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Measuring Large resistances

K

Kreyen

Jan 1, 1970
0
I am having problems trying to measure resistance values above
100MegaOhms. My Laboratory Ohmeter gives unstable values.

Is ther anyway apart from the usual dc bridges of getting accurate
resistance measurements.

Thanks guys.

Kreyen
 
P

pimpom

Jan 1, 1970
0
Kreyen said:
I am having problems trying to measure resistance values above
100MegaOhms. My Laboratory Ohmeter gives unstable values.

Is ther anyway apart from the usual dc bridges of getting
accurate
resistance measurements.

Are you trying to measure a resistor or something else? What
range variations are you getting? Did you consider surface
leakage and other interfering factors? What level of accuracy do
you need?
 
K

Kreyen

Jan 1, 1970
0
Are you trying to measure a resistor or something else? What
range variations are you getting? Did you consider surface
leakage and other interfering factors? What level of accuracy do
you need?

I'm trying to measure 3 resistances to be used for calibration
purposes.. I'm getting variations of several percent while I'm
looking into a t least a 0.1 percent accuracy level.

Kreyen
 
K

Kreyen

Jan 1, 1970
0
Sure, 100 volt in 100 MOhm = 1 uA.
How accurate can you measure current?

Well we got this expensive meter at the Lab at work so I can't see how
accurate its current abilities are ... as I'm now at home sweet home .
But I'll check tomorrow and let you know the values. It is a HP and a
very professional looking device though.

Kreyen
 
S

Shaun

Jan 1, 1970
0
Kreyen said:
I'm trying to measure 3 resistances to be used for calibration
purposes.. I'm getting variations of several percent while I'm
looking into a t least a 0.1 percent accuracy level.

Kreyen

On resistors in those high range, even finger prints on the resistor can
affect accuracy.
Solution: Clean them with isopropyl alcohol and let them dry, then try
measuring them again.

Some meters have a nanoSiemens range which is 1/Resistance, they usually
give accurate readings. Other that that a megohmeter that uses high voltage
to measure resistance is usually used.

Shaun
 
P

pimpom

Jan 1, 1970
0
Kreyen said:
I'm trying to measure 3 resistances to be used for calibration
purposes.. I'm getting variations of several percent while
I'm
looking into a t least a 0.1 percent accuracy level.

I was about to suggest something similar to Jan's but decided to
ask for more details. For that level of accuracy, a bridge would
be your best bet. With the jury-rigged method, it goes without
saying that the supply would have to be stable and accurate to
better than 0.1%, with a similar requirement for the measuring
instrument.

Perhaps you could try your lab ohmmeter again, making sure that
everything is squeaky clean and perfectly dry. In still air.
Varying thermal gradients might also have affected the readings.
 
S

Spehro Pefhany

Jan 1, 1970
0
I am having problems trying to measure resistance values above
100MegaOhms. My Laboratory Ohmeter gives unstable values.

Is ther anyway apart from the usual dc bridges of getting accurate
resistance measurements.

Thanks guys.

Kreyen

100M isn't that high, well, depending on what kind of accuracy you're
looking for.

How much instability are you seeing? How much above 100M?

LMC6042 has 2fA (typical) input bias current and costs a couple
dollars one-off.

A 100M 1% resistor with 50ppm/K tempco runs around five dollars.
A 1G 1% resistor with similar tempco is maybe double that.

Most inexpensive DMMs on the lower ranges have very high input
impedance so the LMC buffer might not be necessary.
 
F

Fred Bartoli

Jan 1, 1970
0
Spehro Pefhany a écrit :
100M isn't that high, well, depending on what kind of accuracy you're
looking for.

How much instability are you seeing? How much above 100M?

LMC6042 has 2fA (typical) input bias current and costs a couple
dollars one-off.

A 100M 1% resistor with 50ppm/K tempco runs around five dollars.
A 1G 1% resistor with similar tempco is maybe double that.

Most inexpensive DMMs on the lower ranges have very high input
impedance so the LMC buffer might not be necessary.

High value resistors are (depends on the model) somewhat unstable.
I currently have a batch of Dale 1G/1%.
They measure fine with 100V bias which is the datasheet measuring
conditions. When measuring them at low voltage, they're all over the
place, from +3ish% to +7ish%. Yep, not even grouped...
On the contrary I've some Caddok rated <0.02ppm/V!

