Re: 1A5 differential comparator 'sponginess'

Albert Otten

Hi Chris,

This probably is thermal "DC shift due to overdrive". You might follow the checks in Section 6 Step 22. Unlike the 7A13 the 1A5 has no adjustments here. Note that is recovery is checked (also in the 7A13) at return to GND level, not at the switch to top level of the square wave (because there can be no long term drift in zero level of the Type 106, PG506 or the special overdrive test calibration fixture 067-0608-00).
The thermal drift could also be in the mainframe. What happens when you check recovery with an ordinary amplifier plugin in stead of the 1A5?
I don't own a 1A5 so I can't do a test myself.


On Wed, Sep 25, 2019 at 11:14 PM, cmjones01 wrote:

I've just become the proud owner of a 1A5 differential comparator
plugin for my 535A scope. It's a wonderful thing and its only real
fault was that the variable volts/cm pot was made of that awful
crumbly Tek plastic and had fallen apart. With that temporarily
shorted out while undergoing repair, the rest works really well after
a dose of DeoxIT D5.

However, it has one odd behaviour which I can't explain. The whole
point of the 1A5 is being able to work with signals much larger than
the scope's screen. However, if I feed this one a large positive
signal, the trace drifts upwards somewhat over a period several
seconds, then drifts back down slowly after that signal is removed.
The same happens the other way with a large negative signal. It
happens on both A and B inputs, and also when applying/removing a
large comparison voltage.

For example, I have it running right now set to 0.1V/cm. With a signal
of anything up to about 1V square wave from the calibrator, the
'ground' level of the trace stays constant, subject to the usual
long-term drift I'd expect. However, if I apply a 10V square wave,
still with the plugin set to 0.1V/cm, the 'ground' level wanders
slowly up about 0.8 of a division. It takes several seconds to do
this. If I turn the calibrator back down to 0.5V, the trace wanders
slowly back down. The signal is well within the plugin's rated
common-mode range.

Can anyone explain this? Are they all like that? My 7A13s, which
presumably have a similar architecture, don't suffer from it.

I'm wondering if there's some leakage somewhere which is charging
something up slowly, when given a large offset input signal.


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