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Post by Bogdan on May 7, 2015 0:45:26 GMT -8
Hello. Someone has a good transimpedance design to be able to measure currents from 1nA to 1mA.
Thank you!
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Post by Gary Lecomte on May 7, 2015 18:09:28 GMT -8
I Built this MANY Years Ago and don't have the schematic anymore. I would need to take it apart and create the schematic now. It's Not Perfect, but fairly accurate. Readout is a 3 1/2 digit LCD, Panel Meter. It's powered with a 12 Volt adapter.(Adapter Plug on back panel)
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Post by Bogdan on May 7, 2015 23:28:32 GMT -8
Hello Mr.Gary. Thank you for your reply. I will be very interested in the schematics.
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Post by Gary Lecomte on May 9, 2015 15:55:48 GMT -8
I Tried to find my Original PCB Layout, But No Longer have it.
So I Photographed the copper side and re-created it. But I Haven't drawn up the Schematic Yet.
However I See a Problem.
The 3 IC's in this are:
LT1012, and this is Available. TL497, and this is also Available. CA3146, But this is NOT Available, Except possibly at an OUTRAGEOUS PRICE.
UPDATE:
I'm Surprised, I have Lots of the TL497's Also Two of the LT1012's And Two of those OBSOLETE CA3146's
It also uses a 1N4739, 9 volt zener A LM385Z-1.2, Voltage Reference And a 78L12 Regulator.
Plus six, 549 ohm, 1% Resistors. (I Have a Few Hundred of these.) And a few 5 % values.
And a Custom Wound Toroid Transformer. BUT SIMPLE to Make This. Just 10 turns primary, 21 turns secondary
I Believe I have these also.
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Post by Bogdan on May 10, 2015 0:41:52 GMT -8
Until now I managed to use one TL084 to measure 4.7nA using 10Mohm resistor gain. The 3 op-amp free are tied as unity-gain. The power supply is made with a 9V batery and an ICL7662 to get the -9V.
I plan to use a tee-resistor network to get 100Mohm to go below nA.
I am very interested in your schematic. I truly appreciate your effort.
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Post by Gary Lecomte on May 10, 2015 7:04:37 GMT -8
A TL084 is NOT Suitable for Tiny Currents. It has Much too High of Input Offset Currents!
The LT1012 is MUCH BETTER.
It Has: Pico-amp Input Current, Microvolt Offset, Low Noise Op Amp.
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Post by Bogdan on May 11, 2015 5:19:14 GMT -8
Please tell me if the MAX4239ASA+ can be used to measure from 1pA to 1nA. Thank you! I have 2 pieces.
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Post by Gary Lecomte on May 11, 2015 7:37:48 GMT -8
Please tell me if the MAX4239ASA+ can be used to measure from 1pA to 1nA. Thank you! I have 2 pieces. Yes that IC Would be Suitable, If you design a proper circuit for it.
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Post by Gary Lecomte on May 12, 2015 8:22:44 GMT -8
Here is the Schematic for my Pico-Amp Meter. I have also Re-Created the PCB for this, If anyone wants it.
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Post by Bogdan on May 15, 2015 7:14:09 GMT -8
Thank you for the schematic!
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Post by Gary Lecomte on May 16, 2015 6:54:02 GMT -8
Thank you for the schematic! Most Important in making a PCB for these Pico or Nano Amp Circuits is Having Proper Ground Guards Around the IC Inputs. And Avoiding any Ground Loops! Here is my PCB.
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Post by Bogdan on Jun 29, 2015 14:32:13 GMT -8
Hello! Now I facing another problem, how do I connect the TIA to the capacitor lead to measure the charging/discharging currents when using 1000V or 1500V or 2000V? I have found this schematic used by someone to measure this currents but no explanations. The circuit I made to measure small currents is this: I use : LOG114 with 1uA reference current on input I1. The power supply for the log is made using an 78L05SMD (12V -> 5V) and after the ICL7660 to get the -5V. The logarithmic output of the LOG114 is scaled to 0V..2.5V and feeded to the 24bit ADC LTC2400. I use an microcontroller to get the readings and send to computer for further processing. The problem is how do I connect this to measure capacitor charging/discharging? If I use for example a simple current source made of a 1.5V battery and a 91Mohm 5% resistor, I get correct readings of current 16nA. Please help me adapt this to measure charging and discharging of a capacitor.
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Post by Bogdan on Jun 29, 2015 14:35:07 GMT -8
One thing I am shure, it needs to be done like this:
But at the signall line pointed by the red arrow we have enough voltage to kill the LOG114.
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ndham
New Member
Posts: 6
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Post by ndham on Feb 15, 2016 14:40:53 GMT -8
A TL084 is NOT Suitable for Tiny Currents. It has Much too High of Input Offset Currents! The LT1012 is MUCH BETTER. It Has: Pico-amp Input Current, Microvolt Offset, Low Noise Op Amp. Try the OP77 low offset 10,000,000,000 gain very linear and only $2 great great grandson of the OP07
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Post by Gary Lecomte on Feb 16, 2016 9:00:06 GMT -8
High Gain is NOT the ISSUE. You Need a VERY LOW OFFSET CURRENT.
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