You are here

When Vref is not equal to Vref | Cypress Semiconductor

When Vref is not equal to Vref


Over the last couple of years I have been asked several questions about the reference system in PSoC3 and PSoC5.  The common assumption is that when you place a Vref component in your schematic and set the reference to 1.024 volts, you will get 1.024 volts +/-0.1% at room temperature. Unfortunately this is not true.  What you will actually get is a voltage that is not exactly 1.024 volts, but with an offset of several mVolts.  But the PSoC datasheet says that the Vref is trimmed to within +/- 1mV, what's the deal? To understand this, you need to first understand the PSoC3/5 Reference Tree.  Below is a simplified version. 

Notice that on the upper left hand side there is a bandgap voltage reference that generates about 1.2 volts.  Its output is buffered then fed into a resistor network to create a voltage near 1.024 volts.  This output is connected to the input of several reference buffers that drive different analog blocks including comparators, opamps, the DelSig ADC (Upper Right), and the SC/CT blocks.  During calibration, the output of the DelSig ADC’s reference buffer is measured to get exactly 1.024 volts +/- 0.1%, while the bandgap voltage is adjusted.  This means that only the DelSig ADC’s reference is the only true calibrated reference.  The input to the other reference buffers may be off by several mVolts to start with.  Each reference buffer will also have its own input offset as well.  If the input offset of all the references buffers were the same, then all the references would match, but this is unlikely.  The input offset of all the buffers will most likely vary from buffer to buffer.  This is why a reference may not be exactly 1.024 volts and why references will not all be the same. 

By Mark Hastings

Comments

patm's picture

Hi Mark,

Thanks for the explanation. If we can't expect 0.1% precision from anything but the del-sig, what can we expect? What is the max. offset for the del-sig and the various reference buffers? Over temperature, of course.

Cheers,

Mr. Muon

ALL CONTENT AND MATERIALS ON THIS SITE ARE PROVIDED "AS IS". CYPRESS SEMICONDUCTOR AND ITS RESPECTIVE SUPPLIERS MAKE NO REPRESENTATIONS ABOUT THE SUITABILITY OF THESE MATERIALS FOR ANY PURPOSE AND DISCLAIM ALL WARRANTIES AND CONDITIONS WITH REGARD TO THESE MATERIALS, INCLUDING BUT NOT LIMITED TO, ALL IMPLIED WARRANTIES AND CONDITIONS OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, TITLE AND NON-INFRINGEMENT OF ANY THIRD PARTY INTELLECTUAL PROPERTY RIGHT. NO LICENSE, EITHER EXPRESS OR IMPLIED, BY ESTOPPEL OR OTHERWISE, IS GRANTED BY CYPRESS SEMICONDUCTOR. USE OF THE INFORMATION ON THIS SITE MAY REQUIRE A LICENSE FROM A THIRD PARTY, OR A LICENSE FROM CYPRESS SEMICONDUCTOR.

Content on this site may contain or be subject to specific guidelines or limitations on use. All postings and use of the content on this site are subject to the Terms and Conditions of the site; third parties using this content agree to abide by any limitations or guidelines and to comply with the Terms and Conditions of this site. Cypress Semiconductor and its suppliers reserve the right to make corrections, deletions, modifications, enhancements, improvements and other changes to the content and materials, its products, programs and services at any time or to move or discontinue any content, products, programs, or services without notice.