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ECL open input default state

The MC100EP142 has "default LOW" inputs with internal 75k pull-down resistors to VEE. Default HIGH inputs also have internal 36.5k pull-ups to VCC which bias them between VIH (min) and VIH (max) but the default LOW (=VEE) is way outside the "operational" VIL range. One might infer it's OK to bias inputs down at VEE, provided they're static; but I wish the datasheet was more explicit on this!

I'm not confident about leaving inputs open. I would rather tie them off to VEE, if that's OK. In the case of inputs defaulting high, where an application requires a permanent high state on the input, and a suitable low impedance source is not conveniently available to drive it externally, I would be tempted to at least add an external decoupling capacitor to help noise immunity.

Reply to
Andrew Holme
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Yup, ECL is happy with inputs at Vee. Most are also happy with inputs at Vcc (strapped high) but some older parts aren't.

That's OK, but it doesn't matter. You'd need an insane amount of noise to pull a pin from Vee up to the logic threshold. I've never seen it happen.

In the case of inputs defaulting high, where an

That case has less noise margin, but I doubt you'd have trouble there either. The '142 datasheet says that the diff input common mode range includes Vcc, so you could just pull it high.

The '142 differential inputs have one pin pulled down to Vee and the other weakly held at about -1.6 in a 5-volt ECL circuit. That's not technically proper "ECL" levels, but it's a solid diff input given the common-mode range of the input circuit. Base currents are pretty high in ECL.

John

Reply to
John Larkin

Strapping ECL inputs at VCC saturates that side of the current switch. If the input is on the inverting side of the switch it can affect the output level, as well. It's better to strap them a diode down from VCC.

Reply to
krw

As I said, some older parts do that. Most newer ECL is happy with a commmon-mode input range that includes Vcc. The datasheets usually specify.

There's usually a Vbb available somewhere, so there's no reason to risk it.

John

Reply to
John Larkin

The '142 has a lot of single-ended inputs. Can these be tied to VCC?

Thanks.

Reply to
Andrew Holme

Looking at the datasheet, pulling single-ended inputs to Vcc is not clearly guaranteed safe. You could ping OnSemi support and ask.

People sometimes generate an "ECL high" buss, a junction drop or so below Vcc, as a hard-high rail, or use some otherwise unused ECL gate section to make a stuck-high rail as the general pullup.

John

Reply to
John Larkin

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