1. Home
  2. Electronics Design
  3. sub mV level shunt
  4. Page 2

sub mV level shunt

Zooming in on my Level=7 simulation, the Ron slope _does_ change at VDS=0, so clamping an AC signal _will_ exhibit rectification effects. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| STV, Queen Creek, AZ 85142    Skype: skypeanalog |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 

             I'm looking for work... see my website.
Reply to
Jim Thompson
Loading thread data ...

I was thinking about when the fet is off, not shorting the signal.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

In SPICE... with your model...

In a real circuit, or with a really good (for that region of operation) model in SPICE, it will be linear at low levels. If you use models that were not checked against measured data in the region concerned, then you can get wrong answers.

If you measure it on real silicon, and find that it is in fact nonlinear below 10mV VDS, then I'll be willing to listen, but until you measure it I won't be very interested. I'd be happy to place a bet that the slope of ID vs VDS changes much less than 1% over that +/-10mV region of VDS (with at least Vth+5V on the gate), provided someone honest and competent is doing the measurement.

Reply to
Chris Jones

This was done on a foundry-level Fairchild Spice model. The slope change was small... might require lab measurement of distortion/spectrum to catch it. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| STV, Queen Creek, AZ 85142    Skype: skypeanalog |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 

             I'm looking for work... see my website.
Reply to
Jim Thompson

Maybe you are seeing thermal EMFs, if you really do need uV. It doesn't take much of a temperature gradient to develop a few uV with random metals etc.

Reply to
speff

What happens is that (for n-channel) increased negative drain voltage essentially enhances the positive gate voltage slightly, turning the fet on a bit more. It's subtle in mosfets, more obvious in jfets and phemts.

formatting link 

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics
Reply to
John Larkin

Further thought... the curve thru zero _must_ have curvature. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| STV, Queen Creek, AZ 85142    Skype: skypeanalog |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 

             I'm looking for work... see my website.
Reply to
Jim Thompson

Yes it will have gentle curvature. Very gentle, for VDS

Reply to
Chris Jones

Believe what you will... I'm seeing enough curvature to cause distortion of a 1mV signal... and, obviously, so is the OP.

...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| STV, Queen Creek, AZ 85142    Skype: skypeanalog |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 

             I'm looking for work... see my website.
Reply to
Jim Thompson

Ok. I believe my measurements. In the past I have built switched attenuators (on chips) with MOS switches. Distortion was only a problem with high signal levels on VDS. There is gentle curvature of the I-V curve of the MOSFETs, but not enough to cause trouble in most circuits, unless VDS becomes a significant fraction of (VGS-Vth). In fact with 1mV VDS, I would be skeptical about whether most SPICE simulators would have the numerical precision to model the distortion, even with the tolerances set very tight. Of course there is some distortion but not much at all.

Reply to
Chris Jones

Jim ignored my measurements of about 45 milliohms, IIRC. Exactly the same, no matter whether the current was -1A, -0.1A, -10mA, -1mA, +1mA, +10mA, +100mA, or +1A, good to within about 1% measurement accuracy. I didn't measure at 10A, because that'd have 5W heating and other issues. Perhaps up there Jim's effect would be observable.

You'd want a MOSFET with Rds(on) low enough to look like zero anyway. Rds had a 30% tolerance in that value, not to mention tempcos, etc., so any supposed 1% pos-vs-neg deviation would be swamped by that.

I suspect that Jim's models aren't so good with power MOSFETs with their deep narrow V-groove channels.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

I'm so glad that those of you who know it all have solved the OP's problem... I'm sure he is too >:-} ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| STV, Queen Creek, AZ 85142    Skype: skypeanalog |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 

             I'm looking for work... see my website.
Reply to
Jim Thompson

Sometimes it is useful to know that other people have done the thing that one is trying to do, and that the other people definitely did not have a certain problem. It prompts one to look for other explanations for the symptoms that one is observing, and might lead to discovering the real problem.

Do you still believe that Rds goes up by a factor of 40 below 1mV VDS? You should try measuring it on a real MOSFET, not a simulator. I am happy to place a bet about what you discover. Do you have a decent multimeter and a couple of power supplies and a few resistors, and a MOSFET, or do you only have simulators?

Reply to
Chris Jones

Jim has better simulator tools than I have, but I have better measurement tools than he has.** He not only has good tools, but access to foundry models, which can be excellent, at least for most issues. I'd have some respect for what they show for power MOSFETs, if he had an vetted foundry model for one of those beasts.

** Not only power supplies and multimeters, etc., but also a Keithley 2612A two-channel SMU (Source-Measure Unit, $10k, but purchased on eBay for $4k), good to 0.02%, and 0.1% accurate for 10A pulsed measurements.
formatting link 
--
 Thanks, 
    - Win
Reply to
Winfield Hill

I didn't say that... I said I observe definite curvature.

Solve the OP's problem. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| STV, Queen Creek, AZ 85142    Skype: skypeanalog |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 

             I'm looking for work... see my website.
Reply to
Jim Thompson
[snip]

For some reason the discrete FET manufacturers choose not to invest in the equipment and staff necessary to model their devices :-(

Indeed !-)

But do you not find some observable curvature as you pass thru zero?

Aside... looks like a major ASIC foundry may build me a custom on-chip JFET for my customer needing low noise !!!! ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| STV, Queen Creek, AZ 85142    Skype: skypeanalog |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 

             I'm looking for work... see my website.
Reply to
Jim Thompson

They know you're not going to be designing a power MOSFET, or maybe they'd rather be sure you didn't!

OK, I'll extend my measurements to below 1mA.

BTW, my +/- 1mA measurement on a 45-milli-ohm part was at Vds = +/- 0.045 mV. Straight line, constant 0.045 ohms.

This is indeed good news, because it opens up all kinds of possibilities.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

[snip]

Absolutely! Marvelous step forward! ...Jim Thompson

-- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

formatting link
| 1962 |

I'm looking for work... see my website.

Reply to
Jim Thompson

I might have uses for something like that, so anything you can share about it would be very interesting. I often need something like a half-size BF862, and there ain't one, at least not with that sort of noise performance. (ON Semi has a bunch of new ones, which I'm investigating.)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

Phil, I'll send you and Win foundry contact information by E-mail. It would be helpful if you two "big names in the biz" would affirm the need for a good on-chip JFET. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| STV, Queen Creek, AZ 85142    Skype: skypeanalog |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 

             I'm looking for work... see my website.
Reply to
Jim Thompson

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.