ADC current draw for TM4C123GE6PMI

Hey guys thanks a lot for taking the time to reply!

cb1_mobile - that is exactly why I posted. I want to make an analog power plane and ferritebead or inductively couple it to my main 3.3V plan that is generated by a buck regulator. I am really trying to figure out how to connect VDDA to this new analog 3.3V plane and how much bulk capacitance to put on the Tiva's VDDA pin so when I kick off recording at max speed I don't starve it and create a dip or any transient issues.

This information was helpful. Thank you Amit and cb1!

Thank you - I'm wondering if your "power plane" might subject you to (more) induced noise - especially if the size of the plane encroaches upon digital switching areas and/or other current carrying areas of your board?

Firm/I have visited clients who, "sweated the details"of such plane - pronounced it "fine" (under "light/restricted test operation") and then found their "plane approach" to prove, "Not so much" when power & high speed switching circuits were @ "full blast."

Short, direct, adequate VDDA pcb trace widths & "close-in" filter components may warrant your consideration...

Hello Robbie,

A reference supply IC like REF5030 is a good candidate. I have used it in one of my designs and it has excellent stability over temp and time. It must be placed close (500 mils) from the device and a bypass capacitor between the output and the GND plane, close to the device will help achieve the performance.

Regards
Amit

What about the REF3033 instead? My target voltage is 3.3 and it looks like the REF50xx doesn't produce that voltage. The 3033 also has low quiescent current and still supports 20mA output current?

If it is the case that this regulator is a good idea would you recommend putting ferrite beads and or an LC filter in series with either the input or output of the part. I kind of feel like ferrite beads at the input would be a good idea as to prevent HF noise from getting into the LDO.

cb1 - are you hinting at less of a copper plane and more so copper pours to support adequate but not over adequate copper area for the analog voltage may be better?

Hello Robbie,

If you are certain that the REF3033 can be the best fit for the application then it should be OK. I would recommend a Capacitor and proximity to the device. It should be good enough.

A common ferrite for the main and the reference voltage generator should be good.

Regards
Amit

Ok. So if I'm understanding you correctly I can put a ferrite bead in series with BOTH the input to the REF3033 and output of the REF3033. Yea, I think that device would work well. All it has to power is the ADC and ~10 op amps with very low quiescent currents.

I also feel like it would be smart to put some bulk capacitance on the analog plane now that it is being fed by the REF3033. Does this seem like a good idea?

A VDDA plane which passes close to high speed digital and/or power switching circuitry may not prove as "noise-free" as you seek. Now - if you are going to "burden" that plane w/ ~10 op amps that plane is (necessarily) expanded in size AND in current delivering demand.

Based upon the power appetite of your 10 op amps - and earlier (safety) recommendation - 20mA output current may not prove adequate...

Ok I understand what you're saying now and I agree. Fortunately I have partitioned my digital stuff off pretty well and there aren't any fast digital signals around my analog circuits. At least, I feel like I have a pretty good idea of that not being well versed in this analog/digital analysis yet.

I also see what you're saying about the amps. I was looking at quiescent current, but that won't represent operational current draws. I need to figure out how to roughly estimate how much current everything on this new analog power plane will pull. I'm thinking that since the amps all drive inputs to other op amps and the last one drives the ADC it wouldn't be too much, but there are two stages that apply a gain of up to 10x.

So just to double check (because it seems like an almost too good to be true number) the max current the ADC will pull through VDDA is 2.5mA? Even running full speed? I know there could be other things pulling current, but if the max current through VDDA is 2.5mA then by default that limits the worst case ADC current draw to 2.5mA?

I don't think so!

The 2.5mA is the spec for the MCU's ADC, alone. You must ADD the individual current requirements of each/every (other) device which is powered from VDDA! You may reduce that current by employing (equal performing) "dual or quad" op-amps. (at the minimum - board layout will reduce/simplify)

I am in favor of a safety factor in powering your MCU's ADC - AND all other analog powered devices... This area is not one where you should, "cost-shave!"

Hello Robbie

I would suggest ferrite bead on the input and capacitor on the output. A 1 uF and 0.1uF capacitor should be able to provide both bulk and instantaneous current source.

Regards
Amit

So do people generally use multiple REF3033 or REF5030 devices in a design? For example, I may have one power my ADC, another one power my amps and another one power my sensor. I was thinking you would want all of your analog stuff (amps, ADC, sensor etc..) powered from the same reference. It just seems more consistent to me.

It seems like it is really a design decision between having multiple voltage references or a large LDO that is a little less accurate, but would allow you to put all of your analog components on the same reference.

Is one of these better than the other?

If I may - one properly filtered and closely located voltage source (dedicated to your MCU's VDDA) has proved best for my small firm. As this critical trace (or plane) expands & "wanders" you are increasing your exposure to "unpleasantness" - are you not?

Hello Robbie,

As I mentioned, for the requirements on the ADC controller I have used REF5030. Based on what the range is, I have seen most customers use the VDDA as the reference

Regards
Amit