Transcript PH4705/ET4305: A/D: Analogue to Digital Conversion

PH4705/ET4305: A/D: Analogue to Digital Conversion
Digital World
Background:
Binary numbers, BCD, Octal, Hex
Computer word size (8,16…..bits)
Digital representation of an analogue signal
is always an approximation
PH4705/ET4305: A/D: Analogue to Digital Conversion
Key A/D Characteristics:
Resolution
Accuracy
Quantization Error, noise
Conversion Speed/Time of Reading
Parallel, serial output
A/D types:
Traditional, modern
PH4705/ET4305: A/D: Analogue to Digital Conversion
Resolution
The number of bits in the
output of an A/D
converter defines its
RESOLUTION
Resolution is commonly
considered in terms of the
Least Significant Bit, LSB,
Vfs=full scale voltage
N=number of bits
PH4705/ET4305: A/D: Analogue to Digital Conversion
A 16-bit converter operating with a
full scale range of 5 V would have
a LSB calculated as
PH4705/ET4305: A/D: Analogue to Digital Conversion
Accuracy versus Resolution
A: both low
B: accurate (all on target)
but low resolution
C: high resolution (tight group)
but not so accurate
D: high resolution and accurate
PH4705/ET4305: A/D: Analogue to Digital Conversion
Quantization Error
Transfer function of an ideal
3 bit A/D.
The straight line is analogue in
analogue out
The staircase is the best an
A/D can do
PH4705/ET4305: A/D: Analogue to Digital Conversion
Because the digital output must assume one of the available
discrete binary codes there will be a deviation from the ideal
whenever that code does not match the analogue value.
This quantization error is ±½LSB
Other sources of noise are still there, the aim is to keep them
<±½LSB
Jitter: The output of an A/D may constantly cycle ±1LSB
PH4705/ET4305: A/D: Analogue to Digital Conversion
Conversion Speed/Time of Reading
A further source of potential error is related to the dynamic
behavior of A/D:
A/D conversion takes time, during that time the analogue
signal may have changed.
Where in the time taken for conversion was the analogue
input sampled? This is usually fixed by the clock signal
of the A/D but exactly where is that?
This time uncertainty is termed APERTURE ERROR.
PH4705/ET4305: A/D: Analogue to Digital Conversion
Simple A/D:
The counter starts at zero,
the output of the D/A is a
staircase that is compared
with the analogue input
by the comparator, when it
equals (or exceeds) it the
comparator output switches
stopping the clock to the
counter
PH4705/ET4305: A/D: Analogue to Digital Conversion
Again using a 3bit A/D as
Illustration. The analogue
input is converted to 101
in 5 clock cycles.
The disadvantage of this
arrangement is that
conversion time is not fixed,
it is longer for higher
Voltages.
PH4705/ET4305: A/D: Analogue to Digital Conversion
Successive Approximation A/D:
This get there faster.
Bits in the counter are toggled
from the MSB (most significant bit)
end, so ½, ¼, 1/8 of full scale are
added or subtracted until the input
value is reached ( to within the
resolution).
The diagram again uses 3 bits to
illustrate the principle.
PH4705/ET4305: A/D: Analogue to Digital Conversion
Modern A/Ds use
much more complex
conversion techniques
to achieve high
accuracy fast.
For discussion see
Bill Klein’s article on
the web site.
To find out more
Wikipedia is a good s
tarting point.
PH4705/ET4305: A/D: Analogue to Digital Conversion
A/D Data Output:
How many bits? Depends on required speed
of conversion, resolution required and
complexity (cost). Typically between 8 and
16 bits, with 8 and 12 bits being common.
(16 bits is, in most applications, much higher resolution
than is necessary as ±1LSB represents and error of
0.00002% and other noise is very difficult to keep down
to that level.)
PH4705/ET4305: A/D: Analogue to Digital Conversion
A/D output cont:
Parallel data output is common particularly
in older designs and at higher resolutions.
Some newer A/D chips have a serial output,
they are intended for microprocessor and
embedded systems and take advantage of
the higher clock speeds of modern ICs.
PH4705/ET4305: A/D: Analogue to Digital Conversion
Auxiliary circuits:
Sample and Hold
Fixes a time varying analogue signal whilst
conversion takes place
Analogue multiplexer
Enables an A/D to sample more than one
analogue input
PH4705/ET4305: A/D: Analogue to Digital Conversion
An Analogue Multiplexer is simply a switch
that connects multiple analogue inputs
signals to a single output one at a time.
It is used to concentrate analogue inputs
into a single information stream for further
processing, either because the analogue
inputs change only slowly so need not be
continuously monitored, or because the
following processing is complex and the
expense of a processor for each signal
cannot be justified.
Analogue multiplexers use FETs as
switches and which input is connected to
the output is usually determined by a digital
address word.
Typical IC devices have either 8 or 16
inputs.
PH4705/ET4305: A/D: Analogue to Digital Conversion
IC analogue
multiplexers come in
two variants. For
single ended or
differential inputs
(matching our two
types of sensor
circuit).
The device shown is
DG406 single ended
and DG407
differential.
Note the addressing
a enable logic.
PH4705/ET4305: A/D: Analogue to Digital Conversion
Another common IC is the DG408/409 with either 8 single ended or 4
differential inputs.
Channel on-resistance: 100Ωmax, leakage (when off): 5nAmax, channel
characteristics closely matched. Digital input TTL/CMOS compatible, Power
supply ±18v max (dual or single, input voltage range- rail to rail, Low power
1.25mW max.
PH4705/ET4305: A/D: Analogue to Digital Conversion
A typical application of an analogue multiplexer, gathering multiple
analogue signals through a sample and hold and an analogue to
digital converter into a digital data stream. The operating principle of
the sample and hold is illustrated, a sample of the analogue input
voltage is stored on the hold capacitor, which is impedance buffered
by the Op Amps on either side. Sample/hold switching is coordinated
with switching the multiplexer.
PH4705/ET4305: A/D: Analogue to Digital Conversion
Typical devices and specification