HP3478A- Few Questions  - Page 1
Forum section: electronics / testgear

author: eevblogfan
author_rank: Frequent Contributor
timestamp: February 21, 2013, 03:07:11 pm
content: hello eevblogersplease excuse me if that's the wrong section ... I wasn't sure where Shall I open that threadit's been a year or so since I bought my 3478 .. and the internal Jumper was on the 100/120 ( Can't remember )so I moved it to the 220 one ... and that's it .. though I've read here somewhere that something like +10VAC on the "higher " end of the voltage rating , is ok for the short term , though bad for the long term of the device/transformer ,the Humm is much Quite nowHence today , after seeing Dave's video ( #427 ) . I just opened my meter and did few things :1) Checked the Battery voltage ( see the pic's below )2) Changed the Jumper position to 240V AC3) Measured the LM399's voltage ( Right on the 2 pins ) the voltage reading ( Via my 287 & rel'd ) was 6.885V ( see in the pic's below .. I compared the 287 and the 3478A , NOTE: It's the same rel as with the LM399 testing , Ie . I didn't turn the 287 of or so .. )Now , Dave mentioned that there should be two transformers right there ( instead of the 2 opto's )and that exactly What my unit haveand I think I saw Op07 near by the LM399 ( in Dave's unit) ... though mine have some other op amp .. ( any idea ? )and as per the battery voltage ... does the voltage of mine is Okay ?BTW .. the last pic shoes the ACmV of the E3610 , Does That measurement correct ? IE ... I don't (yet) have an scope ( I think analog is the recommended here (?) ) ... so Just as A reference ... Does 0.5mVRMS is the correct value ?thank you in advance

author: eevblogfan
author_rank: Frequent Contributor
timestamp: February 21, 2013, 03:08:49 pm
content: I have 6 pic's ( 5 more ) and each is 1.7/8MB ...the limit is 3MBFixed

author: c4757p
author_rank: Super Contributor
timestamp: February 21, 2013, 03:58:22 pm
content: Quote from: eevblogfan on February 21, 2013, 03:07:11 pmMany things could interfere with that reading, like noise pickup in the test leads, background noise in the meter, zero offset in the RMS converter, and the meter's AC bandwidth (though the 3478A is pretty high, at 300 kHz). I wouldn't trust an AC V measurement that low without a more "proper" measurement setup.

author: eevblogfan
author_rank: Frequent Contributor
timestamp: February 21, 2013, 04:12:30 pm
content: heythank youI've noticed that When the mV R.M.S in 1mV~2mV or higher ( Ie .. Higher then 0.5mV ) the circuit is unstable/ have 10% error or so ... ( CC load )When I use proper lm336-5 (5V low noise zener ) as my reference in the CC load , I then gets 0.05% accuracy ( the 0.05% error is the 100mOhm resistor's Tempco ... )before I got the LM336-5 ... the reference was simply the 7805Which generated like 1 or 2 mV R.M.S ( the reading on my 3478A .. )Hence I Thought I shall ask for that .. Because the E3610A spece'd for lower then 1mV or so ... ( Am I wrong ?)sorry for not being clearI ment to ask : If the reading of the 3478A was below 1mV R.M.S (E3610A out put ) doesn't it mean that I can trust that whenever I read below 1Mv it truly is under 1mV ?thank you in advance

author: c4757p
author_rank: Super Contributor
timestamp: February 21, 2013, 04:24:58 pm
content: Er... I'm not sure I understand the question.

author: robrenz
author_rank: Super Contributor
timestamp: February 21, 2013, 07:38:52 pm
content: The 3478 specs for AC are for a sine wave only and greater than 10% of full scale which would be >30mV the accuracy below that is unspecified. The noise on the power supply output is most definitely not a pure sine wave. A scope with a differential amplifier is needed to do a proper noise measurement. The proper setup is shownherelook at the PARD measurement section in this HP power supply handbook.

author: eevblogfan
author_rank: Frequent Contributor
timestamp: February 21, 2013, 08:12:08 pm
content: hey robrenz:)I wasn't intended to do a "proper" noise measurement ... as I said ... I noticed that when the 3478A read lower then 1mV .... the instability of the circuit ( referring to the CC load ) is no longer exist ...And hence asked ( sort of " can it be " ? )any how ... Can some one please reply on the Questions presented on my very first post ?

author: codeboy2k
author_rank: Super Contributor
timestamp: February 22, 2013, 07:07:05 pm
content: and the answer is "sort of" "maybe", "I really don't know, in fact"Without a proper noise measurement of the problem source and the good source, you don't know. You say changing the reference to a good zener reference helped your circuit, OK fine. And you measured your reference with the AC mv range on the 3478 and see less than 1mV. OK fine. But robrenz already said that the 3478 is not specified below 30mV and a true sine wave input, and noise is never a sine wave and you're trying to measure something less than 1mV.So to answer your question, can you trust the 3478? no.If you really want to know, then you must do a proper noise measurement as robrenz said, and thoughtfully gave you a link, and pointers.As for your original post, the battery is likely to read a steady voltage until the end of it's life, and then it drops off. At which point you must change it with the power on, so as not to lose the cal settings. I can't tell what the battery chemistry is, it seems low for a lithium cell, maybe it's NiMh or NiCd or something else.If you bought it used, and are unsure of the battery, I would just change it. As a used device, it was likely sitting around unplugged for a while waiting to sell, so you can't really know the date it was installed or how much life is left. Why not just change it out and be sure?the different op amp is probably nothing to worry about. HP probably just used a different opamp occasionally, depending on available supply.