On Mon, 26 Oct 2015 12:38:44 -0700 (PDT), dagmargoodboat@yahoo.com wrote:>On Monday, October 26, 2015 at 11:43:40 AM UTC-4, John Larkin wrote: >> On Mon, 26 Oct 2015 08:11:45 -0700 (PDT), dagmargoodboat@yahoo.com >> wrote: >> >> >On Monday, October 26, 2015 at 10:03:50 AM UTC-4, George Herold wrote: >> >> On Sunday, October 25, 2015 at 1:46:40 PM UTC-4, kellysb...@gmail.com wrote: >> >> > On Saturday, October 24, 2015 at 7:06:57 AM UTC-6, kellysb...@gmail.com wrote: >> >> > > I am having a difficult problem on a current project so I was wondering if anyone could help. This is a project where I have to measure current generated by a sample at high temperature inside a furnace. The current is in the picoamp-nanoamp range depending on temperature and I have been using a Keithley picoammeter. The furnace is controlled by a PID temp controller connected to a solid state relay. When the relay is off I can get data with no issues but when the relay is on there is lots of noise and I cannot get good data. Unfortunately, the high temperature environment makes it difficult to shield the sample without going to exotic materials like platinum foil. Is it possible to measure picoamps-nanoamps in the presence of lots of noise by using some kind of instrumentation amplifier or something with very good common mode rejection ratio? >> >> > > >> >> > > -Kelly >> >> > >> >> > >> >> > Thanks everyone for your responses! To answer some of your questions: >> >> > >> >> > I am going to about 1050 C in an oxidizing atmosphere so that rules out many traditional materials. Basically I have to use platinum wires to connect to the sample. Nobody that I know makes a high temp coax that goes to 1050 C which is why I have virtually no shielding. >> >> > >> >> > I don't have to make fast measurements, in fact 1 sample measurement per second would be fine. >> >> > >> >> > I have tried ramping to a certain temperature and then unplugging the furnace. All the noise goes away and I can get continuous data with no problem. But that is not a good long term solution since I will need to record data over multiple ramp/dwell/cool cycles which is why I am looking for a better solution. >> >> > >> >> > Someone suggested an isolation transformer but that might be cost prohibitive and same goes for a DC power supply. My furnace runs off 120 V at 14 amps. However, I did find a line filter at the local surplus electronics store but I haven't tried it yet. >> >> > >> >> > I like the idea of integrating a capacitor. Texas Instruments makes the IVC102 which does this but I am sure there are other solutions as well. Has anyone used this IC? Are there other IC's or solutions that someone could recommend? I will be honest, my electronics skills are amateur but I have breadboarded a few things in my time. >> >> > >> >> > Thanks in advance! I really appreciate all your responses. >> >> >> >> Can you look at the noise/ interference on a 'scope? >> >> (or are the signals too small?) >> >> If you knew it was 60 Hz, or a switching transient or >> >> something else then that would help. >> >> >> > >> >If it were a zero-crossing SSR, integrating over an integer number of >> >line cycles would help a lot, theoretically. (Avoids the problem of the >> >PID switching asynchronously to the integration interval.) >> >> To invoke filtering of any sort, you'd have to know that the current >> source is constant or linear on voltage. If it is a semiconductor sort >> of thing, it will rectify the noise and mess up the average current >> measurement. > >You're right, I'd assumed the EMI wasn't interacting with the signal source. > >The furnace itself is typically a brick-lined steel box, which you'd think >would be pretty good first-order shielding from the outside world. > >With these high impedances, capacitive-coupling from the heating element could >be serious; an electrostatic shield could fix that. > >The easy solution is just to turn the thing off for the one-second measurement >interval. The thermal time constant is so much longer that 1s shouldn't make >a nit's difference to temperature control. > >Cheers, >JamesNo, no, that's too easy. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
measuring nanoamps with lots of noise
Started by ●October 24, 2015
Reply by ●October 26, 20152015-10-26