Measure opamp output Z

[Hammy]

For anyone who's read my previous post you know that I'm using an
MCP600x for an EA in an LDO. I first tried to do it in spice but the
FET model and the EA aren't really designed for LDO's.I'm now doing it
in MathCAD but I cant calculate DC gain as well as a pole location
without knowing Ro.

The MCP600x datasheet doesn't have output Z versus frequency graph so
I'm looking for a way to measure the output Z of the MCP600X.

I've found two ways to measure Ro.

This is from Apex

http://i37.tinypic.com/2r5xhn9.png

They also suggest applying a sinusoidal signal to the input and
measuring the rms current while sweeping the input frequency. Are
there any other alternatives for measuring Zo or estimating Zo from
the datasheet?

MCP600X Datasheet

http://www.datasheetcatalog.org/datasheet2/c/0gdg4l76zrrof3l7wxk1rk532qfy.pdf

LDO Showing Cpar (effective input capacitance),and Rpar EA output Ro.

http://i36.tinypic.com/snz0qp.png

DC gain of LDO.

http://i37.tinypic.com/2zs01p5.png

 

[John Larkin]

On Wed, 03 Sep 2008 14:36:00 GMT, Hammy <spamme@hotmail.com> wrote:

For anyone who's read my previous post you know that I'm using an
MCP600x for an EA in an LDO. I first tried to do it in spice but the
FET model and the EA aren't really designed for LDO's.I'm now doing it
in MathCAD but I cant calculate DC gain as well as a pole location
without knowing Ro.

The MCP600x datasheet doesn't have output Z versus frequency graph so
I'm looking for a way to measure the output Z of the MCP600X.

I've found two ways to measure Ro.

This is from Apex

http://i37.tinypic.com/2r5xhn9.png

They also suggest applying a sinusoidal signal to the input and
measuring the rms current while sweeping the input frequency. Are
there any other alternatives for measuring Zo or estimating Zo from
the datasheet?

MCP600X Datasheet

http://www.datasheetcatalog.org/datasheet2/c/0gdg4l76zrrof3l7wxk1rk532qfy.pdf

LDO Showing Cpar (effective input capacitance),and Rpar EA output Ro.

http://i36.tinypic.com/snz0qp.png

DC gain of LDO.

http://i37.tinypic.com/2zs01p5.png

If you're still driving the gate of a mosfet, through a series
resistor, inside a slow loop, the amp's Zout doesn't matter.

Spice should work fine here. Scribbling a Bode plot on a piece of
paper should work too.

John

 

[Hammy]

On Wed, 03 Sep 2008 08:19:54 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

If you're still driving the gate of a mosfet, through a series
resistor, inside a slow loop, the amp's Zout doesn't matter.

Spice should work fine here. Scribbling a Bode plot on a piece of
paper should work too.

John

Thanks John