Some obstacle in designing a chopper op-amp !

[analogtmp]

Hi all,
I am working in the design a chopper op-amp.but I can't continue for some question:
I have designed a two stage amp with 1M bandwidth ,gain is 72dB,PM 61. -3dB bandwidth is 250Hz.compensation capacitor is 1.1pF.I have choose the chopper frequency at 100kHz or 100Hz.With the two frequencies ,the chopper modulators can't reduce the input offset and the 1/f noise .I don't know what is my mistake. anxious for your suggestions!!!

 

[flatulent]

Measure the chopper performance by cascading them without the amplifier between. Your system cannot be better than this measurement.

 

[analogtmp]

Re: flatulent

My chopper modulator is realized with the four simple MOS switch,If they work as that you telled ,Can you tell me what mistake I made ?

 

[flatulent]

1. Does your chopper multiply by +- 1? 1 and 0 will not work properly.

2. The amplifier should have flat gain over the chopping frequency +- the maximum input frequency.

3. Put feedback just around the amplifier between the choppers to make the gain what you want. You cannot put feedback around the far sides of the choppers.

4. To provide the best help for you, upload the schematic of your circuit. and any measurements you have made or simulation outputs.

 

[analogtmp]

Re: flatulent

hi flatulent,
I have uploaded my design ,please help me !
What is the meaning of "Does your chopper multiply by +- 1? 1 and 0 will not work properly. " Can you detail the operation.
"3. Put feedback just around the amplifier between the choppers to make the gain what you want. You cannot put feedback around the far sides of the choppers. "---In case of the two stage op-amp,the output modulator is put between the two stage amplifiers .then how to put the feedback in order to simulate the gain?

 

[nathan]

The chopping should periodically (50% duty cycle) invert the phase of the input signal to achive the proper frequency modulation.

nathan

 

[analogtmp]

Re: nathan

I have done that ,but I still can't get the smaller offset

 

[flatulent]

The choppers should be entirely outside of the amplifier. The signal path is signal source-chopper-amplifier-chopper-load.

One trick in making your IC is to have two metal patterns. One brings each block to the outside so they can be measured independently. The other metal pattern is for the final circuit configuration. Make some ICs with the first and some with the second metal mask pattern.

A good book is Operational Amplifiers by Tobey, Graeme, and Huelsman. Pages 150-164 cover chopper amplifier design.

 

[analogtmp]

Re: flatulent

hi flatulent,
my amp won't be taped out .but I don't understand your last post:

What is the meaning of "Does your chopper multiply by +- 1? 1 and 0 will not work properly. " ?
"3. Put feedback just around the amplifier between the choppers to make the gain what you want. You cannot put feedback around the far sides of the choppers. "---In case of the two stage op-amp,the output modulator is put between the two stage amplifiers .then how to put the feedback in order to simulate the gain?

Excuse me to explain them on the base of my design, Thank you !

At last ,where can I get the book :Operational Amplifiers by Tobey, Graeme, and Huelsman. Pages 150-164 in electrics edition?

 

[flatulent]

I get the impression that this is a project for a class. You seem to have selected a project that is too complex for you to do in the time of the school term. You should go to the professor of the class and negotiate a simpler project. This may result in a lower grade for the class, but you will pass the class and heat toward graduation and making a living.

 

[analogtmp]

Re: flatulent

hi flatulent,

You are right ,I am a graduate student with interesting in analog circuits design. I have no choice in the project.Thank you very much for your earlier suggestions.
Also ,I am glad to see more and more friends will join in the chopper discussing and intercommunicate your design experience.
Thank all !