On 3/8/2015 3:31 PM, Brian Reay wrote:
On 08/03/15 18:46, rickman wrote:
On 3/8/2015 9:53 AM, Brian Reay wrote:
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.

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

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).

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.


Ok Jerry. You can, of course, find the rate of change (slope) by that
method if you know ( or assume) the signal is either only increasing or
decreasing between the samples. (A Nyquist matter).

However, the 1/2 lsb matter I mentioned is more for during the
conversion,
rather that for different samples. It is particularly important for
slower
ADC types, such as SAR implementations.

Can you explain your 1/2 lsb effect? What type of ADC are you referring
to? Different ADC types do require a S/H on the input for signals that
are not *highly* oversampled. For example a flash converter can mess up
and be quite a bit off if the signal is slewing during conversion. Same
with SAR converters. But I don't know of any effect where 1/2 lsb is a
threshold.

What threshold would you expect? As I recall, 1/2 lsb is the limit to
ensure that the conversion would be the 'same' over the conversion time.

I'm not sure what you mean by "the conversion would be the 'same' over
the conversion time", but I don't see how 1/2 lsb is any magic threshold.

If you are working with a flash converter, there are a number of
comparators each with a different threshold. The input signal could be
right at the edge of one of these thresholds so that a very tiny change
in the input signal will cause that threshold to be crossed during the
conversion.

Maybe I'm not understanding your point.

--

Rick