MAX3232 200mV noise - Page 1
Forum section: electronics / projects

author: David Chamberlain
author_rank: Frequent Contributor
timestamp: June 07, 2018, 05:20:42 am
content: I have a PCB with a MAX3232IPW and am getting what looks like the charge pump ramp on both DOUT1 and DOUT2 (RS232 side Transmit pin 7&14 on the chip). As per the scope image below I see this 200mV ramp imposed on all RS232 data out of my device.I have isolated the switching supplies and feed 3v3 direct on to the plan from my bench PSU. Have tried replacing all caps (all are 100nF 16V X7R 0402). I have replaced the RS3232 with new ones from Element. The 3v3 supply looks good (at least on first inspection). I'm not sure what could be wrong here.Here is the layout, I have solid ground planes and plenty of stitching.And with the fillsThe thing 'works' the way it is but I'm worried (actually I'm sure) it will fail EMC with all that junk floating around and it's unnecessary anyway.Any ideas are welcome.Thanks

author: apblog
author_rank: Regular Contributor
timestamp: June 07, 2018, 05:49:30 am
content: The 3232 is noisy but not that noisy.Could you post your schematic? Are your charge pump capacitors the correct values?It looks like you have long traces to some of those caps... they should probably be shorter.Also I always add extra supply bypassing to try to keep the noise out of the supply rail.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 07, 2018, 06:06:05 am
content: Some ripple on the charge pumped supply and thus the output voltages is normal. It might be a little more than normal as the 16 V 100 nF 0402 size caps might loose quite some of there capacitance at around 12 V. So if really an issue it would be about a larger (with a DC bias) capacitance for the charge pump output. However this might need a larger form factor (e.g. 0805). At least space seems to be not an issue.The ripple is at a relatively low frequency and thus not such a large EMI issue. If at all it is the relatively steep falling edge that could be a problem. If the RS232 is not using higher baud rates (e.g. > 200 kBaud) one might be able to mitigate the problem with some capacitance at the outputs.The trance length is not an issue at those frequencies. It is more about the capacitance left under DC bias.

author: David Chamberlain
author_rank: Frequent Contributor
timestamp: June 07, 2018, 06:26:00 am
content: Thanks to the both of you. Yes the capacitance and dc bias sound like a good idea to follow up on.When i replaced the caps we only had 0604 in stock and now i think about it i didnt bother checking the voltage of them. The origional ones should have been 16v 100nF. So ill replace again with larger sized.Now i look at the ground pin i notice the top plane is broken so only have one via in that spot. Could be a problem as well. forgot to mention i see +-6 volt signal levels on rs232 side. Its a 3v3 part so presume thats normal and were not meant to see 12 volts?Also yes i can put caps on the outputs. Infact i removed them while isolating the problem. I know they do reduce the noise but only about 1/2. Will post schematics when im at work in the morning.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 07, 2018, 03:01:46 pm
content: The 12 V level was just a crude estimat, 6.6 V might be OK with RS232 though at the low end. I have not used 0402 caps so far - but small form factor caps are more sensitive to the capacitance to go down with voltage. For EMI the amplitude of the slow ripple is not the problem - it's the slope that might be a problem and here a cap at the output would definitely help.There are no really high currents, so 1 via should still be OK.For GND pin of the chip, there is a small gap in the GND connection toward the decoupling cap. I would normally not leave that gap. It is still probably OK - the MAX3232 is not that fast, compared to normal logic.

author: bson
author_rank: Unknown
timestamp: June 08, 2018, 02:25:34 am
content: How much are you loading the outputs?

author: David Chamberlain
author_rank: Frequent Contributor
timestamp: June 08, 2018, 04:09:31 am
content: The line is about three meters and terminates at another RS232 transever of a known good design. The noise is actually worse with the transmission lines connected (RX, TX and Ground).Here are some interesting differences, all measured from pin 5 of P1 connector below...With RX TX GND : noise p-p 400mV, Ramp freqnency 75kHz, Negative level -6.8V, Positive level 6.4VWith RX line noise p-p 300mV, Ramp freqnency 80kHz, Negative level -6.9V, Positive level 6.4VWith No RX or TX line noise p-p 300mV, Ramp freqnency 8kHz, Negative level -6.6V, Positive level 7.3VWith No connections line noise p-p 300mV, Ramp freqnency 8kHz, Negative level -6.6V, Positive level 7.2Vrelevent parts of schematic. Note the 16V caps were replaced with 50V versions.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 08, 2018, 07:06:59 am
content: The change in Ripple frequency is interesting. It looks like the chip is adjusting the charge pump frequency as needed. This suggests that the 200-400 mV ripple (it is no noise) is normal for this type of chip. Larger caps would likely just lower the frequency (within bounds).I would not worry so much about the slow ripple - 8 kHZ in the idle mode is not even relevant for EMI and also 85 kHz is rather low and not effective antenna here. The tricky part is more those higher frequencies from fast edges one does not see. Charge pumps can cause quite some emissions towards the supply.

author: David Chamberlain
author_rank: Frequent Contributor
timestamp: June 08, 2018, 07:44:17 am
content: Thanks for the reassurance around EMC your all probably correct. Obviously I'm shitting bricks just prior to submitting a prototype for testing and would have preferred everything to be perfect yet the one thing that's letting me down is the simplest part component in a larger design so it's a little maddening.I checked our 'gen 1' design it was 5 volt logic with a ST232C and I'm see perfectly flat noise on the RS232 side down almost to the limits of the scope floor. The major difference I see is that it used linear regulators throughout. whereas the new design main regulator is a TPS54340 3.5A buck (following the reference design like it were the bible) and anyway we don't see that noise on our supply rails just the TX out of the RS232 part.I'm not really in a position now to compare the old ST part with the TI3232 one we have now.. but it did start me wondering if the two things were related. When I trigger on the NOISE and look at 3v3 to the chip I do see a tiny little dip (sorry pictures not available till Tuesday due to queen getting older).The old designer of gen 1 did tie the positive charge pump cap to VCC but I'm sure that's a big no no due to internet whispers of 'latchup' and the like. But that's another story the ST part does show this in it's data sheet but the MAX3232 it's to ground like I would expect. But out of desperation I tried that anyway today and it had no effect on the noise. regarding the smp being at fault i have bypassed this per my first post.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 08, 2018, 09:55:12 am
content: I don't think the SMPS for the 3.3 V is causing the ripple. The small dips in sync with the ripple and Ti3232 charge pump are very likely a signal injected from the TI3232 towards the supply. This is more a question if the 100 nF decoupling at the charge pump is enough.