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Is output noise of 400 uV in bandgap okay?

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surianova

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hi all!

my bandgap output noise is 400 uV rms for frequency from 1 to 1 Mhz,is it ok? pls comment.
 

lijianheng

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Re: noise in bandgap

Which process PDK do you use?

It just fine.
 

gavin168

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noise in bandgap

I am fresh designer.

what is bandgap output noise?

thanks
 

Yanhui

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Re: noise in bandgap

surianova said:
hi all!

my bandgap output noise is 400 uV rms for frequency from 1 to 1 Mhz,is it ok? pls comment.
a simulation result?
or a test result?

the structure, and the source u give, pls~
 

qutang

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noise in bandgap

is it test result?
maybe ok.
 

JPR

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Re: noise in bandgap

surianova said:
hi all!

my bandgap output noise is 400 uV rms for frequency from 1 to 1 Mhz,is it ok? pls comment.
400uV RMS sounds high to me, but it is difficult to say without more information. If you need any accuracy more than about 8-bit using the Vref, you are likely to run into problems with your noise level, depending upon your system and the actual bandwidth of your bandgap.

If your bandgap bandwidth is wider than your signal bandwidth, AND you are using an oversampling A/D or D/A, you might be able to extend a few extra bits from filtering: Assume that the bandwidth for your reference is 200kHz, and you have signal filters at 5kHz. You should be able to squeeze an extra 5 bits or so (ideal = 6.3 bits more) out of this system.

However, if your reference bandwidth is at or below your signal bandwidth, you can not get any extra out of it.

What are the specs and expectations of the system? What is the size and power consumption of the reference? Do you have an external filter capacitor? What is the actual bandwidth of your reference? Do you need better than 8-bits out of the system? Does your reference need to respond quickly (at startup for example)? How fast does your reference need to repsond to a change in load? How much change in load is expected?
 

yorande

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noise in bandgap

To JPR:
Hi, could you please explain about what dose the "accuracy more than about 8-bit using the Vref" mean?

And why should a bandgap circuit has bandwidth?

And why should the change in load affect a bandgap circuit? I think in most circuits they are quite independent from each other.


To surianova:
Hi, why do u simulate the noise from 1--1MHz? any special requirements in this frequency range? And how do u get noise of a bandgap circuit?

Thanks.
 

JPR

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Re: noise in bandgap

yorande,

If there is D/A converter or A/D converter using this reference, the noise on the reference will limit the accuracy of the conversion for most converter types.
An 8-bit A/D at 1.25V will have a resolution of 4.88mV, which relates to a quantization noise of 1.4mV. With 400uV RMS noise on Vref, there will not be excessive noise on the ADC value. However, if we had a 10bit ADC, the quantization noise would be 350uV RMS, which is about the same as the 400uV RMS from this voltage reference. Adding the voltage reference noise would now add significantly to the overall noise performance, limiting the accuracy of the ADC.

As to the bandwidth of the bandgap reference. In most cirucits, there is no need for wide bandwidth from the voltage reference circuit (it is a DC voltage level), so the output will typically have significant filtering applied, meaning that the bandwidth will be low. However, there are applications where the high frequency noise does not impact the results, so the bandgap would not be filtered, or some bandgap circuits that would have poor power supply rejection with filtering so the bandwidth needs to be wide. If this 400uV RMS stems from a wide bandwidth cirucit, then much of the noise will be at high frequencies and may not impact some A/D or D/A circuits (particularly sigma-delta type A/D). However, if the 400uV RMS is on a signal that is already heavily filtered, then the additional filtering from the oversampling type converter would not be of significant benefit.

As for the change in load, there are some circuits (such as an R-2R ladder type D/A) that can produce very different loads on the reference, depending upon the D/A code being converted. If the voltage reference is used with this type of variable load, then the load regulation of the converter would matter. I am not stating that the load regulation matters for ALL voltage references, it is simply a specification that might impact the ability to filter out noise.

My main point was that the noise level sounds high, but, without knowing more about the application or the bandgap itself, it is difficult to determine if it is a reasonable value.
 

abcyin

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Re: noise in bandgap

JPR said:
yorande,

If there is D/A converter or A/D converter using this reference, the noise on the reference will limit the accuracy of the conversion for most converter types.
An 8-bit A/D at 1.25V will have a resolution of 4.88mV, which relates to a quantization noise of 1.4mV. With 400uV RMS noise on Vref, there will not be excessive noise on the ADC value. However, if we had a 10bit ADC, the quantization noise would be 350uV RMS, which is about the same as the 400uV RMS from this voltage reference. Adding the voltage reference noise would now add significantly to the overall noise performance, limiting the accuracy of the ADC.

As to the bandwidth of the bandgap reference. In most cirucits, there is no need for wide bandwidth from the voltage reference circuit (it is a DC voltage level), so the output will typically have significant filtering applied, meaning that the bandwidth will be low. However, there are applications where the high frequency noise does not impact the results, so the bandgap would not be filtered, or some bandgap circuits that would have poor power supply rejection with filtering so the bandwidth needs to be wide. If this 400uV RMS stems from a wide bandwidth cirucit, then much of the noise will be at high frequencies and may not impact some A/D or D/A circuits (particularly sigma-delta type A/D). However, if the 400uV RMS is on a signal that is already heavily filtered, then the additional filtering from the oversampling type converter would not be of significant benefit.

As for the change in load, there are some circuits (such as an R-2R ladder type D/A) that can produce very different loads on the reference, depending upon the D/A code being converted. If the voltage reference is used with this type of variable load, then the load regulation of the converter would matter. I am not stating that the load regulation matters for ALL voltage references, it is simply a specification that might impact the ability to filter out noise.

My main point was that the noise level sounds high, but, without knowing more about the application or the bandgap itself, it is difficult to determine if it is a reasonable value.

it's a great reply, but I still have some confusion about the relationship between the reference and its bandwidth, so, pls, can you share me with some literature about this item, thx very much!
 

yorande

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noise in bandgap

Hi JPR:
THanks for your answer.
However though wideband reference is good for power supply rejection, here there is another probelm if the bandgap is for an amplifier. Cause if no enough filtering is offered, the circuit is tend to oscillate. Shouldn't the power supply ripples be filtered by external caps? Could a wideband bandgap works better for this?
Thanks again.
 

abcyin

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noise in bandgap

another question: how to get the bandgap bandwidth, i mean what characteristics determine the BW of it, and by what simulation, you can get the BW.

Thx again!
 

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