Sampling resistor for current value

[Zhong Pan]

Hello,

I am designing a PCB circuit that gives specific current values to a
device. Unfortunately after the D/A converter I MUST use a voltage
amplifier for some reason, so I lose the information how much current
I am sending.

Previously I would just put a 1-ohm sampling resistor in series in
the middle of the trace and measure its voltage. But I am dealing
with high speed this time - although the frequency on the trace
is below 10MHz, I need to keep a very sharp rising edge that may
require GHz response.

My question: Will the action of putting a series resistor significantly
influence the high-frequency response? If yes, what do people do
in this situation?

Thanks!

Zhong

 

[Robert Baer]

Hello,

I am designing a PCB circuit that gives specific current values to a
device. Unfortunately after the D/A converter I MUST use a voltage
amplifier for some reason, so I lose the information how much current
I am sending.

Previously I would just put a 1-ohm sampling resistor in series in
the middle of the trace and measure its voltage. But I am dealing
with high speed this time - although the frequency on the trace
is below 10MHz, I need to keep a very sharp rising edge that may
require GHz response.

My question: Will the action of putting a series resistor significantly
influence the high-frequency response? If yes, what do people do
in this situation?

Thanks!

Zhong

First, the resistor must be non-inductive; second, if you want GHz
response you must pay close attention to the capacitance across the
resistor ans to other elements *and* most important, make it a part of a
(matched) transmission line.

 

[Al Yeager]

I recomend that you check out application note 908 'Ultra-Low Resistance
Surface mount Shunts' on the dallas/maxim site. You may want to consider a
current transformer instead if you really need that kind of response.

al