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PSoC 4 MCU

Anonymous
Not applicable

Can anybody tell me the minimum external voltage reference that can be used with PSoC4 SAR ADC? 

   

I am trying to to measure small signals +/-20mV pk.  I could amplify them, but that would limit the bandwidth.  The internal amps have a maximum GBW of 6MHz and I need a bandwidth of at least 200kHz which limits the gain to a maximum of 30.

   

A (hopefully!) simple solution would be to use a lower V ref, but I can't find any data which states the minimum value that the ADC will reliably work at.  

   

Any help would be appreciated.

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Esteemed Contributor II

An external Vref for the p<SoC4 SAR ADC can be as low as 0.01V as the configuration window tells. Buffer gain cannot be used with the current PSoC4 devices. When converting +- values they must be shifted to match your supplied Vref as ground. Challingging will be to deliver a temperature-compensated precise (0.1%) low noise supply independent simple reference Voltage in the range of 25 to 50 mV.

   

 

   

Bob 

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Esteemed Contributor

Some thoughts -

   

 

   

1) Input CM range is speced at Vss to Vdda, I think this is an error, and should be

   

Vssa to Vdda, I will get this clarified.

   

 

   

2) The SAR linearity is speced for Vref >= 1 V, so no data on very low Vref values.

   

 

   

3) The input offset is 2 mV max, but no drift values. At low Vref it becomes a major error

   

contributor, especially over T.

   

 

   

Some questions -

   

 

   

a. What dynamic range do you want ?

   

b. What resolution (over T) is needed ?

   

c. What accuracy (over T) required ?

   

 

   

There is a method of doubling small signal GBW of two matched OpAmps, but requires precision

   

external R's. There is no characterization howevere of OpAmp matching, so error budget would

   

be in question.

   

 

   

Attached some low accuracy methods (you have to do the error budget) to level translate

   

signals.

   

 

   

Regards, Dana.

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Esteemed Contributor

There is an approach of removing errors due to T, V, offsets, linearities

   

during production test. Basically a link is established to DUT, a precision

   

V generator is commanded by DUT in a cal routine, and measured values

   

are curve fitted to results, using least squares or power curve fit.

   

 

   

It may seem complicated but actually quite straightforward. But does increase

   

product test time.

   

 

   

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Anonymous
Not applicable

What is the maximum external voltage rating?

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Esteemed Contributor

Can you be more specific, of what, the part Vdda, Vssa, Vdd, Vss, the SAR inputs....?

   

 

   

Most of that info in datasheets -    

   

 

   

          http://www.cypress.com/?app=search&searchType=advanced&keyword=&rtID=107&id=4749

   

 

   

Regards, Dana.

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Anonymous
Not applicable

Sorry what is the max voltage i can put on vref? I want to 5v on it.

   

if i can the datasheet says 0x0000 = Vss and 0x7FF =2*Vref. So to get a range from 0v to 5v Vref has to 2.5v. Setting i want are Single and unsigned. Is this statement correct?

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Esteemed Contributor

If you set the Vdd project rating to 5.5 V in .cydwr tab, then by setting Vdda to 5.5 V

   

and Vref to Vdda you would achieve the following (that would give you a range of 0 - 5.5 V) -

   

 

   

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Anonymous
Not applicable

when i do that in the ADC_SAR_SEQ_P4 General tab? does that mean i physcally have to do it also? By connecting J13 Pin 2 to 5.5v.

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Esteemed Contributor

If you power it off USB then you will get 5 V (not 5.5), from board manual -

   

 

   

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Esteemed Contributor

Post did not work, see attachment. Note set your project .cydwr to 5.0 V

   

in this case.

   

 

   

Regards, Dana.

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