Hi guys I have a question for all.
I've seen quite a few schematics of DACs around the web and I have stumbled upon something which I do not understand.
1. Some people connect the I2S lines from source to DAC, directly.
2. Others use utilise "damping resistors" in series along the lines.
For case 2, most use resistor values such as 22R or 33r but there are also individuals who use values as high as 1K.
So my question is... Are these damping resistor necessary?
If so, how do we decide on the values without a scope?
I've seen quite a few schematics of DACs around the web and I have stumbled upon something which I do not understand.
1. Some people connect the I2S lines from source to DAC, directly.
2. Others use utilise "damping resistors" in series along the lines.
For case 2, most use resistor values such as 22R or 33r but there are also individuals who use values as high as 1K.
So my question is... Are these damping resistor necessary?
If so, how do we decide on the values without a scope?
series termination
they are for matching the characteristic impedance of the transmitter to the transmission line. (PCB trace) basically just damping, as you say.
Many times, if the trace is very short (< wavelength) then any reflections and such will cause no real harm to the signal. If the trace is long, there will almost certainly be reflections if the transmitter is not well matched to the line. (causing all sorts of bad signal quality)
If you know the impedance of the trace, and the output impedance of the driver, you can make up the difference with this padding resistor. However, without a scope you would have no way to verify - so it's best if you can use a scope.
22R to 33R is fairly typical. Most traces and drivers are setup somewhere around 50 ohms give or take. Making the resistor a little bigger than absolutely needed will just slow the edges of the signal a bit more (hence the typical of 33R or so)
1k just seems silly unless the driver was very, very strong. (or extra attenuation was needed for whatever reason) Normally a 1k or 10k might be used as a pull-up on an open drain output - don't know that I've ever seen one as a series termination...
they are for matching the characteristic impedance of the transmitter to the transmission line. (PCB trace) basically just damping, as you say.
Many times, if the trace is very short (< wavelength) then any reflections and such will cause no real harm to the signal. If the trace is long, there will almost certainly be reflections if the transmitter is not well matched to the line. (causing all sorts of bad signal quality)
If you know the impedance of the trace, and the output impedance of the driver, you can make up the difference with this padding resistor. However, without a scope you would have no way to verify - so it's best if you can use a scope.
22R to 33R is fairly typical. Most traces and drivers are setup somewhere around 50 ohms give or take. Making the resistor a little bigger than absolutely needed will just slow the edges of the signal a bit more (hence the typical of 33R or so)
1k just seems silly unless the driver was very, very strong. (or extra attenuation was needed for whatever reason) Normally a 1k or 10k might be used as a pull-up on an open drain output - don't know that I've ever seen one as a series termination...
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