes. Also, if your diagram says 200 feet, you are going to have to worry (a lot) about the leakage in the cable.. in such a case one would strongly wish to have the amplifier at the other end of the cable if at all possible. Also if there is significant voltage across the caps, even a small change in capacitance (due to temperature or physical changes such as pressure or stress) will result in a significant "noise" current.

• Thanks for the respond I meant 20ft not 200 – Shahreza Dec 13 '18 at 22:29
• Still.. that's a lot of cable. 20mm would be better. – Spehro Pefhany Dec 13 '18 at 22:30