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ycm Junior Member Offline Posts: 22	bandgap error amplifier bandwidth Apr 21^st, 2010, 11:53am if a bandgap is loaded with a buffer amplifier, what is the constrain to select the bandwidth of the error amplifier in the bandgap? one thing I can think about is the power, lower bandwidth lower power, what else?
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Mayank Community Fellow Offline Posts: 334	Re: bandgap error amplifier bandwidth Reply #1 - Apr 21^st, 2010, 9:53pm hii Error Amp BW should be as low as possible to filter out noise BUT sufficiently large to accomodate major supply ripple. So, it mostly depends on your supply profile & noise specs. -- Mayank
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ycm Junior Member Offline Posts: 22	Re: bandgap error amplifier bandwidth Reply #2 - Apr 22^nd, 2010, 8:30am thanks, if we put a simple RC filter at the output of the bandgap to solve the noise issue, the bandwidth then mainly depends on PSRR spec? what is the TYPICAL bandwidth requirement, 100kHz?
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HdrChopper Community Fellow Offline Posts: 493	Re: bandgap error amplifier bandwidth Reply #3 - Apr 22^nd, 2010, 6:01pm you are correct... BW will be set by PSRR requirements if bandgap reference is filtered by any means different from the opamp BW... Best Tosei
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RobG

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Re: bandgap error amplifier bandwidth
Reply #4 - Apr 26^th, 2010, 7:22pm

It depends on your application, but I generally favor higher bandwidth than the signals of interest (load and power supply noise). If you have transient loads you probably want a high bandwidth so the output will recover quickly.

Also, as mentioned, you need the bandwidth for PSRR. Filtering with a large cap doesn't help.. the transfer function from the supply to the output is often non-linear so AC noise on the power supply can result in a DC shift in the output voltage.. so you want good AC supply rejection. In this case the shift is DC, so a large filter cap won't help.

rg

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loose-electron

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Re: bandgap error amplifier bandwidth
Reply #5 - Apr 27^th, 2010, 12:22pm

I would keep the bandwidth of the amplifier low and passively filter at the output.

passive filter for the enviropnment noise and power line junk

RobG wrote on Apr 26^th, 2010, 7:22pm:

It depends on your application, but I generally favor higher bandwidth than the signals of interest (load and power supply noise). If you have transient loads you probably want a high bandwidth so the output will recover quickly.

Also, as mentioned, you need the bandwidth for PSRR. Filtering with a large cap doesn't help.. the transfer function from the supply to the output is often non-linear so AC noise on the power supply can result in a DC shift in the output voltage.. so you want good AC supply rejection. In this case the shift is DC, so a large filter cap won't help.

rg

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Jerry Twomey
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HdrChopper

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Re: bandgap error amplifier bandwidth
Reply #6 - Apr 28^th, 2010, 6:33pm

loose-electron wrote on Apr 27^th, 2010, 12:22pm:

I would keep the bandwidth of the amplifier low and passively filter at the output.

passive filter for the enviropnment noise and power line junk

RobG wrote on Apr 26^th, 2010, 7:22pm:

It depends on your application, but I generally favor higher bandwidth than the signals of interest (load and power supply noise). If you have transient loads you probably want a high bandwidth so the output will recover quickly.

Also, as mentioned, you need the bandwidth for PSRR. Filtering with a large cap doesn't help.. the transfer function from the supply to the output is often non-linear so AC noise on the power supply can result in a DC shift in the output voltage.. so you want good AC supply rejection. In this case the shift is DC, so a large filter cap won't help.

rg

Bob:

I agree with Jerry...passively filtering the BG output will prevent AC noise from getting into the opamp and thus will also prevent noise mixing due to circuit asymetries resulting in DC shifts as you suggested. This beyond the low BW for the opamp.
Also a passive RC in seres with the supply line (if acceptable) will have the same purpose on the noise coming thru this line into the opamp...

Best
Tosei

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RobG

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Re: bandgap error amplifier bandwidth
Reply #7 - Apr 29^th, 2010, 8:41am

Sure, if you have external components big enough to keep all the noise from getting to the opamp you can use a poor opamp, but just using a low bandwidth opamp will likely cause problems. Similarly, a passive filter on the output will have series resistance so you are going to have poor DC load rejection. If you want to use an external cap for filtering you can make a case for passive filtering, well if bond wire inductance don't get ya coming back in... but it is hard to do with on-chip caps as a general rule.

So... a poor opamp with external components or a good internal opamp... you need the BW to be way lower or way higher than the signals of interest... that is kind of what it boils down to.

As I mentioned before, the transfer function from the power supply to the opamp output is non-linear so noise on the power supply will cause a DC shift in the output voltage unless there is enough gain to reject it -- even if you have a very large external cap. Be careful... it depends on the application.

rg

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