Jan Fredriksson | 14 Apr 19:46 2014

Re: volt-nuts Digest, Vol 56, Issue 9

It was the April 1989 HP journal that made me post the question. The
article makes really good reading about the core of the 3458. It also made
me think about how one could implement the AD with the components available
today and bench instruments. It should not take that many parts to make a
single voltage range, moderate speed, single shot AD using a bench clock /
counter / timer. Just for the learning.

But about the switches there is not much in that article, just the
paragraph quoted by TH, "A custom chip design.." etc.

The article is otherwise seems like a very good starting point for learning
multislope ADs. It seems like it would almost be possible to set up a
spreadsheet with the data given.

I noted that they use a 390pF integration cap which made me wonder what
kind of switches where used, as any FET capacitance / charge would have to
be compensated / cancelled / nulled somehow.
Jan Fredriksson | 14 Apr 05:36 2014

Switches in integrating ADC

What kind of switches are used in integrating ADC, ie to switch
between voltage sources (ref and external) and to switch in multisloping
resistors? FETs?
frank.stellmach | 11 Apr 11:53 2014

34401A Why 10M ohm default i/p resistance?

In the manual (!), HP reasons the 10M standard input resistance:
"Normally, the multimeter’s input resistance is fixed at 10 MΩ for all dc voltage ranges to minimize
noise pickup." 
 I explained that to myself like this: AC stray fields or noisy high impedance sources induce noise
input currents in the DMM frontend.
The higher its input Z, the higher the noise voltage reading will be.
In that sense, 10MOhm 'shorts' those noise effects.

Anyhow, my 3458A is always programmed to have TOhm input Z as power up state, therefore I have to short the
input jacks, whenever the instrument is not connected, or doing an ACAL. That avoids drifting of the
input and unwanted relays actuations, when left open.

On the 34401A, it is very uncomfortable to set the input to high Z every time, as many keypresses are needed.

So agilent recognized that dilemma, and improved the handling on the new 34461A:
Changing input Z is only one additional keypress, as this feature is assigned to one of the soft keys.

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(Continue reading)

starbook | 5 Apr 04:11 2014

Datron 1281 Selcal Multiplier

These error codes 2292, 2282, 2272, 2262, 2252, 2216, and 2214 all use 
the Selfcal Multiplier circuit.
I have all the error codes listed above, when doing a INT SRCE CAL.
Output at M609 pin-2  is not a dc level, therefore M609 pin-15 is a 
sawtooth waveform.
Output on M609 pin-2 is a 10% duty on time square wave with a positive 
All IC's are functional
What is the problem with this selfcal multiplier circuit?
Tony Greene | 2 Apr 04:13 2014


I have been troubleshooting a HP3456A voltmeter.  

Its giving a stange fault, but I beleive I have nailed it down.  The unit is reading +10.00000 volts input at
+9.99661 volts.  If you reverse the input, it reads correct at -10.00000 volts. All other ranges read
correct on either polarity.

What I need is a source for the FET  A20 Q109 or a suitable substitute.  Its HP part number 1855-0298, it was made
originally by USA mfg 02883 - Siliconix, Inc, Santa Clara, CA.  It crosses to NSN 5961-01-135-1324.  And
Vishay has bought them and the part number is now obselete.

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Sam Reaves | 31 Mar 16:04 2014

3455A ROMS

I have a HP3455A that by all appearances has lost its cookies perhaps due
to bad ROMS. Does anyone have these rom images and a procedure to
replace/refresh them?

Jan Fredriksson | 29 Mar 15:57 2014

Re: volt-nuts Digest, Vol 55, Issue 23

Yes, LM399 good good! Me like!

The dozen LM399 have now spent a day at an average 0.0uV deviation, +/-0.2u
peak to peak probably below 0.1ppm rms (10s averaged), despite the room
temperature changing by something like 5 degrees (C). Condition; Bench
supply 12V, 12 IC paralleled, current limited by 2K7 to each IC, output via
2K7 from each.

I got my LM399 from ebay at $4.99 each. Seller polida2008. (I'm in no way
associated with the seller). Buying ICs on Ebay is risky but these where
Jan Fredriksson | 29 Mar 10:47 2014

(no subject)

I was looking at a few different alternatives for a transportable,
non-permanently powered on, DIY, voltage reference and I picked the LM399
as one of the candidates.

The LM399 is a buried zener reference with built-in temperature regulation
and an integrated thermal insulation cover. It is very simple to
implerment, only one resistor needed.

The downsides:
- it has a nominal drift in the order of 10ppm/sqrt1000h (as good as any
except the LTZ1000)
- Its a bit sensitive to input volgage, due to a 1ohm input impedance.
- Its a bit noisy, about 10uVptp.
- Output voltage tolerance is poor and at around 6.9V

I set one of these up and, after a day of stabilizing, found the noise
about as stated in the datasheet. However, noise in the datasheet is
specified for 10Hz and up. The real problem to was at lower
frequency. Overnight, the 10s averaged values where slowly drifting around
about 10uV ptp.

Now I made twelve LM399 parallelled on a simple breadboard, running at
1.8mA each, plus heating, a total of around 200mA.

Like the first, I use simple 2K7 1% metal film resistors for current
limiting at 12V supply. The resistors do not need to be very high-spec
as errors are attenuated by a factor 1:2700 (1R/2K7). There is another 2K7
resistor per zener for output averaging.

This board, after a few hours stabilizing, measures 0.2uVptp, 10s averaged,
(Continue reading)

acbern | 29 Mar 09:30 2014

Keithley 182-M

starbook | 28 Mar 16:47 2014

Datron 1281 A_D CORRN ERROR 2008

What does A-D CORRN error 2008 , (no other error codes listed ) mean to 
the A_D?
How to troubleshoot this error?

Richard Moore | 28 Mar 06:32 2014

Re: 3458A RAM batteries

My 3458A tested with marginal RAM when I sent it to Loveland for cal. 
Gary advised that I buy the replacement RAM board which uses RAM chips 
that have replaceable batteries (? could they be super caps?) that snap 
onto the tops of the chips. They sent the meter back to me with no 
charge other than for shipping. I bought the upgrade board and new cable 
which is needed because the new board is shorter than the old style, and 
the old cable won't reach. Then I sent it back to Loveland for cal. I 
think the RAM board was around $450 -- old memory in failing RAM chips 
in head.... bad batteries there too.

Dick Moore