HP34401A, 1mA calibrator and 10mA hidden range

[xardomain]

Hello,
last year I acquired both a HP34401A and a Instek 8251A and not used them much.

This year I got a DMMCheckPlus that would give calibrated VDC, VAC, DCV and ACI.

The values are 5VDC(± 0.007%), 5VAC(±0.1%), 1mA DC(±0.1% ), 1mA AC(±0.2%) plus 100k, 10K, 1K, and 100Ohm precision resistors(0.1% - 10ppm). It came with a paper with actual values checked against a traceable source.

Not a full professional calibrator, just to have a quick check about a DMM health.

Most of the stuff seems "okay" on both DMM:
5VDC
Instek    Hp
4.9991 vs 4.99996

5VAC
Instek    Hp
4.9935 4.99346

1mA DC
Instek    Hp
1.0000 vs 1.00003

except when I test 1mA ACI:
On the 8251A I read 1.0964mA (and 0.37mA with probes unconnected, may need calibration as well)

On the HP34401A instead, I read 0.12mA when unconnected, and basically reading does not change if I connect the 1mA ACI source.

If I use the 1mA DC source, it correctly shows  1.0001mA

I tried to understand IF this DMM could measure below 10mA AC and apparently, it can't (error on 1A range is 0.1%) so nothing wrong here.

According to this, this DMM does have a hidden 10mA range(but not a 100mA one!), so this is another incentive to buy a HPIB card:
https://www.keysight.com/main/editorial.jspx?cc=IT&lc=ita&ckey=1000001267:epsg:faq&nid=-31895.536880933.02&id=1000001267:epsg:faq

Could anybody please confirm it? Which is the lowest AC current I could expect to measure reliably with the HP34401a?

I would like to sell the 8251A, thinking one HP34401A would suffice and was way better...now this weird feeling that the Instek does something the HP doesn't....

TIA

Giuseppe Marullo
IW2JWW - JN45RQ

[coromonadalix]

The only thing i can say,  one of my 34401a went thru a calibration  (the one im selling here)   it was tested down to 10 ma DC, 1 amp AC   and the zero offsets where tested ??


From the Keysight link :


Is there a way to measure low-level ac current using the Keysight 34401A?

Yes. The lowest specified measurement on the 1 A ac current range is 1% of the range, or 10 mA (see footnote 4 in the Data Sheet). However, there is an undocumented 10 mA range on which you can measure down to 100 uA. To enable the 10 mA ac current range, send the following command over the remote interface:

DIAG:POKE 25,0,1

Once enabled, you can access the range from the Agilent 34401A front panel or over the remote interface bus. It will typically perform to the 1 A range specifications. The 10 mA range is tested on the production line.

Why isn't the 10 mA ac current range included as one of the standard ranges for the Agilent 34401A? It is because adding a 100 mA range would have added significant cost to the instrument for a measurement that is infrequently used, and implementing the 10 mA range without the 100 mA range would require the long explanation, that you are currently reading. Therefore the decision was made to document only the 1 A and 3 A ranges.

The increased cost is due to the addition of a precision shunt resistor. The selection of shunt resistors for the rest of the design only required .1Ω and 5Ω. With a 100 mA input, the shunt voltage would be either (.1 * .1) 10 mVAC or (.1*5) .5 VAC. The 5Ω shunt cannot be used because a signal wih a crest factor of 5 would generate a burden voltage of 2.5V, and the front-end protection would clamp the volatage to 2 Volts. This leaves the .1Ω shunt; however, at 10% of full scale (10 mA), this generates only 1 mVAC, which has poor accuracy on the 100 mVAC input.

If you ever want to disable this range the command to do it is:

DIAG:POKE 25,0,0


I think it's self explanatory ?

[bdunham7]

The 34401A is way better, except that like many precision DMMs it is light on current ranges.  I had an 8251A and got rid of it quickly--it has plenty of its own shortcomings.  What you need is to construct a simple low-inductance 1 ohm shunt.  An external, unfused shunt will be better for ACI anyway and will give you the lowest burden voltage.  Just get a dual-banana plug and put a precision non-inductive resistor across it.  And then sell the 8215A.

[HighVoltage]

For AC-I and AC-V, the successor model 34410A and 34411A are much better.
This was always a shortcoming of the 34401A.

[Dr. Frank]

For all ACV and ACI ranges, the input signal must be > 1% of full scale, otherwise the 34401A will clip the reading to zero, by software. See specification, remark [4].
This behavior is due to the limitation of the AD637 TRMS converter, and is therefore not applicable for DCV or DCI.

Therefore, you can't measure 1mA in the 1A range, that is already 0.1% of range, so you need to initiate the 10mA range.

Frank

PS: description of effect is in the web archive:
https://web.archive.org/web/20121104004609/http://www.gellerlabs.com/34401A%20AC%20zero.htm

[coromonadalix]

Thks  Dr Frank

Never saw this link : https://web.archive.org/web/20121104004609/http://www.gellerlabs.com/34401A%20AC%20zero.htm