looking for high value resistors

[alan]

Hello people

I am looking for high value resistors (1G, 10G) that have these
additional properties:

low capacitance (I can't find any specs for this)
reasonably linear resistance vs voltage
low excess noise
Easily available in the US
low tempco
(whatever other characteristic might be important)

What type of resistor would have the best properties?

Using this for a high gain, medium speed (<1kHz) current amplifier for
STM and stuff.

 

[Country Loon]

"alan" <no-longer-valid@yahoo.com> wrote in message
news:ckj12j$ej8$1@news.Stanford.EDU...

Hello people

I am looking for high value resistors (1G, 10G) that have these
additional properties:

low capacitance (I can't find any specs for this)
reasonably linear resistance vs voltage
low excess noise
Easily available in the US
low tempco
(whatever other characteristic might be important)

What type of resistor would have the best properties?

Using this for a high gain, medium speed (<1kHz) current amplifier for
STM and stuff.

Never heard of such a resistor but if it exists it is bound to be noisy as
the noise is something like

Vn=sqrt(kTRB)

and R is the resistance.


Tom

 

[Winfield Hill]

Country Loon wrote...

alan wrote ...

I am looking for high value resistors (1G, 10G) that have these
additional properties: [ snip ] Using this for a high gain,
medium speed (<1kHz) current amplifier for STM and stuff.

Never heard of such a resistor but if it exists it is bound to
be noisy as the noise is something like Vn=sqrt(kTRB) and R
is the resistance.

Sorry, high-value resistors are much better for trans-resistance
amplifiers. That's because of Ohm's law, i_n = v_n / R, so the
effective Johnson current noise seen at the input node due to the
feedback resistor is sqrt(kTB/R), which gets better for large R.
That's the right way to use the Johnson-noise math for currents.


--
Thanks,
- Win

(email: use hill_at_rowland-dotties-org for now)

 

[Daniel Haude]

On Fri, 15 Oct 2004 04:27:08 -0700,
alan <no-longer-valid@yahoo.com> wrote
in Msg. <ckocm2$cci$1@news.Stanford.EDU>

I found a glass encapsulated Welwyn 1G resistor in our lab, and I
measured the cap to be .04pF. Maybe I should be more careful about
measuring? Coz this value seems way low compared to yours. I found
another Dale/Vishay 1G resistor that looks like a "regular" 1G resistor,
and that has a cap of about .2pF.

Sorry, I lost a digit there: .03pF is more like it.

--Daniel

--
"With me is nothing wrong! And with you?" (from r.a.m.p)

 

[Daniel Haude]

On 15 Oct 2004 05:59:52 -0700,
Winfield Hill <Winfield_member@newsguy.com> wrote
in Msg. <ckohk801pui@drn.newsguy.com>

That's correct, I'm not sure what Daniel is talking about.

Just about what ELTEC says in their data sheet. Maybe I should switch
suppliers.

Actually, I want minimal noise at a few hundred Hz to take good spectra.
The setpoint is like 100pA. The best I can do with resistors is
actually run 10G and divide the output by 10. Frank mentioned using
capacitors for feedback, which is an idea I have in the back of my head,
but I'm not sure yet how to implement it while still keeping the DC part
of the signal.

Dan suggests 100M instead of 1000G, and I suggest 100G instead of 10G,
if you can get it...

If you want to have "a few hundred Hz" of bandwith with a 100G feedback
resistance, you must also do frequency compensation because the 3dB point
of such a circuit (w/ .02pF stray C) will be at about 40 Hz. That's the
infamous "R-C-R trick" or "Keithley circuit" which has been discussed at
great detail in this group earlier this year.

But to the point of STM/STS: I don't think you're very well off with an
100G STM amplifier because it limits the max currnt at which you can do
any measurement to about 100pA -- and although I made a lot of measurments
on semiconductors with such and lower currents, I still like to be able to
fry my tip with several nA against a metal surface. A low-gain amplifier
will of course impose a higher low limit of usable stabilization currents.
Much of this depends on your specific application.

