Integrator for Sigma Delta Converter Gain Discrepancy

[highpotential007]

I am new to analog design and I have a few questions. I have designed a fully differential folded cascode amplifier, the gain is 70db, bandwidth of 32MHz and a phase margin of 77 degrees. Now I am attempting to create an integrator with this amplifier. I am sampling at 3MHz, and using a switched capacitor network. The problem is that when I calculate what the output of the transfer function is for the integrator I get a gain of 7. When I simulate the circuit I get a gain of 100. I know that I can manipulate the capacitor values to reduce my gain, but I would like to know if there is some other problem with the circuit possibly the amplifier which would cause this discrpancy. The transfer function I am using accounts for finite amplifier gain, and finite bandwidth. Any help would be greatly appreciated.

 

[willyboy19]

You need to understand when talking about SC integrator gain, the specific input signal frequency is very important. You may have a gain of 7 at high frequency while a gain of 100 when the input frequency is 14 times lower.

Please be more specific about what condition you are using in the simulation bench and what assumption you have for the calculated integrator gain. At this point you can safely ignore the finite OPAMP DC gain and bandwidth since their contribution should be much smaller compared with the order of magintude of discrepancy you mentioned.

 

[highpotential007]

I am using a 20Khz input sine wave, and I used a 20khz frequency when calculating the transfer function. Thanks for any help.

 

[avinash]

dear high poteintial,
i am also designing a fully diff opamp.can you tell me how you characterized it for gain ugb and phase margin.

 

[highpotential007]

In order to characterize the opamp for unity gain bandwidth, and phase margin you input an ac signal, and run an ac analysis sweeping the frequency from say 1 to 1Ghz.
This will give you a bode plot, and a phase plot. From their youcan get your gainbandwidth, and phase margin.

 

[avinash]

dear high potential,the method you r telling is for single ended opamp.what about diff o/p opamp.

 

[highpotential007]

Avinash,
The method for differential is exactly the same as for single ended. If you do the math for the transfer function the gains, and frequency response should be completely symmetrical, therefore the factor of two introduced by the differential inout drops from the transfer function giving you the same values as the single sided. Try it, and verify. It works out very nicely. The differential nature basically helps your SNR.

 

[avinash]

dear highpotential,
what should be the connections made at i/ps and o/ps while checking the gain,ugb and phase margin.