Basics...offset triming, and mercury batteries

[PRR]

> I've got two JRL standard cell oven(s) coming to my doorstep

Waitasec... those geeks didn't puck around. You get THREE cells per oven!

You don't have the "man with two watches" problem.

You may be cornering the market on Weston cells.

 

[Freq Band]

I posted this on the HP Agilent yahoo group, but if anyone here wants to offer me a solution....????
(If I find the answer, I'll post it here...if not, consider it unsolved.)

I'm trying a Vref + an alkaline battery, to replace a mercury battery
in a Julie Research Labs potentiometer.
I think I can get away with 1.25v, instead of 1.34v of the original
merc. battery.

http://i5.photobucket.com/albums/y177/Midiot/my_merc_replacement.gif

But I have run into a quandary.....
On my bench, my little circuit works fine with a 100k resistor load,
but in the box, a 100k load drops the 1.25v.... to 0.9 volts.
This voltage is to be sent into a KVD (100k ohm impedance).
Here are two diagrams from the manual showing how the battery is used...
(R7 is the KVD)
http://i5.photobucket.com/albums/y177/Midiot/DSCN3397.jpg
http://i5.photobucket.com/albums/y177/Midiot/JRL_mercury_trim.jpg

I've looked, and measured, and tried different parts, but I cannot
figure this out.
(I get 0.9v at the output of the potentiometer box with either hooking
it up to the KVD, or just a 100k resistor across it's terminals. No
load attached, 1.245v....).

Like an Oregon forest, I'm stumped.
=Randy=

LT1634a, 1.25v
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1002,C1223,P1611,D3209

From top down...
Null Meter
Potentiometer
KVD voltage divider (2ppm).
Bottom unit is a Guildline current source voltmeter, JRL's competitor.

Larger pic... http://i5.photobucket.com/albums/y177/Midiot/DSCN3417-1.jpg

=FB=

 

[Freq Band]

Solved.
Figured it out my self, and it was easy, of coarse.

0.9xxx volts is exactly what I want to see there !! ....adjusting the trim knobs until I reach exactly 1.00000v.

Back at the battery, (before the trim resistors) , sure enough...1.25v.

=FB=

 

[PRR]

I happened across this site: new old-time potentiometers, galvanometers, standards (as well as new digital stuff). In India... so they are maintaining the best British traditions, which were very fine indeed.

http://www.vaiseshika.com/
http://www.vaiseshika.com/RES_DC-POT.php

 

[Freq Band]

Back to my original question about trimming the offset of an "already in-circuit" opamp.....

[quote author="PRR"]
To offset trim your OP77: first bring the inputs up into their working range. Easiest may be to use >+15V power, apply a 9V battery to the + input, strap - input to output, then bridge a floating meter from the battery to the output. Trim for zero. [/quote]

Is this what you are describing? (although different opamp is pictured here)

Are all connections applied directly to the "in circuit" opamp pins (so input/output caps hare not included in the trim measurement ?) ?
Should input and feedback resistors be excluded, or included??

I have tried this on a opamp that was "out of circuit", and it appeared to work as shown above, with no resistors.

=FB=

 

[Samuel Groner]

Didn't read the entire thread but why do you start with a OPA604 for a precision application..? Trimming offset works as shown, but the precision is bound to be benign because of the meter limitation and thermal EMFs; and then this opamp will have significant drift. Better run the opamp at high noise gain for trimming--or use a decent part which you don't need to trim and offers low drift...

Samuel

 

[Freq Band]

Right.
I am not using the OPA604 for my voltage reference application.
Although I use them in other, audio-related circuits. So I was wondering if this method would work for any offset-trimmable opamp, in any application.....and whether to include the gain resistors in the trim adjust measurement.

^^^^^^^^^^^^^^^^^

I am working on other solutions for the volt reference circuit.

~~~~~~~~~~~~~~~
(side note)

As a challenge, I built a precision 1 volt ref.
So far, within a 10 degree F ambient temperature range, in an unshielded plastic box, It has stayed at exactly 1.00000 v for over a week !!

LT1052ai
LM399AH-50
5 ppm TC (or less) resistors


=FB=