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Author Topic: Measuring nanoamps and below like a Ninja  (Read 10480 times)

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Offline BFX

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Re: Measuring nanoamps and below like a Ninja
« Reply #75 on: February 20, 2017, 03:12:00 AM »
Something arrived at Friday  8) 
I'm curious how good is it since I don't believe that plastic box. But I should wait for triaxial cable and connector for my Keithley 602.
 
 

Offline Assafl

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Re: Measuring nanoamps and below like a Ninja
« Reply #76 on: February 20, 2017, 03:51:44 AM »
A calibration box for a hi-pot tester?

http://www.sefelec.com/en/calibration-kit-MG-91
 

Offline BFX

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Re: Measuring nanoamps and below like a Ninja
« Reply #77 on: February 20, 2017, 06:32:41 AM »
A calibration box for a hi-pot tester?

http://www.sefelec.com/en/calibration-kit-MG-91
We will see how usable it is  :D   :-DMM
 

Offline guenthert

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Re: Measuring nanoamps and below like a Ninja
« Reply #78 on: February 20, 2017, 10:18:26 AM »
I like the idea of coiling the leads of the resistors.  Might have prevented my mishap, breaking the glass of a 100GOhm resistor.
 

Offline ocw

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Re: Measuring nanoamps and below like a Ninja
« Reply #79 on: March 04, 2017, 11:47:38 AM »
There has been slow evolution of my ADA4530-1 low current to voltage converter.  It has selectable 1k, 10k, 100k, 1M, 10M, 100M, 1G, 10G, 100G and 1T feedback resistors.  See its attached picture.

I'm trying to find the minimum current which I can measure with the 1T feedback resistor.  I knew that 1.5 pA was easy by using a 1.55 volt battery which was connected to an external 1T resistor.  I added a 1/10 voltage divider inside of my shielded test box to feed 155 mV to that resistor.  The second attachment shows the 165 fA current produced by that combination (1T 10% resistor is actually close to 0.955).  As is typical with these converters, a positive current is converted to a negative voltage.  1 volt = 1 pA = 1,000 fA.

My test box has a switch to change to a 1/100 voltage divider.  The third attachment shows the approximately 16 fA current produced by that.

Time to build a greater voltage divider.
 

Offline VintageNut

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Re: Measuring nanoamps and below like a Ninja
« Reply #80 on: March 04, 2017, 01:17:14 PM »
There has been slow evolution of my ADA4530-1 low current to voltage converter.  It has selectable 1k, 10k, 100k, 1M, 10M, 100M, 1G, 10G, 100G and 1T feedback resistors.  See its attached picture.

I'm trying to find the minimum current which I can measure with the 1T feedback resistor.  I knew that 1.5 pA was easy by using a 1.55 volt battery which was connected to an external 1T resistor.  I added a 1/10 voltage divider inside of my shielded test box to feed 155 mV to that resistor.  The second attachment shows the 165 fA current produced by that combination (1T 10% resistor is actually close to 0.955).  As is typical with these converters, a positive current is converted to a negative voltage.  1 volt = 1 pA = 1,000 fA.

My test box has a switch to change to a 1/100 voltage divider.  The third attachment shows the approximately 16 fA current produced by that.

Time to build a greater voltage divider.

Two things that I am surprised about

1. No guard

2. The banana jacks appear to garden variety. These usually have significant leakage compared to pA current
working instruments :Keithley 260,261,2750,7708, 2015, 236, 237, 238, 147, 220,  Rigol DG1032  PAR Model 128 Lock-In amplifier, Fluke 332A, Gen Res 4107 KVD, 4107D KVD
 

Offline ocw

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Re: Measuring nanoamps and below like a Ninja
« Reply #81 on: March 04, 2017, 01:34:32 PM »
On the ADA4530-1 data sheets Analog Devices warns of possible nonlinearity when using high voltage resistors at low voltages (a poor voltage coefficient).  To try and evaluate the linearity of the 1T ohm resistor which I used as both a feedback resistor, with another used along with a metered voltage to set the current level which is being measured, I fed the external MOX112523100AK resistor with a 5 volt ramp waveform and observed the output of the converter. 

I fed a 3 milliHertz waveform to the converter's input.  While my DC measurements with the battery and resistors located inside of a shielded box produce negligible power line noise, my quick connection to the function generator had some of that noise modulating the 3 milliHetz source.  But, the attached view of the output of the converter still looks like the MOX112523100AK resistor is fairly linear over the 0 - 5 volt range measured.

