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From: John Larkin on 20 Mar 2010 16:24
On Sat, 20 Mar 2010 19:58:24 -0000, "Andrew Holme" <ah(a)nospam.com>
wrote:

>
>"John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message
>news:r81aq51jobb2athvc55u5eqijb752r4p6f(a)4ax.com...
>> On Sat, 20 Mar 2010 15:02:55 -0000, "Andrew Holme" <ah(a)nospam.com>
>> wrote:
>>
>>>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!
>>
>> Yup, ECL is happy with inputs at Vee. Most are also happy with inputs
>> at Vcc (strapped high) but some older parts aren't.
>>
>>>
>>>I'm not confident about leaving inputs open. I would rather tie them off
>>>to
>>>VEE, if that's OK.
>>
>> 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
>>>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.
>>
>> 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
>>
>>
>
>The '142 has a lot of single-ended inputs. Can these be tied to VCC?
>
>Thanks.
>

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

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