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PSoC 5, 3 & 1 MCU

Anonymous
Not applicable

Hi All,

   

Yet another PSoC beginner. I've approximated an SC block as an op-amp to use it as a differentiator. (My Input after external Capacitor goes to P2.3; the o/p port P0.3 is shorted to P2.3 via a feedback resistor).  A very simple application.

   

 

   

I'm giving a reference voltage of 1.25V thru a PGA (Vinp=AGND; Gain=0.5) to the SC block's ACMux. My input is fed to BMux after going through external capacitor.  My differentiator's (SCblock's) o/p has to start at 1.25V.

   

 

   

I face the following problems,

   

 

   

1. The differentiator's o/p starts at 1.6V rather than 1.25V. Reason for this offset ? Also, I measured the PGAs o/p before it was routed to SC block and it gave 1.25V.

   

 

   

2. The differentiator's o/p is routed to another SC block approximated as op-amp which I use as a comparator. This one has a reference from another PGA (1.72V). I observe on the CRO that my differentiator's o/p crosses 1.72V comfortably but still the comparator doesnt switch to 5V. Why  ?

   

 

   

Please clarify,

   

RAM.

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4 Replies
Esteemed Contributor

You might post youir project so forum can take a look at it.

   

 

   

When you say "The differentiator's o/p starts at 1.6V rather than 1.25V.",

   

is this for input floating ? What is the size of the Rfeedback used on

   

differentiator ? Is bias current creating a large offset ? Or is this just

   

creating a follower out of OpAmp, hence with input floating the differentiator

   

will be at Vdd/2 ?

   

 

   

Regards, Dana.

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Anonymous
Not applicable

Couple of points to check:

   

1. What is the power settings and the column clock frequency? Please see application note - AN2223 (PSoC 1 Approximating an opamp with switched capacitor integrator) - http://www.cypress.com/?rID=2630 for guidance.

   

2. If input is floating, the approximated opamp, will act as a voltage follower (with a resistor connected between -input and output). But the output voltage will depend on the CA, CB capacitor values. See equation 8 in AN2223. If the feedback resistor is too high, it can affect the output voltage too, depending on the column clock frequency.  

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Anonymous
Not applicable

For the comparator question: SCBLOCK, approximated as opamp, can only work if operated in a closed loop system. Otherwise it is a pure integrator. Please follow cookbook provided in appendix of application note AN2041 (http://www.cypress.com/?rID=2899) to know the parameters for comparator configuration.

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Anonymous
Not applicable

 @ rjvb, 

   

 

   

Thanks a lot!

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