Good point. The wait time from bias to measurement voltages were
software controlled in my instruments from 1 to 60 seconds before the
measurement voltage source was removed and the capacitor allowed to
discharge. For measurements less than 1E10 ohms, 2-5 secs was generally
sufficient; the longer times being necessary only for higher Rs. The
capacitor discharge time was generally 1-2 sec, so you could have a
cycle time of 5 sec/point for most practical tests. Even so, if you were
opting for the full 96 points, that was 8 minutes/cycle. In practice, I
found the "overnight" test with extrapolation to 7 days was OK at 15
minute intervals per point, although I recommended 5 minutes if
possible. As this was a production QC test, as opposed to a full
qualification test, the number of samples was usually less than a dozen,
so this presented no difficulty.
Brian
Brian
Hernefjord Ingemar wrote:
> Interesting topic. What's your experience of 'stabilizing' before doing readout? Myself, I found that you have to wait, sometimes several minutes after the target is connected. At least when you speak TeraOhm/pico/femtoAmpere. So, if you have many connections, the whole measurement cycle can take time.
> /Inge
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Seth Goodman
> Sent: den 5 december 2007 07:57
> To: [log in to unmask]
> Subject: Re: [TN] SV: [TN] SV: [TN] Low Noise Coaxial Cable Question
>
>
> Joe and all,
>
> My personal preference when doing very low current measurements is to avoid low-noise cables altogether and use the shortest length of PTFE dielectric cable possible. I can't really point to any harm that adding a lossy coating to the exterior of the dielectric would do. It's just a personal bias against adding something that I didn't really need. If there is no way to have the cables stationary for the duration of the measurement, then you may need to use this type of cable.
>
> My background in this was not from SIR testing, but closely related. Around ten years ago, I designed an electrometer for a company that makes ionization chambers for measuring the activity of low-level radioactive samples for medical applications. The ion chambers required an adjustable bias voltage of several hundred volts. The finished instrument had a noise floor of around ten femtoamps and somewhere around six decades of range. We used the Keithley instrument in the lab as a reference and of course studied during the design work. The reason the ion chamber manufacturer wanted a dedicated instrument, rather than a set of instructions for a setup with the Keithley, was that they wanted a semi-automated system that could switch ranges by itself and report results on a serial line. What we wound up with was an autoranging instrument with only two ranges. They've been in production for a number of years now.
>
> The observations in this thread seem right to me. As with any low-level measurement, you should put the first amplifier stage as close to the device under test as the environment permits and keep the cables short. At the femtoamp current level, having the first amplifier stage above room temperature would make it harder than it needs to be, so I agree it is a good choice is to use longer cables, but no longer than absolutely necessary.
>
> Now that I recall, the challenges of making the front-end amplifier board lower leakage than the ion chamber currents are what first brought me to TechNet around that time. It was a valuable resource back then and it is today.
>
> Thanks to all.
>
> Seth Goodman
>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Joe Russeau
> Sent: Friday, November 30, 2007 12:22 PM
> To: [log in to unmask]
> Subject: Re: SV: [TN] SV: [TN] Low Noise Coaxial Cable Question
>
> Hello Seth and Inge,
>
> Seth thank you for that very educational response.
>
> Basically, I am researching all of the different SIR test equipment available. I'm looking into purchasing a newer system (By-the way, this is not an invite to SIR equipment manufacturers to bombard me with product literature). Anyway, one of the groups I have been speaking with recommended
>
> using their low noise coaxial cables. I asked how they classified the cable as low-noise. They responded that they pull the cable taught, drop a weight on it and measure the resulting charge. If no charge, then it is considered
>
> low noise. What I had hoped for with asking my question and the question to
>
> TechNet, was to find out what materials were best for low-noise coaxial cables. I have been contacting different cable manufacturers, who claim to have low-noise cable, to try and determine if the materials sets are consistent from manufacturer to the next. I figured that would allow me to find the cabling and perhaps make the cables myself. What I have found is that each cable manufacturer uses different materials for their low-noise cable. So, now I'm back to square one. So perhaps I should ask the question this way. If you were in the process of measuring low currents, as
>
> is done in an SIR test, and you were looking into coaxial cables as the transfer vehicle from the DUT to the measurement equipment, what materials would you prefer in the cables to give the best data integrity?
>
> Best Regards,
>
> Joe Russeau
>
>
> ----- Original Message -----
> From: "Hfjord" <[log in to unmask]>
> To: <[log in to unmask]>
> Sent: Friday, November 30, 2007 1:27 PM
> Subject: [TN] SV: [TN] SV: [TN] Low Noise Coaxial Cable Question
>
>
> Exellent! However, Joe has still not told us WHAT he wants to do with the
> cable. I doubt he is on a level, that dielectric intrinsic noise will play a
> role. Few test engineers work with such problems. If he isn't a Nobel Prize
> aspirant of some kind. I suggest someone finds remedy against Tinnitus.
> He/she will get the prize, I'm sure. And the inventor will create a lot of
> noise...he-he
> /Inge
>
> -----Ursprungligt meddelande-----
> Från: TechNet [mailto:[log in to unmask]] För Seth Goodman
> Skickat: den 30 november 2007 18:55
> Till: [log in to unmask]
> Ämne: Re: [TN] SV: [TN] Low Noise Coaxial Cable Question
>
> Joe,
>
> Some of what has been mentioned is really part of other cable
> specifications and has nothing to do with noise created in the cable
> itself. For instance, penetration of electromagnetic fields inside a
> cable shield is often described by the manufacturer's specification for
> shielding effectiveness. To the extent that the cable has in imperfect
> coaxial shield, it can convert ambient electric or magnetic fields into
> a loop voltage or flowing current. If external fields are the source of
> your problem, you want cable with high shielding effectiveness as
> opposed to low-noise cable. For electric fields, improving the shield
> means thicker braid, finer wire in the braid, better conductivity of
> braid wire and plating, more than one braid and/or a foil shield. For
> magnetic fields, a shielded twisted pair may do better.
>
> The traditional electronic noise sources, which are Johnson (thermal)
> noise, shot noise and a group of unrelated mechanisms that produce 1/f
> noise, do not produce appreciable noise in cables. When cable
> manufacturers list a cable as low-noise, they usually mean triboelectric
> noise. If the connected circuit puts a dc potential on the cable, then
> mechanical flexing can change the capacitance between conductors, which
> will cause current to flow in the external circuit. The triboelectric
> and capacitance change with motion can together be called microphonics,
> as they are both motion-related. Another source or noise in
> high-voltage cables is leakage.
>
> Regards,
>
> Seth Goodman
>
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