ADR421 Vref IC Low Short-Circuit Current!

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ROFLCat
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ADR421 Vref IC Low Short-Circuit Current!

Post by ROFLCat » Mon Sep 07, 2020 12:48 am

Hey guys!

So I got this ADR421BRZ votlage reference IC for one of my projects and I seem to have a problem with it. The accuracy is just fine and I get 2.499V and when divided by a precision voltage divider, it comes out 1.2493V when divided by 2 on my 4 1/2 digit multimeter but, the short-circuit current is nowhere near the stated current in the datasheet. At 5-9V input, I get about 4.5mA out and at 12.9V input, I get 5mA out while the datasheet states that it should be able to do 27mA shorted to ground.

One other weird thing is that it only draws 80uA at 12.9V input and the datasheet says it should be taking more than 390uA. What the...

Anyway, can anyone help me with this? Should I contact the distributor about this and try to get a new one? It was an expensive chip and I have only bought one, so I don't know if they'll replace it or not.

Thank you for your reply! :D

dyarker
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Re: ADR421 Vref IC Low Short-Circuit Current!

Post by dyarker » Mon Sep 07, 2020 2:31 am

The spec sheet I'm looking at
https://www.analog.com/media/en/technic ... 23_425.pdf
says 27mA is typical, not minimum. There is no minimum. So, the IC you got limits to a lower current to protect itself when a short is detected.

A short is an error condition. Why would you want more short circuit current?

Again, quiescent current of 390uA is typical, not minimum; and less is better.

You have your divider and the voltages are good. I see no problem.

Cheers,
Dale Y

ROFLCat
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Re: ADR421 Vref IC Low Short-Circuit Current!

Post by ROFLCat » Mon Sep 07, 2020 3:37 am

You are absolutely correct! I had not noticed the "typical" figure part.
But then again, it says "The ADR42X family of references are guaranteed to deliver load currents to 10mA with an input voltage that ranges from 4.5V to 18V."

I can't even have the output stable with a 1k Ohm load; it quickly drops down to less than 2V from 2.5V.

With my DMM's limited bandwidth of about 50kHz, I'm measuring less than 10uV AC on the 2.5V output with the breadboard fully shielded with aluminum foil and grounded, which crudely suggests that it's not oscillating.

I will be using this as a precision Vref on high impedance circuits which is just fine with this "issue".

Do I need to worry about the chip not being stable or acting up later on?

Thanks!

dyarker
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Re: ADR421 Vref IC Low Short-Circuit Current!

Post by dyarker » Mon Sep 07, 2020 3:13 pm

The spec sheet title is ADR421. The "BRZ" on your parts might mean low power.

A 1k load is a ridiculously heavy load for a precision reference. It is not a power supply regulator.
--------------------------------------------------------
Yes, probably not oscillating. Did you place 2 small capacitors like spec sheet says?

Cheers,
Dale Y

ROFLCat
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Re: ADR421 Vref IC Low Short-Circuit Current!

Post by ROFLCat » Mon Sep 07, 2020 4:42 pm

As I remember, the "BRZ" is only related to the precision of it (0.04%) and not the power consumption?

It is true that it shouldn't be able to do source much current on the output but, it has an op amp in its design and should be self-regulating. It drops regulation even when I put a 23.5k resistor to ground. It's find with a 47k though.

Yes, I have a 0.1uF on the input and another 0.1uF on the output.

dyarker
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Re: ADR421 Vref IC Low Short-Circuit Current!

Post by dyarker » Mon Sep 07, 2020 8:11 pm

Now I missed a line on spec sheet - Load Regulation, Conditions column; which shows 0ma to 10mA.

Where did you get the IC? Discount places sell parts that failed some test but still function.
Dale Y

ROFLCat
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Re: ADR421 Vref IC Low Short-Circuit Current!

Post by ROFLCat » Tue Sep 08, 2020 1:08 am

I got it from a local supplier. It wasn't on a discount or anything. Where I live, Mouser or Digikey and stuff don't ship here.

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haklesup
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Re: ADR421 Vref IC Low Short-Circuit Current!

Post by haklesup » Tue Sep 08, 2020 12:58 pm

Where are you seeing 27mA in the spec. The DS says 10mA output and figure 9 indicates supply current is quite sensitive to operating temperature this the typical spec. If I had one, I could hook it up to a curve tracer and let you test it remotely by zoom meeting (but I don't have that part)

dyarker
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Re: ADR421 Vref IC Low Short-Circuit Current!

Post by dyarker » Tue Sep 08, 2020 2:17 pm

Where are you seeing 27mA in the spec.
Last item in specs for each part number. In this case on page 5 of 24. And resolved subject line problem in post 2.
Dale Y

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Re: ADR421 Vref IC Low Short-Circuit Current!

Post by ROFLCat » Thu Sep 10, 2020 6:00 am

So I submitted a support ticket on my distributor's app/website 3 days ago and still waiting for a response. They reply really fast usually, in a matter of a day.

Meanwhile, I've been mocking around with it using low pass filters, different caps in different places, shielding, and even going on the supply to as high as 15.5V but I still get a max short-circuit current of 5.5mA @ 15.5V.
I have noticed that the output short-circuit current scales with the input voltage, but not much. Vin=5V, output Isc=4.5mA. Vin=15.5V, output Isc=5.5mA.

One other thing I noticed is that in the datasheet, it says that it doesn't need any decoupling caps on the input nor the output. This thing oscillates with and without caps, it's crazy! It only stabilizes when you've got a low pass filter on the output and in my case it's an RC filter with 47k resistor and a 2uF film cap.

I hope they can send me another chip so that I can see of that's faulty too or not.

dyarker
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Re: ADR421 Vref IC Low Short-Circuit Current!

Post by dyarker » Thu Sep 10, 2020 7:39 am

Not getting 10mA (max) into a load is a problem. It looks like IC fails in this respect.

Getting a smaller short circuit current is a good thing. Why are you worried that? When output voltage is forced to zero (by short) the IC probably limits the current so it does not destroy itself.
Dale Y

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