At what frequency does one start impedance matching?

[Nemo]

I have a purely analog circuit operating at up to 1MHz, though it is not
inconceivable that at some point in the future it will be modified to
work at 10MHz. [Just future proofing here - I already saw one project
where Marketing asked if they could stick a 0 on the end of the spec.
This is better than some places I've worked where they didn't ask!]

Signal levels are in the mV region at the start of the circuit, rising
to a couple of volts at the output.

I intend using stripline to impedance-match the output to the cable it
drives. My question is, for the earlier stages, is it better to try and
use series resistors and impedance-match the tracks between stages, or
simply to use big fat tracks for minimal inductance? The IC's will be
about an inch apart (much less if I can manage it). The IC's involved
have significantly higher GBW's than the signal.

Thank you,

 

[Uwe Hercksen]

I have a purely analog circuit operating at up to 1MHz, though it is not
inconceivable that at some point in the future it will be modified to
work at 10MHz. [Just future proofing here - I already saw one project
where Marketing asked if they could stick a 0 on the end of the spec.
This is better than some places I've worked where they didn't ask!]

Signal levels are in the mV region at the start of the circuit, rising
to a couple of volts at the output.

I intend using stripline to impedance-match the output to the cable it
drives. My question is, for the earlier stages, is it better to try and
use series resistors and impedance-match the tracks between stages, or
simply to use big fat tracks for minimal inductance? The IC's will be
about an inch apart (much less if I can manage it). The IC's involved
have significantly higher GBW's than the signal.

Hello,

if impedance matching is necessary depends not only on the frequency, it
depends on the length of the cable measured in wavelength at the given
frequency.
If the cable is very short compared to the quarter of a wavelength on
the cable at the given frequency, the matching ist not necessary.

Example: at 1 MHz, the vacuum wavelength is 300 m, the wavelength on the
cable is about 200 m, a quarter is 50 m, a cable shorter than about 5 m
should be no problem without impedance matching. At 10 MHz, the
quarter wavelength is only 5 m, the cable should be shorter than 0.5 m.
The same is true for striplines instead of cables.

Bye

 

[BillyGates]

I have a purely analog circuit operating at up to 1MHz, though it is
not inconceivable that at some point in the future it will be
modified to work at 10MHz. [Just future proofing here - I already saw
one project where Marketing asked if they could stick a 0 on the end
of the spec. This is better than some places I've worked where they
didn't ask!]
Signal levels are in the mV region at the start of the circuit, rising
to a couple of volts at the output.

I intend using stripline to impedance-match the output to the cable it
drives. My question is, for the earlier stages, is it better to try
and use series resistors and impedance-match the tracks between
stages, or simply to use big fat tracks for minimal inductance? The
IC's will be about an inch apart (much less if I can manage it). The
IC's involved have significantly higher GBW's than the signal.

Thank you,

There is a formula, L > Tr/2Td where Tr is the signal's rise time and Td is
the propagation delay(depends on materials).

Td ~= 1.5ns/ft for some pcb mats.

 

[Nemo]

Thank you everyone, those were very clear and useful answers.

 

[Nemo]

You need to provide a lot more information in order to get a satisfactory

answer.

In the rf world, devices can be damaged and/or needed power can be lost.
In the digital world, data setup and hold time can be diminished and clock
rise/fall edge fidelity can be compromised.
In the high fidelity audio world, it doesn't matter because nobody can hear
the difference.

What happens in your circuit if a certain percentage of the signal gets
reflected back and forth?

I think it's going to be more like the hi fi audio world. The driving
IC's can source /sink about 50mA, allegedly, the signal levels are low
and frequencies well under the ICs' GBW limits. The important thing is,
the other answers have given me confidence that the best FIRST approach
is wide, low-inductance tracks, rather than try to control their
impedance, and keep the IC's as close as possible. It'll work well
enough for the circuit to get going, and I can probe around and add
series resistors if it experiences unexplained problems, and re lay out
the PCB if that makes a measurable improvement.