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From: Robert Baer
X-Mailer: Mozilla 4.75 [en] (Win98; U)
Subject: Re: Opamp GBW tempco ?
References: <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org> <3D8235D8.DD57FF10@earthlink.net> <email@example.com> <firstname.lastname@example.org>
Date: Wed, 18 Sep 2002 08:00:28 GMT
NNTP-Posting-Date: Wed, 18 Sep 2002 01:00:28 PDT
Organization: EarthLink Inc. -- http://www.EarthLink.net
Ian Buckner wrote:
> "fred bartoli" wrote in message
> > Robert,
> > Opamps for my app may not be as silly as it looks at first.
> > My gain stages are 30dB and 10dB, I have 3x30dB+3x10dB switched on
> or off
> > depending on the wanted gain and I'm seeking 0.01dB, 0.01° accuracy
> over the
> > full gain range for the IF-detector block. This is about a 30ppm
> > for the the phase accuracy. Not a trivial task for a discrete
> > stage.
> > Also, the IF section has switched gain stages, which means load
> > (yes they can be made small, but compared to required accuracy...)
> and the
> > input and output impedances of opamps is definitely a plus in this
> > Also the lock-in detector is sensitive to IF harmonic contents. It
> > rejects even orders harmonics but odd orders harmonic are taken into
> > accounts (1/3 for H3, 1/5 for H5...) and the full scale voltage is
> set as
> > high as possible for obvious reasons. That sets the distorsion
> > pretty high for single transistor stages.
> > Also the VNA has to provide gated measurements which means highly
> > operating conditions (IF levels) and asks for the best transfer
> > stability over time.
> > IMHO we are definitely in the modern Opamp domain.
> > For your remark about the 100kHz "which is almost DC", its
> > depends on how accurately you look at this.
> > Just consider the 350MHz (TI's THS4021) opamp I've found, set for a
> > gain.
> > In a non inverter gain block of gain G, an opamp of gain bandwidth
> GBW will
> > give a phase error (when angles are small) of :
> > Alpha = G freq/GBW * 180/Pi
> > This is about half a degree at 100kHz for a 350MHz GBW opamp.
> > Of course I could set the feedback loop to correct that 0.5° error,
> but that
> > will not compensate for GBW variations.
> > Taking the derivative versus GBW gives :
> > d_Alpha / d_GBW = -Alpha/GBW
> > which means that for a best case 20% variation of GBW over the
> > range that will give a 20% variation of Alpha, which is 0.1° (per
> > stage).
> > A 100MHZ GBW will give 0.35° per stage.
> > While it's not bad for a standard IF use (radio, SA), for a
> precision VNA
> > like my app, *only one* gain stage is at least tenfold off from the
> goal for
> > all the IF section.
> > In fact, even a single 100MHz GBW follower will just give a
> variation of
> > 0.01° over the temperature range.
> > Of course I could use active feedback and achieve a near perfect 0
> > error, but for the input stage it's 3db more noise, and for the
> > filters stages the passive tempcos will implies real time phase
> > And one more point in favor of opamps : this is a homebrewing
> project and I
> > don't have access to a precision VNA to make sure the IF perfs are
> OK over
> > temp. Also I need to optimize the oneshoot-prototype-hardware-cost
> and a few
> > opamps are cheaper than a new PCB.
> > It's definitely time for opamps IF block, and furthermore with real
> > calibration.
> > Regards,
> > Fred.
> Hi Fred:
> You might look at, for example, the OPA687 or the AD8351. These have
> GBW products of several GHz. The open loop phase shift for the OPA687
> is only about 35 deg at 100kHz. There are a number of other
For 0.01dB gain accuracy, over temperature would require rather
precision resistors that also have rather low tempcos.
And that translates to expensive wire wound resistors; a specialty
item - i vaguely remember "Kelvin" as a brand name but i could be wrong
and if correct they mey not be in business now (that was 30+ years ago).
As far as phase shift goes, for that kind of accuracy, one might need
trimmers to compensate for strays and have *very* careful layout, and
capacitors will have to have rather low tempcos also.
I think your phase "calculation" based on GBW is in error, as the
phase margin of an opamp VS frequency ain't linear; it tends to be
constant from DC to some value, and rolls "near" the 3dB point (in the
The phase VS freq diagram may have glitches in it (a number of wiggles
up and down) as it tends towards the 90 degree critical point; that is
usually not at the gain=1 point.
Also, it may turn away from perfect 180 degree much earlier than
expected if one attempts to derive the phase margin from the open loop
However, i still stand by my "Bill Gates pocket lint" comment; you
will have more phase problems due to capacitances and *time delay* thru
A single discrete transistor is far superior in that case........
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