Regarding:

Quote
1. No guard

2. The banana jacks appear to garden variety. These usually have significant leakage compared to pA current

The construction was a learning project.  My choice of construction at least appears adequate for the pictured 16 fA current, besides a significantly higher pA current...
« Last Edit: March 04, 2017, 01:38:55 PM by ocw »
 

Online tggzzz

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Re: Measuring nanoamps and below like a Ninja
« Reply #82 on: March 04, 2017, 07:49:43 PM »
On the ADA4530-1 data sheets Analog Devices warns of possible nonlinearity when using high voltage resistors at low voltages (a poor voltage coefficient).

I've picked up a couple of 1Tohm 3% "resistance transfer standards", without having anything to use them for, which implies TEA, FUSS, GAS, but since CAP didn't kick in, neither did DBA.

Anyway, on the side there is a typewritten sellotaped label
R1R2
100V1.0261.026
200V1.0271.023
500V1.0011.016

The corresponding 10Gohm 2% resistors lack such a label, unfortunately.

« Last Edit: March 04, 2017, 07:51:23 PM by tggzzz »
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline ocw

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Re: Measuring nanoamps and below like a Ninja
« Reply #83 on: March 06, 2017, 07:55:59 AM »
Quote
I'm trying to find the minimum current which I can measure with the 1T feedback resistor...
Time to build a greater voltage divider.

Besides the prior 1/10 and 1/100 voltage division, I added 1/1,000 and 1/10,000 options.  That allowed measurements of 1.6 fA and 160 aA via my ADA4530-1 current to voltage converter using its largest 1T feedback resistor.  The ADA4530-1_cal picture shows my converter with my grounded battery powered calibration box attached.  There's not much unshielded space for power line noise to influence the reading.  While the banana connectors aren't anything special, I'm using Teflon insulated wires.

The 1.6 fA measurement didn't look too bad after the reading stabilized--see the 1.6fA attachment.  Due to the large value resistors used with the small current, the stray capacitance in my circuit produced a significant time constant and delay before the reading became steady.  While the reading did stabilize close to the expected 1.6 mV output level for 1.6 fA, it wasn't steady enough for me to rate it as a success.  With the accuracy at 16 fA adequate to satisfy me, about 10 fA may be the limit for my circuit to have reasonable accuracy.  Based on my results in using increased value resistance, feedback accuracy and my recent results using the ADA4530-1 IC, it's likely if I paid $68.96 for a 10T resistor I would be able to have accurate readings down to 1 fA.

With the mentioned results at 1.6 fA, I wasn't expecting much luck in my final measurement at 160 aA.  I'm not used to dealing with attoamps.  As the 160aA attachment shows, it wasn't a complete failure.  The reading did center around 0 to the desired -0.16 mV to indicate 160 aA.  But, it only points to a very vague low current range.  The big jumps on both sets of today's readings could be due to power supply spikes.

I also wanted to take some readings using my ADA4530-1 circuit with my 1T ohm current limiting resistor along with up to 1,000 volts [with 2 mA current limiting] to take a comparative set of readings using my circuit's different feedback resistors.  The difference in the indirectly measured resistance of my external 1T resistor would partially depend on the voltage coefficient of the external resistor and also on the relative accuracy of my at least 1% accuracy feedback resistors (except for both 1T resistors being 10%).  I don't have any way of confirming which was more influential on my results other than what the 1% accuracy resistors suggests.  The results were:
  FB R  |   V to R    |   Voltage Out  |  Measured R
   1T       1.0000V        -1.0013V           0.9987T
 100G     10.000V        -1.0451V           0.9568T
  10G      100.00V        -1.0561V           0.9469T
   1G       1000.0V        -1.0915V           0.9162T

This has been an interesting project.

 

Online MasterTech

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Re: Measuring nanoamps and below like a Ninja
« Reply #84 on: March 09, 2017, 06:24:19 AM »
The way I measure picoamps and very high resistances, great equipment  ^-^
Didnt found anything on this forum about this gear, so I will post a short review about it




Test resistor,




1 Volt across 100G:


« Last Edit: March 09, 2017, 02:30:02 PM by MasterTech »
 

Offline ocw

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Re: Measuring nanoamps and below like a Ninja
« Reply #85 on: March 10, 2017, 08:19:18 AM »
While there are always good (and sometimes not so good) deals available, it seems like building a ADA4530-1 current to voltage converter has a hard to beat performance to price ratio for pA and below current measurements and G to T ohm measurements.  Using a much discussed U1252B meter to measure the converter's output, 10 aA (0.01 fA) current resolution is possible on the lower level currents.  One digit increased resolution is available with my 34401A.  Without zeroing either the converter or the meter connected to it, I had about a 2 fA current reading.  Zeroing the meter was only required for readings where that level of current was significant.

