Speeding optoisolators

Yawnnnn... :O Now..2.1Ghz RF coupling digital isolators are interesting.. Tprop < 10nS

D from BC

How about this?

If the transient led current is large but the dc current is low, the transistors will switch fast but not saturate too hard. Of course, b-e resistors always help, too.

John

isolation

Hey, 100kHz, isolated, for pennies and 2 parts is a decent upgrade for a dogmeat junk-box opto.

All I needed was 20kHz, so 'good enough' is good enough, right?

Besides, using transformers is cheating. :-)

Cheers, James Arthur

Cute. I might rig one up, just for fun.

I s'pose the next step would be doing clever anti-sat stuff to the phototransistor, if we had access to its base (not available on the MOCD217 (which I have a reel of (my junk box is well-stocked))).

Cheers, James Arthur

isolation

I'm wondering if LED capacitance is significant...

D from BC

I once made an inverter like that, using 2N4401/03. Nice and sharp edges (in the 40ns range IIRC), gotta be careful not to leave the input disconnected, in an ambiguous logic state, though! ;-)

Tim

-- Deep Fryer: A very philosophical monk. Website @

Well, to go really fast you'd want a phototransistor (or better yet photodiode) to work in pure current mode, into a TIA.

This uses a silicon (for 850 nm) or InGaAs (for longer stuff) pin diode detector driving a current-mode opamp as the tia. Analog bw is about 200 MHz, which would be faster if I'd bothered to put more voltage across the pin detector. I should have done that.

All non-avalanche photodiodes tend to have a sensitivity in the ballpark of 0.5 amps/watt. If you illuminate them from an led, they have to be big and slow to capture the fuzzy light. If you use a laser, through fiber maybe, they can be a lot smaller and faster.

John

Sure, especially c-b (Miller) capacitance. And there's nothing to discharge the base here except base current itself.

John

isolation

Hmmm. Let's see: Fairchild's datasheet says 18pF, x 1.5k is about a

Storage time in the phototransistor is the main limitation, so optimizing its drive & load is a good first step. Next would be preventing all transistors (photo, especially, and Q1) from saturating.

John's totem-pole actively jerks the phototransistors out of saturation, so that'd be a nice speed-up that's not too critical about exact photocurrent and load--just rip the things 'off'. It takes two opto channels and bipolar drive for their LEDs, but it's brutally elegant for speed.

All of this is pretty simple and low-power, which is nice.

Cheers, James Arthur

'The J730 is compatible with the Highland J720 E/O Conveter,'

My spell sjeker says it is ConveRter.

It's amazing how hard it is to get every last technical and spelling/punctuation error out of datasheets and web pages. I sure hope this one hasn't cost us a lot in sales.

John

This will smoke eventually. The opto current is not limited.

Much better idea however it takes two optocouplers instead of one and will smoke if the input is unconnected. BTW I wouldn't be concerned about the speed of the LEDs compared to that of the photo transistor.

Once we had a problem with decoupling of RS-232 via H11L1. There was a big difference in the speed (several times) depending on the particular make and the particular part. The board was already made; the trivial solution was just pick the fastest parts from the box :)

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

Yep. That's the reason for Q21, which serves as a kind of fast TIA on the cheap.

Photodiode-mode would be much faster, eliminating phototransistor storage time, but my opto doesn't offer up the base. If it did, substituting the back-biased photojunction and adjusting R23 should speed things up a lot.

Nice, as usual for HTI gear.

James

Not so. The opto's current transfer ratio (CTR) limits the phototransistor's current. Since the LED current is about 2mA, the output current is about the same, and the opto's phototransistor will dissipate 10mW, typical. The detector in the specified opto can safely dissipate 150mW, which would be quite impossible given the combination of limited LED current, CTR of roughly 100%, and 5v supply.

No, it won't smoke for the same reason as above. Remember, these are being run at especially low drive currents, which changes the game. In this case, two LED drops plus two limiting resistors set the LEDs' current to much less than half if the input is open. The two output transistors can easily take it.

Agreed, at least at this level of performance.

That's one way, for sure!

Cheers, James Arthur

I feel suspiciously about the circuits with the potentially unlimited escalation of the current. The datasheet guarantees the minimal CTR but it says nothing about the maximal.

Yes, it won't smoke at once, but it will smoke eventually.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

"Brutally elegant"?

Sounds a little creepy. ;-)

Cheers! Rich

I'm available to do proofreading - I'm the self-appointed chief of the Apostrophe Police, after all. ;-)

And, of course, at times I branch out to the Spelling Police (I used to win contests), Grammar Police, and "Does-it-make-any-sense" police. ;-)

Cheers! Rich

My maxim is a twist on Murphy... if there's ANY chance of smoke, no matter how minute, there WILL be smoke.

...Jim Thompson

Minimum CTR at 10mA is 100%, typical is 130%. From the datasheet(*), at 2mA LED current CTR is typically 75% for an output current would be about 1.5mA. 30mA would be the maximum allowed, dissipation-wise.

(*)

Those spreads look pretty tight to me, so I'm perfectly comfortable with my 2,000% safety margin, but if you aren't, you could add a small collector resistor to the opto. I wouldn't, but you could.

Let's do the math. With input floating, if John used 1.5k resistors, the LED current would be from 4v - (2 led drops) passing through 2 x

Cheers, James Arthur