sidiy said:
Hi,
I have never implemented that circuit before, but I saw it in the demo board schematic, so unless there is an error in the schematic, I assume it works ok.
I Actually do think the impedance of the circuit is 75 ohms, at least at the working frequency.
Maybe someone more familiar with this input buffer can chime in.
Cheers!
Russ
Hi Dustin,
Indeed we'd be in deep trouble if that type of circuit would not work. Every CDP needs a data slicer afterall (this because despite using NRZI coding, targeted to eliminate the DC content, the RF signal from the laser diode still 'rides' on a varying level of DC)
I drawed quickly two alternatives. Just a note on the bias through R2 or R7: the voltage drop across them, mainly determined by R3 and respectively R8, must be sensibly higher than the hysterezis of the comparator.
I'm using the second one in one of my DACs, inspired from the demo-board of the PMD100 digital filter.
Hope it helps, cheers!
Indeed we'd be in deep trouble if that type of circuit would not work. Every CDP needs a data slicer afterall
(this because despite using NRZI coding, targeted to eliminate the DC content, the RF signal from the laser diode still 'rides' on a varying level of DC)I drawed quickly two alternatives. Just a note on the bias through R2 or R7: the voltage drop across them, mainly determined by R3 and respectively R8, must be sensibly higher than the hysterezis of the comparator.
I'm using the second one in one of my DACs, inspired from the demo-board of the PMD100 digital filter.
Hope it helps, cheers!
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sidiy said:
No, I'm sorry, I disagree with your statement. (grin)
The circuit didn't reflect back to the source. That is why it worked.
The term (+ something (high) to ground) is a high impedance in parallel with the 75 ohm.
The incoming AC coupled RF signal doesn't really care about it.
RF signals don't care about 'ground' in general. That is why they are so hard to conceptualize. They will leave the circuit board and travel through the air at a whim.
The terminating impedance was close enough. +/- 10% is usually close enough.
I wouldn't have done it that way, but it was good enough and it worked.
You can't argue with that.
It looks like you can order some directly from Maxim.
or:
http://www.newark.com/68K9424/semic...ATED-PRODUCTS-MAX3280EAUK-T&_requestid=118439
Newark has 16000 in stock!
or:
http://www.newark.com/68K9424/semic...ATED-PRODUCTS-MAX3280EAUK-T&_requestid=118439
Newark has 16000 in stock!
The coupling cap was required in the comparator circuit because the comparator action operated at 1/2 VCC.
Where is the SPDIF coming from, inside the box or outside?
The MAX part has a -7V to +12V Common-Mode range.
The SPDIF + doesn't actually need a coupling cap and the SPDIF - on the cable doesn't need to be tied to ground. UNLESS it will violate the common mode range.
If there is a chance of the common mode range being violated, then an input signal transformer would solve the problem.
Where is the SPDIF coming from, inside the box or outside?
The MAX part has a -7V to +12V Common-Mode range.
The SPDIF + doesn't actually need a coupling cap and the SPDIF - on the cable doesn't need to be tied to ground. UNLESS it will violate the common mode range.
If there is a chance of the common mode range being violated, then an input signal transformer would solve the problem.
Well my goal is to make a module that will be flexible. So I don't want to assume that SPDIF is coming from inside or outside. Or even that it is TTL level or 500mv.
Signal transformer is certainly an option, but raises costs, but it does have the benefit of galvanic isolation.
mulling it over.
Signal transformer is certainly an option, but raises costs, but it does have the benefit of galvanic isolation.
mulling it over.