On 3/8/2015 9:03 AM, Jerry Stuckle wrote:
On 3/8/2015 7:35 AM, Brian Reay wrote:
Jeff wrote:

I will finally point out that your use of the term "slope detecting ADC"
is invalid. Google returns exactly 4 hits when this term is entered
with quotes. The name of this converter may have slope in it, but that
is because the circuit generates a slope, not because it is detecting a
slope. Please look up the circuit and use a proper name for it such as
integrating ADC or dual slope ADC. The integrating converter is not at
all sensitive to the slope of the input signal, otherwise it would not
be able to measure a DC signal which has a slope of zero.

I'm only replying so that others are not confused by your misstatements.



He is probably referring to a CVSD, otherwise known as a Delta Modulator.

Jeff

I don't think so. In fact, I have to say Jerry seems a bit confused in this
particular area, perhaps I have missed something.

ADC tend to have a sample and hold prior to the actual ADC convertor, thus
the value converted is that at the beginning of the sample period OR if
another approach to conversion is used, you get some kind of average over
the conversion period. (There are other techniques but those are the main
ones.)

If you think about, a S/H is required if the rate of change of the input
signal means it can change by 1/2 lsb during the conversion time for a SAR
ADC. This limits the overall BW of the ADC process. (I recall spending
some time convincing a 'seat of the pants engineer' of this when his design
wouldn't work. Even when he adopted the suggested changes he insisted his
design would have worked if the ADC was more accurate. In fact, it would
have made it worse.)


No, Brian, I am not confused. It is a form of delta modulation, but is
used in an ADC. Two samples are taken, 2 or more times the sample rate
(i.e. if the sample rate were 20us, the first sample would be taken
every 20us, with the second sample following by 10us or less). The
difference is converted to a digital value for transmission. On the
other end, the reverse happens.

That is not what you have been describing. Now you are saying that the
ADC samples the amplitude of the signal just as I have been saying, but
now you are adding a step in which the delta is calculated which is what
I was describing with ADPCM (although I should have used the simpler and
more like your approach DPCM).

I have never heard of using it in the way you are describing though.
Even in DPCM the samples are taken at a fixed interval and the delta is
calculated on *every* pair of adjacent samples, not just every other.
So a sample stream of x0, x1, x2, x3, etc would produce delta values of
d0, d1, d2,... not just d0, d1...

You describe two samples being taken for each data sample transmitted,
ignoring the change in signal between x1 and x2. The signal could not
be reconstructed with this data missing.


Yes, the signal can change by 1/2 lsb - but that's true of any ADC.

The sample and hold issue is a red herring and in fact, is counter
productive in a dual slope converter whose point is to average
(integrate) the signal over a period of time filtering higher frequency
content.


For any sufficiently high sample rate (i.e. 3x input signal or more),
this method is never less accurate than a simple voltage detecting ADC,
and in almost every case is more accurate. However, it is a more
complex circuit (on both ends), samples a much smaller analog value and
requires more exacting components and a higher cost (which is typically
the case for any circuit improvements).

The sampling method you describe is *not* different from a voltage
detecting ADC and therefore can't be better. All you are doing that is
different is the analog circuitry is obtaining the slope of the signal
over a short interval and is losing the slope of the signal between the
samples being ignored. Can you explain how it could be *more* accurate?

I suspect you are confusing the efficiency of the data rate with
accuracy. DPCM does provide some compression of the data rate when the
signal is over sampled as you seem to be describing. But it does
nothing to make the samples more accurate.


As I said - we studied them in one of my EE coursed back in the 70's. I
played with them for a while back then, but at the time the ICs were
pretty expensive for a college student.

Does this technique have a name? Any references?

--

Rick