With STS, all boils down to how much time you want to spend on your
measurement. You can use a high-gain, low-noise, low-bandwidth amplifier,
but that'll require low modulation frequencies and in turn long lock-in
integration times. A low-gain amp will yield a higher bandwidth allowing a
higher modulation frequency, but at the cost of more noise -- which you'll
have to filter again with a long time constant. I haven't done the math; I
wonder if the two cases (10G @ 200Hz vs. 1G @ 2kHz) are equivalent. If so,
use the smaller R and have a more versatile amp.

The most important point in hogh-resolution STS is to keep the noise
across the gap low, because this you can't filter with the lock-in.

For the < 100ueV energy resolution that can be achieved with a jellybean
transresistance amplifier comprising a OPA627BM and a 1G feedback resistor
looking into almost 1 nF of input capacitance, check out an upcoming
article in the Review of Scientific Instrumentation (I can give you the
exact rev once the issue is out in November).

--Daniel

--
"With me is nothing wrong! And with you?" (from r.a.m.p)

 

[alan]

alan wrote:

I found a glass encapsulated Welwyn 1G resistor in our lab, and I
measured the cap to be .04pF. Maybe I should be more careful about
measuring? Coz this value seems way low compared to yours. I found
another Dale/Vishay 1G resistor that looks like a "regular" 1G resistor,
and that has a cap of about .2pF.

Actually, I just looked at Welwyn's web page, and for what I believe to
be the same resistor (it's their only glass coated one) they quote a
capacitance of .2pF. So I don't know what's up with that.

They also have some planar resistors that they claim to be low
capacitance, but they won't give any values

 

[Winfield Hill]

alan wrote...

what, am I supposed to run this in a 100V amplifier and
divide the output by 10?

That's what Keithley does in some of their models. :&gt


--
Thanks,
- Win

(email: use hill_at_rowland-dotties-org for now)

 

[Daniel Haude]

On Mon, 18 Oct 2004 04:05:52 -0700,
alan <no-longer-valid@yahoo.com> wrote
in Msg. <cl08ij$hni$1@news.Stanford.EDU>

But to the point of STM/STS: I don't think you're very well off with an
100G STM amplifier because it limits the max currnt at which you can do
any measurement to about 100pA -- and although I made a lot of measurments
on semiconductors with such and lower currents, I still like to be able to
fry my tip with several nA against a metal surface. A low-gain amplifier
will of course impose a higher low limit of usable stabilization currents.
Much of this depends on your specific application.

just switch the gain.

Hah! Not so easy to do in the first stage. What would you propose? I
already threw the reed relay out of my amp.

The time you want to spend per spectra sets how wide you have to open
the badwidth on the lock-in.
Given that you have a fixed setpoint current, you're better off running
the higher gain since the spectral noise density is lower and thus the
S/N is better.

Yes, but a higher gain (as long as we're only talking about the standard,
two-component transresistance amp) automatically lowers your bandwidth,
forcing you to use low mod frequencies and long integration times.

I don't understand what you said above about using a
higher bandwidth and then filtering it down.

A higher-bandwidth, lower-gain amplifier permits higher mod frequencies at
the cost of more noise, still requiring you to use long integration.

Isn't that the same as
using a lower bandwidth in the first place?

That's what I said -- if in fact it is the same (which I suspect but don't
know) a low-gain high-BW amplifier has some advantages: higher currents
for tip preparation, higher scan speeds in topo mode, easier to obtain
feedback resistors. My hunch is that the optimum resistance, all things
considered, is between 100 and 1000M.

heh, you need to have better wiring

In this setup there's no way to get to the STM through less than seven
meters of wiring...

--Daniel

--
"With me is nothing wrong! And with you?" (from r.a.m.p)

 

[Guy Macon]

Daniel Haude <haude@kir.physnet.uni-hamburg.de> says...

alan <no-longer-valid@yahoo.com> wrote

just switch the gain.

Hah! Not so easy to do in the first stage. What would you propose? I
already threw the reed relay out of my amp.

Why?

