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dsPIC33FJ128MC804 Voltage Reference problem

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bugwhiskers
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2009/03/15 18:31:47 (permalink)
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dsPIC33FJ128MC804 Voltage Reference problem

My prototype board has been lashed up with the Analogue Comparator Voltage reference output hard wired to the VREF+ input of the ADC.
Everything works fine if I specify VSS-VDD for the ADC reference inputs but if I change to VREF+ and VSS as the reference sources the voltage output of the Analogue Comparator Voltage reference nearly halves. Given that the spec sheet states that the ADC VREF inputs consume only 0.5 mA I find this quite astounding as many people will want to use this arrangement to save an additional voltage reference chip or a buffer.
 
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bugwhiskers
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    beaker
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    RE: dsPIC33FJ128MC804 Voltage Reference problem 2009/03/17 05:37:31 (permalink)
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    Did you fix this yet? It seems what you are trying to do is eminently reasonable. After all, the Comparator Voltage Reference is essentially a low-resolution, 4-bit DAC. Should work.
     
    I have ruled out hardware as a source because it works some of the time. The only two things I can think of are an incorrect configuration of the CVRCON register (unlikely), or an out-of-spec voltage value for the VREF+ ADC input. The minimum is 2.7V. What voltage did you set the CVR to produce?
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    bugwhiskers
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    RE: dsPIC33FJ128MC804 Voltage Reference problem 2009/03/19 03:51:59 (permalink)
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    Hi beaker,
     
    The problem is with the silicon. The CVREF is run to the output pin and to an internal buffer then internally on to the Analogue Comparators.
    This makes the output pin unbuffered so the tiny current draw of connecting it to VREF+ (0.5mA) is enough to load it to the extent that the voltage is halved.
    Why the designers didn't put the CVref to the buffer input and the buffer output to the CVref output pin beggars belief. It must have been late on a Friday when that decision was made.
     
     
    regards
    bugwhiskers
     
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