Current Source Confusion
I’m about ready to call the BMOW video system design officially complete, and hopefully I’ll be posting specs soon. I’ve run into one last snag, however. The UM70C171 palette chip requires a reference current to calibrate the DAC output. The official datasheet shows a few examples of how to do this, and I’m following the example that uses a LM334 current source in a temperature-compensated configuration shown on page 3-109. Their formula for how to choose the resistor values to get the desired current doesn’t seem to make sense, though, and doesn’t agree with the LM334 datasheet. Anyone see where I might be misinterpreting things? John Honniball mentioned that he had a copy of the IMS G171 datasheet, which should be pin-compatible. Does it say anything different?
The UM70C171 datasheet says I need a 4.44mA reference current for a singly-terminated output, and that the relationship of the resistor R1 to the current is IREF = 33.85mV / R1. Solving for R1 yields about 7.5 ohms, which is the value they show in the accompanying diagram. Also shown in the diagram is a second R2 resistor with a value of 75 ohms, which isn’t mentioned in the text.
In contrast, the LM334 datasheet on page 5 shows an example of using it in the same temperature-compensated configuration, with different results. It says the ratio of R2 to R1 should be 10:1, and IREF = 0.134V / R1. Solving for R1 yields R1 = about 30 ohms and R2 = 300 ohms. That’s a substantial difference from what the UM70C171 datasheet says.
The coefficient in the formula from the LM334 datasheet is almost precisely 4 times the coefficient from the UM70C171 datasheet, which I’m sure is significant somehow. I feel like I must be missing something obvious.
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Well… A jfet could be used. Not temp compensated though… Or a bjt config.
I’d go with the UM datasheet first though. Or test obth, and see what actual measurable current you get with each.
Good to hear video is pretty much done. It’d be neat if you had a great place to show off this project.
Hi!
As it happens regarding the formulas for the LM334 problem, you are correct. The one that the datasheet gives is indeed correct.
I’ve used it many times on everything imaginable.
The LM334 datasheet is correct? Or the UM datasheet example that uses the LM334? I have a hard time believing either is wrong, and I keep looking for some detail that’s different between their two example circuits that might explain why they derived different formulas.
I guess I’ll just have to try different resistor values and see what happens experimentally, but I hate not understanding things like this.
Indeed.
The datasheet from NSC is correct. And the user who wrote the descriptions for the palette chip may have thought he was wiring it correctly. And for his use it worked.
Which worked for you?
Study the typical use schematic that NSC provides and the formula for it, and match it against the the schematic used by the palette chip. It (the LM334) will work with precision 1Ohm resistor all the way to the physical limit of resistors for that size. The big problem is how the user understands the datasheet. Which is why I’ve put down all of this here.
I’ve read the LM334 datasheet and it makes sense to me, and leads me to believe I should be using 30 and 300 ohm resistors to get 4.44ma, temperature-compensated with a diode as shown in their example. I’ve got the actual parts on order, so I’ll try it and see when it arrives.