Could some electronics guru please help ? In a flash ADC, there is a resistor chain, whose voltages at each node are the reference voltages for the quantization levels. So, let us suppose that I want to digitize a sine wave, with +ve and -ve lobes - in this case how is the polarity of the reference voltages going to change, with each zero crossing ? In commercial designs, is there a toggle scheme to achieve this ? Any hints, suggestions would be of immense help. Thanks in advance.
"Daku" schrieb im Newsbeitrag news: snipped-for-privacy@q6g2000prl.googlegroups.com...
Hello Daku, You constantly supply +Vref on an end and -Vref on the other end of the chain of resistors. If you only have a single supply, your signal's zero will be somewhere in the middle of the supply voltage. Best regards, Helmut .
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If he only has a single supply for a reference and his AC signal
swings + and - WRT 0V, then his signal's zero will be at 0V and only
half of the input signal will be digitized, yes?
I haven't studied any recent flash ADC designs, but I seem to recall that the early ones were unipolar (0 to +V only). This was also pretty standard for all ADCs, including the more-popular SARs. For bipolar signals you had to add an offset voltage to center it in the unipolar range.
Best regards,
Bob Masta DAQARTA v6.01 Data AcQuisition And Real-Time Analysis
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True flash ADCs aren't common any more. Fast ADCs are, in roughly the
100 Ms/s and range, usually multi-stage pipeline structures, and use capacitive networks rather than resistors. It's hard to make good resistors on-chip, but people can make superb capacitors.
Pipeline ADCs usually have differential inputs, and can typically digitize something like +-Vref. They are fairly complex internally.
In a classic flash ADC, the input signal range is equal to the span of voltages across the resistor string. That tended to make the range unipolar.
you set the input (ADC) at 50% of the span. Readings you get inside are represented as +/- depending where the levels are.. for example, Lets say you have a 16 bit ADC, the zero cross over point as you say, would be at $8000 /0x8000 Any thing below $8000 is the positive side and anything above $8000 is the negative side.. If you were to move that value into a 16 bit integer, then you could use +/- numbers to represent your sine wave in languages like C for example.
the 15th (16) bit is used as the sign and also be can used to indicate the - or + side of the wave.
This is all assuming I understand what your question was? I see ADC in the mix, I can only assume you are trying to determine how one would decode the sine wave. Just think of it as a large DC component single sided results. You just split the possible max value in half and that is your center point.(cross over)
Many ADC's may use a 16 bit word to contain its value how ever, that does not mean it has the 16 bit detail resolution, in which case, the lower bits are most likely masked off. This is where you need to read the docs on how the values are queued. Some do not do it this way from a register point of view and you need to shift it into a integer type number to better work with it.
But then I again, I could be completely off on a different road as to what you are trying to find, here.
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