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Voltage compliance of the programmable current source with REF200

I am designing a low current source (decades to hundreds of microamps) and I found Burr-Brown's REF200 satisfying. I read BB's application bulletin AB-061 "IMPLEMENTATION AND APPLICATIONS OF CURRENT SOURCES AND CURRENT RECEIVERS", in which Fig. 14 shows a programmable current source consisting of two resistors, an REF200 and an external op amp. It says in the text, "Because compliance of the 100uA current source is 2.5V, the current source, Figure 14, can only COMPLY WITHIN 2.5V OF THE NEGATIVE RAIL-even if the op amp can go further." This sentence is above my comprehension. I read the datasheet of REF200 but cannot yet find any clue. As far as I know, the so-called "voltage compliance" refers to the valid range of the load terminal potential. It seems to me that the load terminal potential has to be at least 2.5V HIGHER than the negative rail, which contradicts the original text. Any instruction or information is welcome.

Reply to
Jiaqi
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bottom of page five, left ahnd column.

You are right - the sentence is nonsense. What the author presumably meant to say is that the current source will only deliver the rated

100uA for loads presenting a voltage at least 2.5V above the negative rail,

This is determined by the performance of the REF200 - if you look at page three of the data sheet

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there a two currves for "currrent source - output current versus voltage". The one on the right shows the performance in the range 0V to 5V.

Application notes are written by fallible human beings. Even Jim Williams at Linear Technology is not always 100% right - though he comes pretty close.

--
Bill Sloman, Nijmegen
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bill.sloman

Thank you very much. I agree that LT's Jim Williams is a great man. I enjoy several technical documents written by him. Regards, Jiaqi Shen

Reply to
Jiaqi

Me too. As for the REF200, it's an interesting part, but note TI's detailed information about its status in the "Pricing/Packaging/CAD Design Tools/Samples" and "Inventory" sections of the REF200's web page,

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Here you learn the miniDIP package version has been discontinued. This happened long ago, and only the soic-8 package is still offered. That's unusual and interesting information for a start. You also learn there are NO factory REF200 parts in stock and NO parts in progress, which would also be unusual for an "active status" part. Finally you learn that anyone ordering enough parts to trigger a factory production run (I wonder how many that is) would have to wait 20 weeks.

TI says Arrow Southern Europe, whatever that is, has 291 parts in stock, but I wasn't able to track them down.

The REF200, Jiaqi, is basically dead. Too hard to make, with it's laser trimming, too impractical with it's three separate laser-trimmed circuits, and too easy to replace with cheaper alternatives. R.I.P.

So, as an aspiring engineer, what you learn is that in "designing a low current source (decades to hundreds of microamps)" you should NOT find Burr-Brown's REF200 "satisfying," but rather simply note the concepts they present, and seek other ways to solve your problem.

For example, you might consider the technique shown in AB165, figures 15A and 15B. This techniques uses a voltage reference with an adjustable resistor, which makes more practical sense than using a current-source reference with an adjustable resistor.

BTW, where did you find the AB-061 app note? I found your circuit figure 14, and the same description, in AB165, a 30-page app note.

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--
 Thanks,
    - Win
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Winfield Hill

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