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Design >> Analog Design >> ideal OTA in cadence
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Message started by yixiusky on May 28^th, 2008, 1:43am

Title: ideal OTA in cadence
Post by yixiusky on May 28^th, 2008, 1:43am

hello, i would like to ask one question:

We can make ideal opamp in cadence by using VCVS (in analog library)+lpf_1storder(in ahdl library). VCVS set the gain of

Opamp, and the lpf_1storder can set the unity gain bandwidth of Opamp. then we can get ideal Opamp to simulate.

My question is here: I want to get ideal OTA to simulate OTA-C filter. I choose VCCS in analog library to simulate, but i can

not get result, and i can not set the bandwidth of OTA also. So how can we get ideal OTA in cadence ?

Thank you very much ^^

Title: Re: ideal OTA in cadence
Post by buddypoor on May 28^th, 2008, 5:01am

Probably, there are several aleternatives.
One method is to place a lowpass in front of the VCCS (and to use - if necessary - an additional subtracting unit as differential input).
Dont forget to specify a certain transconductance gm for the VCCS.

Title: Re: ideal OTA in cadence
Post by yixiusky on May 28^th, 2008, 6:53am

:)thank u very much ^^ I make like this shown in attached picture. Is this ok ? Thank you very much !

I am sorry to ask what's "an additional subtracting unit" ? ^^

Title: Re: ideal OTA in cadence
Post by Eugene on May 28^th, 2008, 8:09am

I have one comment unrelated to your original question. With buffers on either side of the filter you should not need to waste nodes on differential symmetry. I would remove one filter and reference the middle circuitry to ground.

Title: Re: ideal OTA in cadence
Post by buddypoor on May 28^th, 2008, 12:06pm

I am sorry to ask what's "an additional subtracting unit" ? ^^

The VCCS has a differential input, has it not ?
If you place a lowpass in front of the pos. input (neg. input to ground) you have lost one input.
Therefore, you have to add a unit which replaces the lost diff. input. And that is simply an adder with one neg. sign to produce the difference between two signals.

Title: Re: ideal OTA in cadence
Post by vivkr on May 29^th, 2008, 11:12pm

yixiusky wrote on May 28^th, 2008, 6:53am:

:)thank u very much ^^ I make like this shown in attached picture. Is this ok ? Thank you very much !

I am sorry to ask what's "an additional subtracting unit" ? ^^

Hi,

If you want to make an ideal OTA, then I would suggest that you use the same structure which you would use to do the simplest hand analysis of your OTA:

A VCCS + output resistance in parallel. I would imagine that there is no need to build in the lowpass filters. When you connect the external capacitive
loads to this OTA model, the bandwidth will be automatically set, and will in fact more accurately track the load capacitance than if you plug in an ideal LPF
in between as you have done here. If you have a 2-stage opamp with internal compensation, a similar structural model suffices. In my experience, this is the most
accurate and simple model. You can even set limits on the current of the VCCS to model slewing if you want !!!

Keep it as simple as possible but no simpler...

Regards
Vivek

Title: Re: ideal OTA in cadence
Post by yixiusky on Jun 1^st, 2008, 4:01am

Thank u very much vivkr :)

Here i attach the OTA u suggest, is it right ? ^^ i attach two circuit, both are for differential input and differential outout.

Which one is right ? :)