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Design >> Analog Design >> Bandgap issues - 1/f noise and ground bounce
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Message started by Alexandar on Nov 10^th, 2010, 2:19am

Title: Bandgap issues - 1/f noise and ground bounce
Post by Alexandar on Nov 10^th, 2010, 2:19am

Dear all.

Suppose I have a classical CMOS bandgap in a large mixed signal environment where the ground is bouncing. The ground bounce is a few milivolts, at a certain beat frequency which is not very high. The current solution is to have the bandgap voltage directly connected to a pad where some large capacitor decouples it. This capacitor is sufficiently large, and keeps the voltage steady in this environment.

Now the issue is that we see 1/f noise popping up when we operate the chip on long time intervals (say miliseconds to tens of seconds). What to do?

Obviously, enlarging components does not suffice. The components are already quite large. I would like to use chopping to remove the 1/f noise. But since the capacitor is in the loop, the bandwidth of the bandgap circuitry is very low. So in that case the chopping frequency should also be low (correct me if I'm wrong), to allow the circuit to settle. But as the chopping frequency becomes lower, the improvement in terms of noise is also reduced. =(

Anyone an idea to solve this trade off between the filtering of ground bounce and removing 1/f noise?

Title: Re: Bandgap issues - 1/f noise and ground bounce
Post by loose-electron on Nov 14^th, 2010, 3:33pm

Chopping structures should be viable within the loop.

Your switching gets rid of your flicker by mixing (multiplying) it up to an out of band frquency.

Last I checked, you can have a chopper system within a closed loop just as long as the chopping HF effects are suitable attenuated.

Title: Re: Bandgap issues - 1/f noise and ground bounce
Post by Alexandar on Nov 15^th, 2010, 5:20am

Thanks for your reply.

So suppose I need a certain chopping frequency to get my noise spec.
This will limit the size of the capacitor I can put at the output node.
The capacitance will determine how resistant the circuit is against ground bounce.
But suppose the capacitance is too low to get rid of the ground bounce. Then, what can I do to obtain both specs?

Title: Re: Bandgap issues - 1/f noise and ground bounce
Post by thechopper on Nov 15^th, 2010, 6:24pm

You could eventually chop inside the loop with minor modification to it in such a way the capacitor is left outside the chopped circuit:
for example, if you cascode the current mirrors bouncing the ptat current from the bandgap reference then you could demodulate the current going out of them before going into the cascoded devices. This way, due to the cascodes, you are isolating the low BW node (the capacitor) from the chopping demodulation node; thus you can chopper faster.
Although the cascode devices are left outside the chopper their offset contribution is negligible since the gain is already large at that point.

Best
Tosei

Title: Re: Bandgap issues - 1/f noise and ground bounce
Post by Alexandar on Nov 16^th, 2010, 6:57am

Thanks for the idea Tosei.

I'll have to verify what the largest 1/f contributors are for the topology you propose, but I think you are correct. =)

Title: Re: Bandgap issues - 1/f noise and ground bounce
Post by RobG on Nov 17^th, 2010, 8:56pm

If your 1/f is coming from the opamp it is just a matter of using a chopper opamp. Normally this would produce a ripple on the output which is a problem, but your filter cap will filter the ripple so it is a "no-brainer."

Be aware that resistors can also contribute 1/f. Hi res poly for sure, and I think I got bit once by a diffusion resistance.

However, what you are describing sounds more like burst noise or something else. Perhaps it is coupling in from another circuit. I'm surprised your filter cap worked -- are you sure you are not just measuring across the cap and "thinking" the problem went away? The noise could still be present on the chip itself.

Title: Re: Bandgap issues - 1/f noise and ground bounce
Post by Alexandar on Nov 18^th, 2010, 1:42am

Hello RobG

Your observation about burst noise is quite correct. In fact, what I describe as ground bounce, you could also name burst noise (correct me if I'm wrong). The burst noise present on the ground/substrate is due to the fact that the power consumption of our digital processing block is signal dependent. Now, for multiple channel application, the effect is multiplied. Suppose the block works with a series of high input value and then low values and then again high values. In that case the ground bounce is large, and becomes visible in our reference voltages.

The 1/f noise is something we observed orthogonally to the ground bounce. I know that high resistive poly can have some 1/f, but this is quite poorly modelled at the foundry. I will try to incorporate some numbers from other foundry to see whether it is significant.

By the way: do you think I need to fear the combination burst noise - chopping? Is it possible that they multiply and downconvert?

Title: Re: Bandgap issues - 1/f noise and ground bounce
Post by RobG on Nov 18^th, 2010, 8:25am

If the noise is produced from another circuit I would not call it burst noise. (http://en.wikipedia.org/wiki/Burst_noise)

I expect the main source of your 1/f is from the FETs in the opamp, but the noise from the resistors won't go away if you chop the opamp so you should at least estimate its level.

Are you sure the noise is really 1/f and not caused by noise coupled in from the other circuits? Is it present when the system clock is shut off? AC noise coupled into the BG circuit gets rectified which can make it look like low frequency noise.

Chopping will downconvert noise near the chopping frequency so, yes, you do need to be careful.

Title: Re: Bandgap issues - 1/f noise and ground bounce
Post by Alexandar on Nov 19^th, 2010, 2:16am

If I apply zero input signal, and if I sweep over time, at the digital output higher noise is seen as function of time. If I plot the variance over time, it really gives me the 1/f characteristic.

Your remark about the rectifying capability of the bandgap is an interesting one! The only coupling can occur via substrate/ground or supply, as other disturbing signals are >50 um away. Most of the wires are shielded with ground. So if I understand you correctly, this ground bouncing (basically a 5mv pulse of a few us every 100us), could show up as as low frequency noise at the output of the BG? Interesting! I will check this effect with a simulation on the extracted version.

I will have a range of chopping frequencies available, so I can always select another frequency externally if some noise source appears to be on the chopping frequency.

Title: Re: Bandgap issues - 1/f noise and ground bounce
Post by RobG on Nov 19^th, 2010, 7:35am

Let me expand on the rectification a bit so you know what I'm talking about. If you have a ripple on the substrate (or power supply) you will see a ripple on the bandgap output since the PSRR is finite. However, the transfer function is non-linear so you will get harmonics, INCLUDING a DC shift. The DC shift will depend on the amplitude (and frequency) of the AC noise on the power supply. Thus this DC shift will vary if the supply ripple varies, and could be mistook for 1/f or other noise.