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Design >> Analog Design >> switched-capacitor circuit need your help
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Message started by lhlbluesky_lhl on Sep 16^th, 2010, 5:52am

Title: switched-capacitor circuit need your help
Post by lhlbluesky_lhl on Sep 16^th, 2010, 5:52am

i have designed a sc circuit, as the figure shows, Cs=Cf, Vout=Vout+ - Vout- = (Vin+ - Vin-) - (Vref+ - Vref-), Vin = Vin+ - Vin- = (0V~2V), Vref = Vref+ - Vref- = 1V, Vout = Vout+ - Vout- = (-1V~+1V), my question is, when the output is small (150mV or small), the settling accuracy decreases a lot, for example: when (Vin+ - Vin-) - (Vref+ - Vref-) = 1.2V, the settling accuracy is 14bit (Vout = 1.19994V), but when (Vin+ - Vin-) - (Vref+ - Vref-) = 100mV, Vout = 99.85mV (9 bit accuracy), when (Vin+ - Vin-) - (Vref+ - Vref-) is even smaller, the settling accuracy is 8bit or less, why? what is the possible reason?

Cs=1.3pF, opamp is two stage (fold cascode + common source) with cascode compensation.

please help me, thanks all.

Title: Re: switched-capacitor circuit need your help
Post by RobG on Sep 16^th, 2010, 7:48am

First of all, how much error are you expecting due to finite opamp gain?

Part of the error is probably signal independent. In other words, an offset. If you were to check it with zero input you'd have an error but that doesn't mean it is only good to zero bits.

Your errors are 60uV and 150uV respectively and it is pretty easy to get that much offset from charge injection or simulation errors.

rg

Title: Re: switched-capacitor circuit need your help
Post by carlgrace on Sep 16^th, 2010, 2:32pm

I think your amplifier is probably doing OK. The accuracy is defined in terms of LSBs, not absolute voltages. Otherwise, you would expect you amplifier to be getting more accurate at smaller inputs.

Here is a thought experiment to explain what I mean. Imagine you wanted your amplifier to settle to 10b (about 1 part in 1000). Your full-scale is 1V. So if you put in a volt, and have unity gain, you would expect an output of 999mV, for a 1mV settling error (1 LSB at a 10-bit level). Now put in 10mV. For 1-part-in-1000 settling you now expect only an error of 10 uV! Your settling in that case would probably be upset by thermal noise (I don't know the performance of your op amp)

Carl

Title: Re: switched-capacitor circuit need your help
Post by lhlbluesky_lhl on Sep 17^th, 2010, 7:58am

the opamp performace is good, GBW and PM , DC gain are all OK.

as the above says, accuracy is related to LSB, that is, for full scale of

2V, 1LSB=0.5mV for 12 bit accuracy, when the input is 100mV, and

the output is 99.8mV, it is also 13bit accuracy, am i right?

in another word, my simulation result above (100mV - 99.85mV) is

OK for 12 bit accuracy, am i right?

please correct me if i'm wrong, thanks all.

Title: Re: switched-capacitor circuit need your help
Post by thechopper on Sep 19^th, 2010, 6:54pm

I think it is natural for every circuit (not only yours) to have smaller accuracy as the signal level decreases, and the reason is simple:
since the noise (or error) source remains constant the signal level varies. Thus the signal to noise ratio diminishes and therefore the equivalent number of bits gets reduced; where ENOB is calculated as the log2(SNR)

Best
Tosei

Title: Re: switched-capacitor circuit need your help
Post by lhlbluesky_lhl on Sep 20^th, 2010, 5:14am

thanks all, any other ideas?