Replacement A9 boards for the HP 5065A
Well I hunted up the change pages for the A9.
The cap is polycarbonate, the original A9 was polyester.
If I can't find the polycarbonate I'll look into the PPS.
Cheers,
Corby
The key properties relevant to long-TC integrator capacitors are
insulation resistance (leakage) and dielectric absorption (DA). (One
response on this thread opined that DA is not important in this circuit,
but that is not correct. DA is most definitely a factor in long-TC
integrators, including this one.) If maintaining an absolute time
constant is important, then tempco enters the picture (but the absolute
time constant of the A9 integrator is not a critical parameter).
Consider polypropylene (PP) capacitors as the baseline for long-TC
integrator applications.
Polycarbonate (PC) capacitors are very scarce these days, because the
sole source of PC capacitor film discontinued it over 15 years ago.
Even if they were available, the advantages of PC over PP in this
application are small.
PPS has a lower tempco than PP, and may have lower leakage (depending on
construction and processing). They are generally not so easy to find,
particularly in larger values, but Robert already directed you to the
Kemet SMR series, which are available in suitable values and are
conveniently packaged.
Polystyrene (PS) is the best dielectric for long-TC integrators except
for Teflon (PTFE), but PS caps are extremely hard to find in large
values, are generally huge compared to PP and PPS, are easily damaged
during soldering, and are not conveniently packaged.
PTFE is the best dielectric for long-TC integrators, but PTFE caps are
expensive and huge, and they are unnecessary for this application.
Your best choices are PP and PPS. The DA of PP and PPS are similar. The
leakage depends as much on the construction and processing of particular
caps as on the bulk resistivities of PP and PPS, so look at the
datasheets carefully to see what is guaranteed by the manufacturers.
Best regards,
Charles
On 2/24/2018 8:40 PM, cdelect@juno.com wrote:
Well I hunted up the change pages for the A9.
The cap is polycarbonate, the original A9 was polyester.
If I can't find the polycarbonate I'll look into the PPS.
Cheers,
Corby
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Returning to the topic of what op-amp to use:
Bert asked if the LT1793 is a good choice. I suggested the LT1012 as
superior in the specific parameters that will provide best performance
in the HP circuit
(https://www.febo.com/pipermail/time-nuts/2018-February/108964.html).
Attila suggested the LTC6240HV.
I continue to believe that the LT1012 is the best choice (or,
alternatively, the similar OP97 -- but the LT1012 would be my first choice).
As Attila noted, the 6240 is limited to +/- 5v supplies. I do not know
if the output voltage of the A9 ass'y needs to be able to swing beyond
+/- 5v, but I would not be inclined to change this without a lot of
careful thought (and adding another op-amp does not excite me). But
really, that is not necessary. Even if the 6240 could supply outputs up
to +/- 15v, the LT1012 would still be a better choice.
Given the relatively low resistances at the op-amp inputs (10k ohms),
the ultra-low input "bias" (leakage) current of the 6240 is simply
unnecessary. Any offset due to the input currents (within the general
range of any of these op-amps) is insignificant compared to the op-amp's
offset voltage. Thus, offset voltage, offset voltage tempco, and offset
voltage long-tem drift are the critical parameters (as Poul-Henning
pointed out). And here, the 1012 is clearly the best of the three. In
addition to having the lowest input offset spec, the 1012 has guaranteed
maximum specifications for these important parameters. The 6240 (for
good reason) is not even rated for long-term stability (drift).
(Long-term offset stability is a particular weakness of CMOS op-amps.)
Finally, as I noted before, the 1012 has an overcompensation pin that
may be useful to improve the damping and the wideband noise of the
integrator. (This would, of course, require a small amount of design
work to place overcompensation pole(s) and possibly zero(es) to best
advantage, but may provide significantly improved performance.)
Of all the op-amps I know and use, the LT1012 would be my first choice
for the A9 integrator board.
Attila also advocated using the "much cheaper and easier to solder TPS7A49
[and] TPS7A3001" voltage regulators, as opposed to the LT3042 and a
negative-regulator-to-be-named-later that I mentioned. I concur with
his suggestion.
