xDAP 7420 Terminology
- A logarithmic representation for the ratio of
signal power in two signals. For each incremental increase
of 10 decibels, the ratio increases by a power of 10. For each
incremental decrease of 10 decibels, the ratio decreases by a power
- A measurement scheme using an
input amplifier having two balanced inputs, a plus input
and a minus input. The amplifier responds in proportion
to the difference between the plus input and minus input signals.
- Common Mode
Voltages presented to the inputs of a differential amplifier can be
decomposed into two parts, a common mode component that is
halfway between the two input voltages, and a balanced differential
mode component that is added or subtracted from the common mode
component. When both inputs of a differential amplifier are
connected to the same voltage signal, the amplifier is said to be driven
in common mode. The ideal response of a differential amplifier
to the common mode input component is zero.
- Common Mode Rejection (Common Mode Rejection Ratio, CMRR)
The ratio of the signal power in the response of a differential amplifier
driven in common mode to the signal power that would be
observed when measuring the signal in a normal differential configuration.
- Multiplexed Sampling
- Analog to digital converters operate
in a multiplexed configuration when multiple input signals
are selected in succession by a digitally-controlled analog switch, and
routed to an analog-to-digital converter. This strategy allows hardware
devices to process more signal channels, but with the disadvantage that
a longer sampling time interval is typically necessary to allow
settling time after each switching transient.
- A strategy employed by Microstar Laboratories to
increase the number of signal channels available with multiplexed sampling.
- Resolution (Quantization)
- The number of bits available to
represent a digitally captured analog signal. The first bit is able to
resolve the range into two parts: the positive half and the negative half.
The next bit can resolve each of these halves into two parts, an upper
and lower part. Continuing this process,
N bits resolve the input
2N distinct and equally-spaced
- Full Range
- For a converter that can produce N distinct output
codes, full range is used somewhat interchangeably to mean
the full set of possible output codes, or the corresponding differential input
voltage swing required to cover all of these output codes.
- Dynamic Range
- The range of signal levels that can be distinguished
from the low-level noise floor. (See Signal to Noise Ratio.)
- Conversion ratio
- The ratio of the number of converter output
codes to the input voltage range required to produce them. The ideal
conversion ratio is the ratio of the number of converter output codes to
the nominal input voltage range. The actual conversion ratio
is the observed number of output codes divided by the voltage range that
produces them. For a perfect conversion, the converter output values will
exactly equal the input differential voltage times the ideal conversion
- Offset Error (bias)
- With a differential amplifier input driven
in common mode, a consistent nonzero measurement is an offset error.
Calibration is effective for nulling offset errors.
- Gain Error
- After correcting for offsets, the gain error
is the difference between an observed conversion ratio and the ideal
conversion ratio, normalized by the ideal conversion ratio. Calibration is
effective for correcting gain errors.
- Nonlinearity Error (Integral Nonlinearity Error, INL)
error can be considered the component of digitization error that does not
arise from random effects. After making a best correction for gain and offset
errors, any non-random differences between actual conversion results and the
results predicted by the conversion ratio are nonlinearity errors.
Because the converter must quantize the measurement according to the
number of available bits of resolution, a nonlinearity error of at least
0.5 bits is unavoidable.
- Ground Noise (Static Noise, Background Noise, Noise Floor)
Random effects in conversion results reflect the combined effects of
quantum noise in devices, radio frequency interference, and so forth.
Noise effects cause a significant portion of the errors observed in
digitized measurements. Static or quiescent noise
is observed by measuring a steady reference voltage during periods of
relative inactivity. Dynamic ground noise, also called
background noise or noise floor, is observed during periods
of intensive sampling activity in the presence of changing high-level
- Signal to Noise Ratio (SNR)
- The ratio of the signal power in
a full-range sinusoidal input signal to the total power in the dynamic
background noise. This is a measure of how much random effects interfere
with measurement accuracy in each sample. It is a best-case estimate;
background noise effects tend to remain about the same level while
the signal power drops significantly as the magnitude of the signal is
reduced below full scale.
