AD584J: strange behavior - Page 1
Forum section: electronics / general

author: siealex
author_rank: Regular Contributor
timestamp: August 13, 2023, 11:37:59 pm
content: Recently I purchased a reference module with AD584J from Aliexpress. 2.5 V, 5 V, 7.5 V work perfectly, but 10 V behaves strangely. If I set the jumper to 2.5 V, then switch it immediately to 10 V, the output voltage drops down to 2.1..2.2 V, then slowly (within 10..30 seconds) crawls up to 4..5 V and then jumps to 10.00 V (screenshot 1, 5 s/div!).To get 10 V instantly, I need to set 5 V (or 7.5 V), then switch to 10 V (screenshot 2, also 5 s/div). Is it normal for this chip? The module is powered off a 12 V 0.25 A transformer (not switching) power supply.

author: Andreas
author_rank: Super Contributor
timestamp: August 14, 2023, 05:02:38 am
content: Quote from: siealex on August 13, 2023, 11:37:59 pmHello,Stabilized 12 V or unregulated?The data sheet says nominal 15 V. (minimum VOut + 2.5V)with best regardsAndreas

author: siealex
author_rank: Regular Contributor
timestamp: August 14, 2023, 08:27:05 am
content: Stabilized, in fact slightly higher than 12 V (~12.5 V).

author: magic
author_rank: Super Contributor
timestamp: August 14, 2023, 05:25:07 pm
content: Is the board fitted with a noise reduction capacitor between CAP and VBG? Or an unusually large output capacitor? Maybe something is wrong with those caps?Anything weird at the STROBE pin?I don't have this chip to test. But...sudden removal of the 10V-2.5V jumper inserts the internal /4 divider into the feedback path and causes 2.5V tap voltage to dip below 1V if output is currently at 2.5V. This may have an effect of reducing bias on Q3,Q4 and slowing down the regulation loop, which may perhaps take a moment to recover, particularly if the CAP node is abnormally loaded by a defective capacitor, board leakage or whatever.

author: siealex
author_rank: Regular Contributor
timestamp: August 14, 2023, 09:36:33 pm
content: The board is fitted with three capacitors: one electrolytic 470 μF 25 V at the input (connected to the input socket via a 1N4007), one 0.1 μF connected to the first one in parallel and another 0.1 μF connected to pins 6 and 7. The strobe pin (5) is not connected anywhere.

author: siealex
author_rank: Regular Contributor
timestamp: August 14, 2023, 09:41:33 pm
content: QuoteYes, the output voltage dips for a very short time, but then returns to 2.5 V and slowly (20 seconds!) crawls up to approx. 4 V, then suddenly jumps to 10 V.Quote20 seconds? And why does it jump to 10 V immediately if I touch (even slightly) the 5 V or 7.5 V pins with the jumper?

author: iMo
author_rank: Super Contributor
timestamp: August 15, 2023, 05:20:24 am
content: QuoteThe question is what chip you have really got..

author: magic
author_rank: Super Contributor
timestamp: August 15, 2023, 09:32:23 am
content: Quote from: iMo on August 15, 2023, 05:20:24 amI doubt there are any pin compatible substitutes.Maybe if those boards start to sell in millions on AliExpress somebody will make a clone in ChinaQuote from: siealex on August 14, 2023, 09:41:33 pmNot the output voltage but the 2.5V pin voltage. It dips below 1V, at which point Q5 can't feed base current into Q3,Q4 at all. It frankly looks like the regulation loop completely shuts down and only base current of the Q13,Q14 darlington charges the noise filter capacitor. This may take a while.Quote from: siealex on August 14, 2023, 09:41:33 pmDoing so increases 2.5V pin voltage.It also applies a positive voltage step to VBG, which is AC coupled into CAP, which is maybe enough positive feedback to unlock the chip. Note that the output is simply the CAP voltage buffered by a few emitter followers.Try without the CAP-BG capacitor. You probably don't care about noise above 100Hz anyway.

author: iMo
author_rank: Super Contributor
timestamp: August 15, 2023, 09:53:27 am
content: Also doublecheck the pcb has been cleanup , sometimes they do not clean the pcb and the residual resin/flux between the pads may cause parasitic leakages..

author: TizianoHV
author_rank: Regular Contributor
timestamp: August 15, 2023, 01:34:30 pm
content: Could be an issue with one of the caps values:Seehttps://www.markhennessy.co.uk/ad584_references/(Unit #2):"But note the capacitor between pins 6 and 7. The datasheet advises a maximum value here of 100n. This needs a closer look...""I changed the capacitor from 330n to 100n. This shortened the time the unit spent drifting, but it didn't reduce the occurance of the slow drift. So I changed the capacitor to 10n, and this meant that the "drift period" was reduced to less than half a second."

author: siealex
author_rank: Regular Contributor
timestamp: August 15, 2023, 11:10:02 pm
content: QuoteOn my board it's 100n. Tomorrow I'll try to find a smaller one.

author: siealex
author_rank: Regular Contributor
timestamp: August 15, 2023, 11:13:49 pm
content: QuoteMarkings on the chip:▶️ AD584JH9338△The chip is in a metal 8-pin circular case.

author: magic
author_rank: Super Contributor
timestamp: August 16, 2023, 12:52:50 pm
content: For checking DMMs this capacitor isn't necessary or useful at all. A smaller value is even more useless than a larger one, as shown in the datasheet.Sufficiently bad leakage current from those pins will affect accuracy, but I don't know how bad is bad enough.I have previously had bad experience with MAX6350 and X7R noise filtering capacitorMarkings can be erased and changed. But if it quacks like AD584 it should be AD584; besides Maxim's second source there seem to be no other chips with exactly the same function. Probably harvested from e-waste.

author: siealex
author_rank: Regular Contributor
timestamp: August 19, 2023, 07:44:48 am
content: Replacing that capacitor by a 10 nF almost solved the issue, now the switch time is less than 0.5 s.