A friend of mine worked at, IIRC, Vishay/sfernice on that specific high
value resistors 'feature' and went auditing some of the 'production'
lines (the quotes are his). He said the voltage dependency was mostly a
prod issue. High value Rs is a very small niche market and as such,
production inherited some highest tech tools, like hand lapping and the
likes. Being labor intensive it was relocated in low labor cost
countries, with poor buildings (he said, almost backyard :) and less
than ideal handling cleanliness.

That could explain a lot some of the strange behaviors...
 
S

Shaun

Jan 1, 1970
0
Kreyen said:
I am having problems trying to measure resistance values above
100MegaOhms. My Laboratory Ohmeter gives unstable values.

Is ther anyway apart from the usual dc bridges of getting accurate
resistance measurements.

Thanks guys.

Kreyen


I just measured some Megaohm resistors with my Fluke 867b. accuracy 0.5%

I measure two - 66 Megaohm resistors at 20% tolerance (not very accurate)

measured 1 - 65.839 Megaohms
measured 2 - 69.742 Megaohms

Also I measured one 200 Megaohm resistor that came out of some medical
equipment, unknown tolerance

measured 196.46 Megaohms

then readings were stable +/- .1 count in nanoSeimens and repeatable.

You maybe working in an electrically noisy environment, try moving you meter
to a different location where there is no extra cables around, maybe a noise
filter would help too.

Shaun
 
S

Spehro Pefhany

Jan 1, 1970
0
*That* is a color of a different horse..the DVM used would have to
have that accuracy, leaves out handhelds!

Maybe *your* handhelds.
 
S

Spehro Pefhany

Jan 1, 1970
0
Nonsense;they have the same impedance on their lowest range as the
others,unless they disconnect the input divider.They don't.

Hmm.. I think you're right on that point. I do have one that does, but
probably most do not. Benchtop instruments like my Agilent 34401A
meters do have an "infinite impedance" setting.
(I did that on a TEK DM501 to make cal measurements for a 577/177 curve
tracer.)

also,I checked a Harbor Freight DMM and it was only 1 Megohm input R.
(and the first one was way out of calibration,read a 1.5v alkaline cell at
1.9v)

1M? What a POS. What do you expect for dollar store prices. Even the
Chinese can't make a worthwhile meter for what Harbor Fright wants to
pay.
seems like they could just use carbon film deposition on glass substrates
and laser-trim them,all machine handled.

Carbon? AFAIK something like Ruthenium oxide is more common in chip
resistors. Ohmcraft makes their using a kind of printing method which
is automated, so it should be consistent. Here is a white paper on
their process:

http://www.ohmcraft.com/Datasheets/03_White_papers/WP_HighValuePrecisionChipResistors.pdf
 
S

Spehro Pefhany

Jan 1, 1970
0
OK, i will bite; what brand(s) and model number(s)?


Fluke 87-5, for example. DC voltage accuracy is 0.05% + 1 count on the
6V range.

But only 1% in the 60nS range (infinity to 16.667M ohm),
unfortunately. A 100M resistor would have a conductivity of 10.00nS so
the resolution is pretty much there, but not the accuracy.

Probably lots of other 4-1/2 digit handheld meters too.
 
K

Kreyen

Jan 1, 1970
0
Hi guys
Thanks for all the enlightening on the subject.
The model of the meter I use at the Lab is the Agilent 34401A which
Spehro incidently mentions has an infinite resistence setting. I
couldn't find that ... which means I'll have to browse through the
thick manual. It doesn't seem to deliver more then then a 10V output
though.
I suspect it might be measuring the limitations in its accuracy
readings or the thermal noise in the resistors.

Kreyen
 
J

JosephKK

Jan 1, 1970
0
I've got a fluke 189 multimeter that reads 100M ohms just fine.
(Ohmite brand.) Since the meter has 10 Meg input impedance they must
do some tricks in software. Which suggests that you might measure a
good 10M ohm resistor and then try putting the 100M in parallel. See
if you get better numbers... of course you will have reduced accuracy,
but it might work. At least you might learn if it's the meter or the
resistors that are flakey.

George H.

Oh drear, it has a 10M ohm input when measuring voltage; measuring
resistance is entirely different.

The measurement problems may be exactly that, measurement method. For
100 Mohm and up i suggest putting the DUT in a closed conductive box
with holes for the test leads. And use minimum length test leads, not
touching anything if possible.
 
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