- Harmonic Frequencies (Harmonic Distortion)
- Applying a
nonlinear function to a pure sinusoidal input signal results in a
distorted signal having additional frequencies at twice the original
signal frequency, three times the original signal frequency, and so forth.
The introduced frequencies are called harmonics.
- Total Harmonic Distortion (THD)
- With an input converter
driven by a full-range sinusoidal input signal, total harmonic
distortion is the ratio of the signal power in the sinusoidal
input signal to the total signal power in the harmonic frequencies
resulting from nonlinearity.
- Spurious-Free Dynamic Range (SFDR)
- With an input converter
driven by a full-range sinusoidal input signal, the spurious-free
dynamic range is the ratio of the sinusoidal input signal power to
the signal power at any other frequency having the next-highest signal
power. Most often, the spurious frequency is one of the harmonic
frequencies caused by nonlinearity. Since the THD measurement includes
several harmonic frequencies, but SFDR only includes one, the SFDR
figures tends to be slightly higher.
- Signal to Noise+Distortion Ratio (SINAD)
- SINAD is
ratio of the signal power in a full-range sinusoidal input signal to the
total power in all other frequencies resulting from the conversion
process. This is similar to SNR, except that the signal power in the
harmonics is included along with the background noise. SINAD provides an
estimate of measurement errors to expect from consistent and random
effects in combination.
- Effective Number of Bits (ENOB)
- ENOB is an equivalent way to
present SINAD in terms of powers of 2 and "bits" rather than decibels. An
ENOB of 14 bits for a 16 bit converter can be loosely interpreted as
meaning that variations can be expected in the last 2 bits of each
conversion due to nonlinearity and random effects. Accuracy can be
extended beyond the levels suggested by ENOB, however, if measurements
are repeatable and averaging can be applied to reduce the random effects.
- Settling Time (Transient Response)
- When using multiplexed
inputs or dedicated inputs subjected to large step transients, the
drive current and bandwidth limitations of amplifiers prevent tracking of
instantaneous step edges. Settling time describes the length of
ts that should be allowed to
give the conversion circuits time to reach the correct values within
- Slewing (Rate Limiting)
- A condition in which an input amplifier
is subjected to a large input level change and uses all available current
internally for driving its output stage. This results in a fast linear
ramp output behavior, rather than tracking the input change accurately.
Sinusoidal waveforms with sufficiently large magnitude and frequency
will be distorted if the time derivative of the waveform exceeds the
slew rate limit.
- Variability in the times at which discrete
events takes place. Jitter causes uncertainty about which precise
moments a signal sample is captured or a switching event is detected.
- Hold Time
- The amount of time that a signal must be
held at a constant level to be sure that other circuits will respond
- Setup Time
- An amount of time that must be allowed after
sending a signal for changes to take effect.
- A delay in receiving the results of a
conversion. For example, a sample might require 1 microsecond to be
transported from the converter device, 2 milliseconds to be moved
to buffer memory, and 25 milliseconds to be transferred to host memory.
The delays for receiving the data typically do not matter unless it is
important to respond very quickly.
- A side effect of stray coupling between signal lines,
resulting in small measurement changes that increase apparent correlation
between signals measured on the lines. Such effects are typically
introduced by capacitive and inductive coupling between lines that are
physically close in cabling and on circuit boards.
- Charge injection
- An amount of charge capacitively coupled into
converter inputs each time a signal is captured. Though the effects of
each charge injection are very small, when the device is clocked at a
very high rate, the cumulative effects act like a current flow,
interacting with the input circuit impedance and producing an observable
shift in the measurements of input signals.
- Charge cross-injection
- A contributor to crosstalk resulting
from multiplexer operation. When connected to one line, stray capacitances
at the switching device charge up to balance with the line voltage.
When switched to a new signal source, the voltage on this capacitance
is temporarily out of balance with the new line, causing a very small but
rapid surge in charge until voltages again equalize. With poorly conditioned
input signals, the effects can be amplified by high impedances and stray
dynamics. The effects are greatest when multiplexer switching occurs at
a high rate.