As the attachment shows I saw excellent accuracy with the resistors which I used as feedback resistors at 1gigaohm and below.  The 100 pA and above current measurement typically had around 0.3% accuracy and below.  My lower current accuracy verification was limited by the less accurate feedback and the external resistors which I used to set that current level.  I used a voltage level measured by a 34401A meter and a 0.01% 1M and below resistors and 1% resistors with values up to 100M.  I was limited by using a 10% 1T ohm resistor, which actually had a value close to 0.955T.

It was difficult to accurately verify the accuracy of even lower current measurements.
« Last Edit: March 10, 2017, 04:35:14 PM by ocw »
 

Offline TiN

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Re: Measuring nanoamps and below like a Ninja
« Reply #86 on: March 21, 2017, 02:10:52 AM »
Let's see if people pay attention what I write. Anyone know where these beauties are from?  8)

xDevs.com | Have test gear documentation to share? Upload here! No size limits, firmware dumps and teardown photos welcome.
 

Online CalMachine

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Re: Measuring nanoamps and below like a Ninja
« Reply #87 on: March 21, 2017, 03:55:12 AM »
Let's see if people pay attention what I write. Anyone know where these beauties are from?  8)



Your Keithley 6621!
 

Offline TiN

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Re: Measuring nanoamps and below like a Ninja
« Reply #88 on: March 21, 2017, 10:03:14 AM »
You are no fun.

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Online CalMachine

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Re: Measuring nanoamps and below like a Ninja
« Reply #89 on: March 21, 2017, 12:18:50 PM »
Haha whoops!  ^-^   Very nice though!  How stable is that reading? 
 

Offline ocw

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Re: Measuring nanoamps and below like a Ninja
« Reply #90 on: March 26, 2017, 12:41:48 PM »
The attachment shows the output of my ADA4530-1 current to voltage converter when it is fed 250 pA AC current at 500 kHz.
At that frequency/current it has lost its conversion accuracy without taking its high frequency roll-off into account.
 

Offline ocw

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Re: Measuring nanoamps and below like a Ninja
« Reply #91 on: April 24, 2017, 01:42:30 PM »
I recently completed the modifications of my ADA4530-1 based current to voltage converter.  A picture of it and my accuracy verification figures for it are attached.  It has the conversion ranges of from 1 pA input current producing 1 volt output, to 1 mA producing the 1 volt output.  Think of the current resolution which is provided in the 1 pA range.  Rather than the leakage resistance from my Johnson/Cinch banana input jacks or the Grayhill 56D30-01-1-AJS range switch, I had more problems at the lower currents due to noise and other currents coupling to any unshielded wires outside of its enclosure.  My alignment box solved things by having a N cell battery feeding a 1.0 volt regulator and a current limiting resistor inside of an earth grounded metal box.

My first version had two ranges with restricted frequency bandwidth.  Not finding them beneficial I added two non-decade ranges in my available switch positions.

The accuracy of the unit is primarily limited by the precision of the feedback resistors.  I have their values and accuracy shown on attachment.  The currents being measured are voltage dependent currents, NOT constant currents.  That's what I'm most frequently measuring.  I want to know what the worst case accuracy is, not what the accuracy is with ideal constant currents.  The current being measured was produced by a regulated 1.0000 volt and a current limiting resistor.  The accuracy of that resistor is shown on the attachment under the "Standard" "Accuracy" column.  So, the ADA4530-1 Accuracy column includes the insignificant amount of burden resistance/voltage in that IC.  The single digit fA input bias current of the op amp didn't effect zeroing the circuit's output until that amount of current became significant.  The external voltage meter can be zeroed if that value becomes important.

With an added external voltage or resistance and a simple calculation, resistance and voltage can also be measured.  The circuit has a very high input impedance for voltage measurement.  I've measured up to 1,000 volts using it.

I am surprised that a company doesn't use a similar circuit for their commercial circuit.  It would have a much smaller burden resistance and voltage for current measurements than what they are using now.  Greater value resistors could be measured.  And the circuit would have a higher impedance for voltage measurements.  I'd design the meter so that a 10M resistor could be added during voltage measurements for times when its high impedance causes more harm than good.  And, given the relatively low cost of the components needed, it could be used in a multi-meter with a cost significantly less than that of the typical electrometer.
« Last Edit: April 24, 2017, 04:34:00 PM by ocw »
 


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