RE: Differential voltage probes 1

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Wall Richard Wayne by way of Terry Fritz <teslalist-at-qwest-dot-net>" <rwall-at-ix-dot-netcom-dot-com>

Terry,

Excellent project.  Maybe later you can adapt it to measure differential
electrostatic TC fields. They're quite a bit different than computer
simulated ES fields.

I have designed and built dozens of these solid state differential
amplifiers over the past several years.  I have a few suggestions.

	1.	While the TL082 j-fet op amp is a venerable old veteran and the over
riding goal is to keep it simple, there are several later generations of
cheap op amps that will produce better and more accurate differential
amplifiers.  In your TL082 configuration there are several problems that
need to be addressed.  On these j-fet high input impedance op amps it's
almost impossible to make the battery +/- supply rails discharge evenly.
The batteries will discharge at different rates.  This makes a big
difference in biasing the op amp.  Even at a small 0.5 volt supply
difference there is often a high input bias.  A rock solid "virtual ground"
at zero volts has to be established.  This will get rid of this annoying
supply bias problem.  The TL2426 "Rail Splitter" precision virtual ground
can be obtained from Digikey for ~$1.50.  Alternately, you can build a
virtual ground rail splitter with a single op amp.  It's just an op amp
follower with about 100 ma output.  Put a 100k pot across the +/- rails.
The pot wiper goes to the non-inverting input and tie the output back to
the inverting input.  That's it.  Adjusting the pot voltage divider sets
the output virtual ground.  Usually we want it to be 50-50 of the total
rail supply.  Since the pot voltage divider is a set ratio, the voltage of
the virtual ground always remains at this set ratio - even as the batteries
discharge.

(While it's not good practice, I have found that if there is a lot of
supply bias, a common voltage signal can be applied to both the
differential inputs and the virtual ground pot can be adjusted a little so
the virtual ground output is zero volts.  The virtual ground won't quite be
50-50, but it sure gets rid of the supply voltage bias.)

	2. While TL082 input impedance is 10^12 ohms, you've essentially shunted
its impedance to ground at 1k.  It's unlikely though that the non-inverting
input impedance is the same as the 1k inverting input impedance.  Due to
output impedance, transorb and output measuring device, the non-inverting
impedance may differ greatly from the 1 k inverting impedance.  This
likewise introduces input bias.  A single differential op amp design isn't
the best.  The solution is to have two high input impedances from two
individual op amps as separate front ends.  These two drive a third
differential op amp.  If you prefer TL j-fets I suggest TL084 (quad) op
amps.  Another advantage wound be to choose another op amp with built in
shunting diodes across the inputs.  I haven't done it, but maybe simply
putting a transorb across the inputs would work.  This would depend on the
impedance of the nonconducting transorb though.  I have done quite a bit of
work with mosfet op amps in the past and for the most part they seem a
little easier to use than the j-fet versions.   There are a lot of high
impedance differential precision instrument op amps with most of these
features built in.  Digikey has them for ~$5.00, give or take.  Finally, I
have been using chopper op amps almost exclusively for a year now.  They
are cheap, accurate and do away with almost all the biasing problems
associated with high input impedance op amps.


RWW	


 > Hi Richard,
 >
 > Such a probe would be for measuring things like gate drive or capacitor
 > voltages...  Since the input impedance is fairly low, "fragile" signals
 > would get loaded down.  Finn just does not want to float the whole scope
 > (anymore ;-)), buy a $1000 probe, or mess with the two probe A - B
channel
 > thing (gets odd on digital scopes anyway).  SS coils have a lot of say
600
 > volt things that float in reference to ground.  This probe could go
across
 > a 600 volt cap with the voltage at say 300 and 900 volts above
 > ground.  That is actually pretty messy to do, but a differential probe
 > makes such things easy and the signals stay cleaner if ground is not
involved.
 >