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low noise Op Amp Design, need help

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tia_design

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low noise cmos op amp design

You guys,

I'm designing a low noise CMOS Op Amp, now I can reduce thermal noise (input referred) down to 1.5nV/rhHz, this is schematic simulation result. I don't know how much additional noise will be added after layout. Does anybody do the simular research so we can discuss it?
 

alvays

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the additional is the disturb, not noise.
 

mitgrace

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Excuse me . How do you run the input referred noise ? Which tools do u use it ? I am looikign for this problems , Thanks
 

xuel

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I think your chip with a carefully designed layout will be a litter better than the worst-case postlayout simulation results .
 

snoop835

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Hi mitgrace!

The simulate for input referred noise, you will need to run the simulation in parallel with ac analysis. The syntax are (for HSPICE);

.AC DEC 100 1 10G *****Sweep frequency from 1Hz to 10GHz*********
.NOISE (VOUT) VIN
.PRINT NOISE INOISE ONOISE
***INOISE=Input Referred Noise*****ONOISE=Output Referred Noise*********

see HSPICE manual for details.

The output waveform should be X-axis = Frequency
Y-axis = nV/sqrt(Hz)

GOOD LUCK!!
 

jake

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snoop835 said:
Hi mitgrace!

The simulate for input referred noise, you will need to run the simulation in parallel with ac analysis. The syntax are (for HSPICE);

.AC DEC 100 1 10G *****Sweep frequency from 1Hz to 10GHz*********
.NOISE (VOUT) VIN
.PRINT NOISE INOISE ONOISE
***INOISE=Input Referred Noise*****ONOISE=Output Referred Noise*********

see HSPICE manual for details.

The output waveform should be X-axis = Frequency
Y-axis = nV/sqrt(Hz)

GOOD LUCK!!
a bandgap, for example, how to add the ac source?
 

Vamsi Mocherla

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Dear Jake,

You can put an ac source on the supplies.

Dear tia_design,

One way to reduce your thermal noise is to increase the bias current in the circuit.
 

jake

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Vamsi Mocherla said:
Dear Jake,

You can put an ac source on the supplies.

Dear tia_design,

One way to reduce your thermal noise is to increase the bias current in the circuit.

Thanks, I do so.
I encounter a big problem about the bandgap noise. Attachment is my circuit. Who can help me to analysie how to reduce the noise(the size of the diff transistor is already big )?
 

Vamsi Mocherla

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Dear jake,

I was looking into your circuit and i am noticing that you have very less current in the output branch. It is about 0.1u A. Is it right? I mean, that will cause a lot of thermal noise at the output. And another thing. The W/L aspect ratio of the input differential pair on the output amp is comparitively low. This will affect the relative noise in terms of gm. I would suggest that the gm of the differential pair be higher than the current mirrors for better noise performance. Finally, the lengths of the devices are a bit high, why dont you reduce them a little bit. It will give rise to a lot of unwanted parasitics...... I hope that I am clear on this.
 

jake

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Vamsi Mocherla said:
Dear jake,

I was looking into your circuit and i am noticing that you have very less current in the output branch. It is about 0.1u A. Is it right? I mean, that will cause a lot of thermal noise at the output. And another thing. The W/L aspect ratio of the input differential pair on the output amp is comparitively low. This will affect the relative noise in terms of gm. I would suggest that the gm of the differential pair be higher than the current mirrors for better noise performance. Finally, the lengths of the devices are a bit high, why dont you reduce them a little bit. It will give rise to a lot of unwanted parasitics...... I hope that I am clear on this.

THank you very much, I am sorry that I have mislabelled the output branch current, and I have improved it into 10uA now. I think that reducing the lengths of the devices is a good suggestion. I will try to do it now, and I will tall you the results.
 

jake

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Vamsi Mocherla said:
Dear jake,

I was looking into your circuit and i am noticing that you have very less current in the output branch. It is about 0.1u A. Is it right? I mean, that will cause a lot of thermal noise at the output. And another thing. The W/L aspect ratio of the input differential pair on the output amp is comparitively low. This will affect the relative noise in terms of gm. I would suggest that the gm of the differential pair be higher than the current mirrors for better noise performance. Finally, the lengths of the devices are a bit high, why dont you reduce them a little bit. It will give rise to a lot of unwanted parasitics...... I hope that I am clear on this.
Dear Vamsi Mocherla,
I have tried to reduce the lengths of the devices, they contribute little for the noise. Some one tell me that the op amplify the noise, however I reduce the gain of the OP, it can't reduce the noise. in fact, it increase the noise. Do you have another method to reduce the noise? Maybe, changing a new structure can reduce the noise. Attachment is my new structure, however, the output voltage vary with temperature seriously, why?
 

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