# FW: [SI-LIST] : Output Impedance

Zenklusen, Fred ([email protected])
Wed, 1 Apr 1998 13:46:14 -0800

George,
I like your method but I believe your formulas are slightly =
incorrect.
Shouldn't it be?

Rdrv(rise) =3D Rline(Vfinal/Vstep-1)
Rdrv(rfall) =3D Rline(Vfinal/Vstep-1)

Fred

George wrote:

> Alla,
>=20
> Kevin's method will measure dc source resistance of the driver.
> AC source impedance can be measured by connecting a long ideal t-line
> (eg 25ns) and fairly llow Zo (40 ohms works well, higher if driver =
has
> very hi Z)) to the output of the driver, and applying an edge signal
> to
> the input of the driver. The size of the step (plateau) obtained for
> positive and negative edges can be used to determine driver source
> impedance:
>=20
> Rdrv(rise) =3D Rline(Vfinal/(Vstep-1))
>=20
> Rdrv(rfall) =3D Rline(Vfinal/(Vstep-1))
>=20
> (This method was recommended by Hyperlynx for use with their .mod
> generation but is equaly applicable to any source R measurement)
> =20
> George
> ----------
> From: Kevin Weldon [SMTP:[email protected]]
> Sent: Wednesday, April 01, 1998 12:59 PM
> To: alaa alani
> Cc: [email protected]
> Subject: Re: [SI-LIST] : Output Impedance
>=20
> Hello Alaa,=20
>=20
> =A0 What I do is connect the input node of the output buffer=20
> to Vcc.=A0 Then I=A0connect the output node to Vcc through=20
> a resistor.=A0 Then I simply change the value of the resistor=20
> until I measure Vcc/2 at the output node of the buffer. The=20
> value of the resistor at this point is equal to the output impedance=20
> of the buffer.=A0 HSpice's Goal Optimization works well for this.=20
> =A0=A0=A0 Is there a simpler way to do this?=20
>=20
> =A0--=20
> Kevin Weldon=20
> Bay Networks=20
> 3 Federal St. BL3-02=20
> Billerica, MA 01821=20
>=20
> alaa alani wrote:=20
>=20
> Hi,=20
>=20
> What is the simplest way to measure the output impedance of a
> buffer=20
> given just a spice netlist?=20
>=20
> Thanks,=20
>=20
> Alaa Alani=20
> LSI Logic
>=20
> =A0
>=20