Best regards,
Charles
On Sat, 24 Feb 2018 23:44:16 -0500
Charles Steinmetz csteinmetz@yandex.com wrote:
Given the relatively low resistances at the op-amp inputs (10k ohms),
the ultra-low input "bias" (leakage) current of the 6240 is simply
unnecessary. Any offset due to the input currents (within the general
range of any of these op-amps) is insignificant compared to the op-amp's
offset voltage. Thus, offset voltage, offset voltage tempco, and offset
voltage long-tem drift are the critical parameters (as Poul-Henning
pointed out). And here, the 1012 is clearly the best of the three. In
addition to having the lowest input offset spec, the 1012 has guaranteed
maximum specifications for these important parameters. The 6240 (for
good reason) is not even rated for long-term stability (drift).
(Long-term offset stability is a particular weakness of CMOS op-amps.)
Oh.. right, I didn't think about long term behaviour. Thanks for the correction!
BTW: How about using an LTC2057 then? Its input bias current and
GBW spec is similar to the LT1012, but its offset voltage and drift
are far superior. Or would its charge injection noise be too large
for this application?
Attila Kinali
--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson
We have decided to go with the 1012 do we need to do any thing with pin 5Bert kehren
Sent from my Galaxy Tab® A
-------- Original message --------From: Attila Kinali attila@kinali.ch Date: 2/26/18 9:53 AM (GMT-05:00) To: Discussion of precise time and frequency measurement time-nuts@febo.com Subject: Re: [time-nuts] Replacement A9 boards for the HP 5065A
On Sat, 24 Feb 2018 23:44:16 -0500
Charles Steinmetz csteinmetz@yandex.com wrote:
Given the relatively low resistances at the op-amp inputs (10k ohms),
the ultra-low input "bias" (leakage) current of the 6240 is simply
unnecessary. Any offset due to the input currents (within the general
range of any of these op-amps) is insignificant compared to the op-amp's
offset voltage. Thus, offset voltage, offset voltage tempco, and offset
voltage long-tem drift are the critical parameters (as Poul-Henning
pointed out). And here, the 1012 is clearly the best of the three. In
addition to having the lowest input offset spec, the 1012 has guaranteed
maximum specifications for these important parameters. The 6240 (for
good reason) is not even rated for long-term stability (drift).
(Long-term offset stability is a particular weakness of CMOS op-amps.)
Oh.. right, I didn't think about long term behaviour. Thanks for the correction!
BTW: How about using an LTC2057 then? Its input bias current and
GBW spec is similar to the LT1012, but its offset voltage and drift
are far superior. Or would its charge injection noise be too large
for this application?
Attila Kinali
--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson
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Moin,
On Sat, 24 Feb 2018 23:44:16 -0500
Charles Steinmetz csteinmetz@yandex.com wrote:
Returning to the topic of what op-amp to use:
Bert asked if the LT1793 is a good choice. I suggested the LT1012 as
superior in the specific parameters that will provide best performance
in the HP circuit
(https://www.febo.com/pipermail/time-nuts/2018-February/108964.html).
Attila suggested the LTC6240HV.
I continue to believe that the LT1012 is the best choice (or,
alternatively, the similar OP97 -- but the LT1012 would be my first choice).
As Attila noted, the 6240 is limited to +/- 5v supplies. I do not know
if the output voltage of the A9 ass'y needs to be able to swing beyond
+/- 5v, but I would not be inclined to change this without a lot of
careful thought (and adding another op-amp does not excite me). But
really, that is not necessary. Even if the 6240 could supply outputs up
to +/- 15v, the LT1012 would still be a better choice.
Coming back to an old thread...
While looking for a suitable opamp for another application, I stumbled
across the ADA4077. It is basically the modern equivalent of the LT1012.
All but the input bias current and input voltage range specs are better:
(worst case where available, typical else)
LT1012 ADA4077 (B grade)
Offset voltage 120µV 65µV
Offset tempco 1.5µV/°C 0.25µV/°C
Offset longterm 0.4µV/month 0.5µV/10kh (13months!)
Bias current 0.2nA 1.5nA
Noise voltage density 14nV/√Hz 7nV/√Hz
Low Freq Noise 0.5Vpp 0.25Vpp
GBW 0.8MHz 3.8MHz
Slew rate 0.2V/µs 1.2V/µs
The SO-8 packages are pin compatible, beside the missing offset compensation.
Unless you are using a chopper opamp for drift compensation, the ADA4077
might be worth considering. The higher GBW and slew rate would be benefitial
in an integrator application like the A9 board, as it improves linearity.
Attila Kinali
--
<JaberWorky> The bad part of Zurich is where the degenerates
throw DARK chocolate at you.