How to receive 200mV signal, convert to 3.3V levels

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Jepael
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Posted by Jepael: Tue. Jul 31, 2012 - 12:50 PM
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Damn datasheet errors hit me again. A chip claims to output LVTTL level output for a received signal, but in reality the signal output AC level is same as received signal, riding on a VCC/2 bias.

So I need to convert a 200mV-600mV AC signal, riding on a 1.65VDC bias (or independent of that), to 3.3V LVTTL. Signal is around 3Mbps biphase, or in other words, very much like SPDIF. Source impedance unknown.

1) use RS422/RS485 receiver, good for 10MBps but most are 5V chips. Proper solution but expensive? Any 3.3V chips you could suggest?

2) use a hack with 74HCU04 or 74HC04 as linear amplifier. Hack but cheap, what if no signal is input, will it fry the chip as current flows through the input FETs as Vin=Vout? Which one? HC1G chips? Should I put a schmitt trigger after HC/HCU? Separate HCU04 and HC14, or single LVC1GX04 which contains both? Oh wait it has only buffers, not schmitt trigger?

3) Comparator? Too many extra components?

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Learning and Information, General Electronics
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meslomp
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Posted by meslomp: Wed. Aug 1, 2012 - 05:39 AM
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rough schematic..

signal goes to series cap to kill the DC offset.
other side of the cap goes to new level DC offset.
behind that is a comparator made with an opamp (not comparator) Opamp has feedback resistor to create hysteresis.
other pin of opamp has fixed DC setting such that signal is clipped to one or other side of supply.
then you would get a square wave out.
one amp 5 resistors and 2 caps(one with amp supply)

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jayjay1974
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Posted by jayjay1974: Wed. Aug 1, 2012 - 12:42 PM
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You need a bloody fast opamp; the input signal is 3MHz , squarish I guess.

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Jepael
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Posted by Jepael: Wed. Aug 1, 2012 - 04:43 PM
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I think it would require a really fast op-amp, meaning so expensive that there must be a single-chip solution that would directly solve this like those RS422/RS485 receivers.

Plus I would most likely need an external schmitt trigger after op-amp so the signal slew rate is fast enough for logic chips.

However, as comparators mostly have open-collector outputs, I think I still need a Schmitt trigger to get good rise time, even if I used a comparator.

Even though I am a bit worried how HCU04 will survive, every datasheet in the world shows examples how it can be used as a linear amplifier, and the quiescent current is within limits for supply pins. At 3.3V, when Vout=Vin, it should consume about 10mA quiescent current, or waste 33mW as heat. However I need to receive two separate signals, so I don't think it is a good idea to use single 74HCU04 for two signals because of current consumption and possible crosstalk. So I could use four HC1GU04 gates, but that could be too many components. Ah those could be two HC1GU04 driving two HC1G14 schmitts maybe, plus I would get non-inverted signal out.

So if no 3.3V RS422/RS485 receivers come around cheaply, I think flooding the board with HC1G chips would work with slightly worse power consumption.

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