J
Jerry Avins
- Jan 1, 1970
- 0
It seems to me important to agree on criteria for deciding whether a
particular circuit or signal is digital or analog
One criterion is intended use; there seems to be general agreement about
that, so I don't address it here. Another criterion is the nature if the
signal or circuit itself, without reference to intentions. That is the
topic of this short essay.
*SIGNALS*
A digital signal consists of a sequence of a fixed number of discrete
states, with no intermediate states are allowed. Outside the realm of
quantum mechanics, a continuous signal is not digital. It can be made
digital by quantizing it, and different quantizers will convert the same
analog signal into different quantized signals. As far as I know,
digital signals exist only as abstractions and in computer circuits. I
would be delighted to learn of exceptions.
*CIRCUITS*
A circuit intended for a digital application has a fixed number of
discrete input and output states. The useful states are limited to those
common to both input and output. For simplicity of design, the number of
states is usually chosen to be 2, although other arrangements are
possible and some have been utilized. States are represented at outputs
as voltage or current ranges, and recognized at inputs in the same way.
The thresholds need not be the same at input or output. For example, the
specification for the 74LS logic family requires a high-level (1) to
equal or exceed 2 volts and a low level (0) to be no more than .8 volts
at the input, and guarantees that a 1 will be at least 2.4 volts and a 0
no more than .5 volts at the output. That specification makes the
devices well suited for digital use, but it defines them as analog
devices by defining voltages which are, as far as states go, ambiguous.
A CMOS CD4011B makes that point better. It is a quad 2-input NAND gate.
With a 15-volt supply, a 1 is 11 volts or greater, and a 0 is 4 volts or
less. The output swings between .05 and 14.95 volts when lightly loaded,
and can sink or source more than 5 ma at 4 and 11 volts. The /intended/
use of a NAND gate makes it digital devices, but these are inherently
analog by construction. Their inherent analog nature seen by connecting
a 1 megohm resistor from output to the inputs tied together. You will
have an analog amplifier with 23dB voltage gain and much higher power
gain. With capacitive coupling, the four gates can be connected as two H
bridges that will deliver 18 milliwatts per channel into 1500 ohms. To
be truly digital, a device must have discrete states, and be incapable
of exhibiting any other state.
That situation is approximated by cross coupling a pair of the gates to
make a set-reset flip-flop. Now there are only two stable states, but
the in-between states still exist as transient states passed through
when the device is in transition. Rise and fall times greater than zero,
and the well known but often ignored metastable state attest to that.
http://www.interfacebus.com/Design_MetaStable.html As far as I know,
digital circuits exist only as abstractions and on schematics. I would
be delighted to learn of exceptions.
Jerry
particular circuit or signal is digital or analog
One criterion is intended use; there seems to be general agreement about
that, so I don't address it here. Another criterion is the nature if the
signal or circuit itself, without reference to intentions. That is the
topic of this short essay.
*SIGNALS*
A digital signal consists of a sequence of a fixed number of discrete
states, with no intermediate states are allowed. Outside the realm of
quantum mechanics, a continuous signal is not digital. It can be made
digital by quantizing it, and different quantizers will convert the same
analog signal into different quantized signals. As far as I know,
digital signals exist only as abstractions and in computer circuits. I
would be delighted to learn of exceptions.
*CIRCUITS*
A circuit intended for a digital application has a fixed number of
discrete input and output states. The useful states are limited to those
common to both input and output. For simplicity of design, the number of
states is usually chosen to be 2, although other arrangements are
possible and some have been utilized. States are represented at outputs
as voltage or current ranges, and recognized at inputs in the same way.
The thresholds need not be the same at input or output. For example, the
specification for the 74LS logic family requires a high-level (1) to
equal or exceed 2 volts and a low level (0) to be no more than .8 volts
at the input, and guarantees that a 1 will be at least 2.4 volts and a 0
no more than .5 volts at the output. That specification makes the
devices well suited for digital use, but it defines them as analog
devices by defining voltages which are, as far as states go, ambiguous.
A CMOS CD4011B makes that point better. It is a quad 2-input NAND gate.
With a 15-volt supply, a 1 is 11 volts or greater, and a 0 is 4 volts or
less. The output swings between .05 and 14.95 volts when lightly loaded,
and can sink or source more than 5 ma at 4 and 11 volts. The /intended/
use of a NAND gate makes it digital devices, but these are inherently
analog by construction. Their inherent analog nature seen by connecting
a 1 megohm resistor from output to the inputs tied together. You will
have an analog amplifier with 23dB voltage gain and much higher power
gain. With capacitive coupling, the four gates can be connected as two H
bridges that will deliver 18 milliwatts per channel into 1500 ohms. To
be truly digital, a device must have discrete states, and be incapable
of exhibiting any other state.
That situation is approximated by cross coupling a pair of the gates to
make a set-reset flip-flop. Now there are only two stable states, but
the in-between states still exist as transient states passed through
when the device is in transition. Rise and fall times greater than zero,
and the well known but often ignored metastable state attest to that.
http://www.interfacebus.com/Design_MetaStable.html As far as I know,
digital circuits exist only as abstractions and on schematics. I would
be delighted to learn of exceptions.
Jerry