 

[alan]

Daniel Haude wrote:

On Mon, 18 Oct 2004 04:05:52 -0700,
alan <no-longer-valid@yahoo.com> wrote
in Msg. <cl08ij$hni$1@news.Stanford.EDU

just switch the gain.


Hah! Not so easy to do in the first stage. What would you propose?

A jumper. Coz otherwise I'd turn the gain knob on my Keithley 427.

The time you want to spend per spectra sets how wide you have to open
the badwidth on the lock-in.
Given that you have a fixed setpoint current, you're better off running
the higher gain since the spectral noise density is lower and thus the
S/N is better.


Yes, but a higher gain (as long as we're only talking about the standard,
two-component transresistance amp) automatically lowers your bandwidth,
forcing you to use low mod frequencies and long integration times.


A higher-bandwidth, lower-gain amplifier permits higher mod frequencies at
the cost of more noise, still requiring you to use long integration.

I don't know how you take your spectra or lock-in measurements, but I
don't think the mod frequency makes a difference in how "fast" you can
take data, given that it is above some minimal level. The bandwidth on
the lock in determines this, because it sets how fast the output of the
lock in can fluctuate. Given that this bandwidth is now fixed, the low
gain amplifier is always worse since the spectral noise density is
higher, so the lock-in always ends up integrating more noise. The only
way to get less noise is to decrease the bandwidth, but that will make
your measurement slower.

Isn't that the same as
using a lower bandwidth in the first place?


That's what I said -- if in fact it is the same (which I suspect but
don't
know) a low-gain high-BW amplifier has some advantages: higher currents

No, the signal to noise goes as sqrt(gain) for a given measurement speed.

I just realised that maybe your situation is that you need to take data
so fast that you use almost the entire amplifier bandwidth for the
lockin bandwidth? In that case, I think the procedure should be:

Determine how fast you want to take spectra. This sets the lockin time
constant/bandwidth.
Set the gain as high as possible such that you still have bandwidth to
do this. This maximizes the signal to noise.
Measure and publish.

heh, you need to have better wiring


In this setup there's no way to get to the STM through less than seven
meters of wiring...

that's still 140pF per meter...

 

[alan]

Daniel Haude wrote:

On Mon, 18 Oct 2004 13:39:11 -0700,
alan <no-longer-valid@yahoo.com> wrote

that's still 140pF per meter...


Correct, but do you know of any UHV compatible coax with less than 1mm
diameter that has significantly lower capacitance?

if it has to be less than 1mm the entire 7 meters, then desert
cryogenics has one that is 94pF/m.

 

[Daniel Haude]

On Mon, 18 Oct 2004 13:39:11 -0700,
alan <no-longer-valid@yahoo.com> wrote
in Msg. <cl1a59$div$1@news.Stanford.EDU>

I don't know how you take your spectra or lock-in measurements, but I
don't think the mod frequency makes a difference in how "fast" you can
take data, given that it is above some minimal level. The bandwidth on
the lock in determines this, because it sets how fast the output of the
lock in can fluctuate. Given that this bandwidth is now fixed, the low
gain amplifier is always worse since the spectral noise density is
higher, so the lock-in always ends up integrating more noise.

Correct.

The only
way to get less noise is to decrease the bandwidth, but that will make
your measurement slower.

Correct.

No, the signal to noise goes as sqrt(gain) for a given measurement speed.

Correct.

I just realised that maybe your situation is that you need to take data
so fast that you use almost the entire amplifier bandwidth for the
lockin bandwidth? In that case, I think the procedure should be:

Determine how fast you want to take spectra. This sets the lockin time
constant/bandwidth.
Set the gain as high as possible such that you still have bandwidth to
do this. This maximizes the signal to noise.

Correct. This way I've ended up at 1G.

Measure and publish.

Ummm... correct.

that's still 140pF per meter...

Correct, but do you know of any UHV compatible coax with less than 1mm
diameter that has significantly lower capacitance?

--Daniel

--
"With me is nothing wrong! And with you?" (from r.a.m.p)