As we know,a normal 15M ohm resistance has parasitic inductance, but how much is it??

I mean a normal resistance, not a surface mounting one.

## how much parasitic inductance of a 15M ohm resistance?

Leaded metal film 0.25W resistor about 5nH not including the leads. Leads roughly 1nH per mm.

Would have thought that the inductance of a 15M resistor would not be important. We only worried about low R values, ie <100 Ohms. But then we were working at 150-500MHz. What frequency are you considering?

I am interested in the frequency between 20kHz and 200kHz

Do you speak about 15 mOhm (milli-)

or 15 MOhm (Mega-) ?

A wirewound 15mOhm shunt may have a higher

inductance and you would have to consult

the resistors datasheet !

For a 15Mohm resistor I think the inductance

in your frequency range is neglegible,

5nH at 200kHz are 0.006 Ohms (inductive).

> Leads roughly 1nH per mm.

Really per mm? I recently inquired about inductance of copper leads

on FR4 PCB, and the typical data I could find was about 4 nH per cm.

I'd assume a straight wire in air to have somewhat less.

@jörg

In most formulas for single-turn inductance

loops there is a logarithmic dependence of the

inductance from the wire-diameter. The smaller the

wire-diameter the larger the inductance.

http://members.aol.com/marctt/CV...

I think a copper-trace often is reatively "broad"

while wires are relatively "thin". Perhaps that

accounts for the higher value for the leads.

But its really "Guesstimating"

Those guesstimates appear to come up with approximately 0.5 nH per mm

(or 0.5 µH per m), which is quite a bit closer to my PCB guesstimates

(4 nH per cm). The dependency from the wire diameter given in that

paper is about ±20 % or so, rather than +100/-50 %.

Still looks a bit strange that the entire resistor (which is some 7

or 8 mm long and usually has a few "turns" on it, in particular for

a hi-resistive one) is supposed to come out with about the same

inductivity as the usual (shortened to a couple of mm) wires.

I completely agree though that this is all peanuts for a frequency of

200 kHz.

With a resitor in the magaohms range, parasitic capacitance is important, but you can neglect the inductance.

In order to get very high resistances on cylindrical forms, it has been common to deposit the film on the form, then cut a spiral grove to make the path length many wraps around the form. This makes the bulk of the resistance long and narrow (both increase R). But it also creates inductiance.

This is, I think, the source of the original question. Part of the problem is understanding what the appropriate model is for this compoent. That is, is the inductance in series or in parallel with the resistive part. It MUST be in series since the DC resistance is still R.

Thus, while there may be substantial inductance, it will be highly damped (that is, very low Q) at such low frequencies. For example, lets imagine that the inductance is 100nh. At 200KHz, this has a reactance of 120 milliohms! So, even if this guess is off by 3 orders of magnitude, the reactance would be only 120 ohms and that is still "zero" compared to megohms.

At such high resistances, stray capacitance CAN be a real problem.

Jim

If the actual value is important to your design, you should consider measuring the inductance with an LCR meter. The handheld LCR/ESR meter I use measures L & C at several frequencies from 100Hz to 100kHz. As an additional feature, it measures ESR useful to characterizing capacitors and inductors as well as 4-wire Kelvin measurements for more accurate measurements of low resistance components.

To the OP, If your question is about a 15 Megohm resistor, then why need such a big value in the essentially audio spectrum? One rule of thumb I was taught is that anything over 1 megohm in audio is unnecessary, I am not saying that the rule is right, just curious as to the need for such a high value if that is your case.

Jim

I tend to agree with the other Jim.

Unless you are measuring femtoamps, a multi-megohm resistor just does not make much sense. If the OP is just trying to get a LOT of voltage gain from an op-amp with a fairly high input R, there are (much) better ways.

Also Jim