High Speed Analog-to-Digital Converters Glossary
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A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Acquisition Time
This term relates to sampling A/Ds, which use a track/hold amplifier on the input to acquire and hold (to a specific tolerance) the analog input signal. Acquisition time is the time required by the T/H amplifier to settle to its final value after it is placed in the track mode.
Analog Bandwidth
The analog input frequency at which the spectral power of the fundamental frequency (as determined by the FFT analysis) is reduced by 3 dB.
Aperture Delay
The delay between the 50% point of the rising edge of the ENCODE command and the instant at which the analog input is sampled.
Analog to Digital Converter (ADC, A/D Converter)
This device converts real world analog signals into a digital format that can be processed by a computer.
Aperture Uncertainty (Jitter)
The sample-to-sample variation in aperture delay.
Conversion Speed (Rate)
The number of repetitive conversions per second for a full-scale change to specified resolution and linearity.
Crosstalk
Coupling onto one channel being driven by a low level (-40 dBFS) signal when the adjacent interfering channel is driven by a full-scale signal.
Differential Analog Input Resistance, Differential Analog Input Capacitance, and Differential Analog Input Impedance
The real and complex impedances measured at each analog input port. The resistance is measured statically and the capacitance and differential input impedances are measured with a network analyzer.
Differential Analog Input Voltage Range
The peak to peak differential voltage that must be applied to the converter to generate a full scale response. Peak differential voltage is computed by observing the voltage on a single pin and subtracting the voltage from the other pin, which is 180 degrees out of phase. Peak to peak differential is computed by rotating the inputs phase 180 degrees and taking the peak measurement again. Then the difference is computed between both peak measurements.
Differential Nonlinearity (DNL)
The deviation of any code width from an ideal 1 LSB step.
Dynamic Range
The ratio of the maximum output signal to the smallest output signal that can be processed in a system. Usually expressed logarithmically in dB. Dynamic range can be specified in terms of harmonic distortion, signal-to-noise ratio, spurious-free dynamic range, or other AC input-based performance criteria.
Effective Number of Bits (ENOB)
The effective number of bits (ENOB) is calculated from the measured SNR based on the equation:
Encode Pulse Width (Duty Cycle)
Pulse width high is the minimum amount of time that the ENCODE pulse should be left in logic "1" state to achieve rated performance; pulse width low is the minimum time ENCODE pulse should be left in low state. At a give clock rate, these specs define an acceptable Encode duty cycle.
Fast Fourier Transform (FFT)
A computationally efficient mathematical technique that converts digital information from the time domain to the frequency domain for rapid spectral analysis. FFTs generally use a "time weighting" function to compensate for data records with a non-integer number of samples; some popular weighting functions are Hanning Window and 4-term Blackman-Harris.
Flash Converter
Flash ADCs are the fastest type of ADC and use large numbers of comparators. An N-bit flash ADC consists of 2N resistors and 2N-1 comparators.
Full Scale Input Power
Expressed in dBm. Computed using the following equation:
Gain Error
Gain error is the difference between the measured and ideal full scale input voltage range of the worst ADC.
Gain Matching
Expressed in %FSR. Computed using the following equation:
Harmonic
A frequency that is a multiple of the fundamental.
Harmonic Distortion, Second
The ratio of the rms signal amplitude to the rms value of the second harmonic component, reported in dBc.
Harmonic Distortion, Third
The ratio of the rms signal amplitude to the rms value of the third harmonic component, reported in dBc.
Integral Nonlinearity (INL)
The deviation of the transfer function from a reference line measured in fractions of 1 LSB using a "best straight line" determined by a least square curve fit.
Least Significant Bit (LSB)
The right-most bit in a data converter code. LSB size is related to converter resolution.
Low Voltage Differential Signaling (LVDS)
Standard defined in ANSI/TIA/EIA-644 and IEEE 1596.3
Maximum Conversion Rate
The encode rate at which parametric testing is performed.
Minimum Conversion Rate
The encode rate at which the SNR of the lowest analog signal frequency drops by no more than 3 dB below the guaranteed limit.
Monotonicity
A converter is monotonic if the output either increases or remains constant as the input increases.
Most Significant Bit (MSB)
The left-most bit in a data converter code.
Noise
Where Z is the input impedance, FS is the full scale of the device for the frequency in question, SNR is the value for the particular input level and Signal is the signal level within the ADC reported in dB below full scale. This value includes both thermal and quantization noise.
Nyquist Theory
This theory says that if a continuous bandwidth-limited signal contains no frequency components higher than fC, then the original signal can be recovered without distortion if it is sampled at a rate of at least
2 * fC.
Offset Binary
Natural binary code in which the code represents analog values between -FS and +FS. The code 0000…0000 corresponds to -FS.
Offset Error
Offset error is the difference between the measured and ideal voltage at the analog input that produces the midscale code at the outputs. Offset error is given for the worst ADC.
Offset Matching
Expressed in mV. Computed using the following equation:
where OFFmax is the most positive offset error and OFFmin is the most negative offset error.
Output Compliance Range
The range of allowable voltage at the output of a current output DAC. Operation beyond the maximum compliance limits may cause either output stage saturation or breakdown resulting in nonlinear performance.
Output Propagation Delay
The delay between a differential crossing of CLK+ and CLK- and the time when all output data bits are within valid logic levels.
Out of Range Recovery Time
Out of range recovery time is the time it takes for the ADC to reacquire the analog input after a transient from 10% above positive full scale to 10% above negative full scale, or from 10% below negative full scale to 10% below positive full scale.
Power Supply Rejection Ratio
The ratio of a change in input offset voltage to a change in power supply voltage.
Resolution
Number of bits representing an analog signal - generally ranging from 6 to 24. The higher the number of bits, the higher the resolution of the converter. Generally more accurate too.
Signal to Noise and Distortion (SINAD)
The ratio of the rms signal amplitude (set 1 dB below full scale) to the rms value of the sum of all other spectral components, including harmonics but excluding dc.
Signal to Noise Ratio (SNR)
The ratio of the rms signal amplitude (set at 1 dB below full scale) to the rms value of the sum of all other spectral components, excluding the first five harmonics and dc.
Spurious Free Dynamic Range (SFDR)
The ratio of the rms signal amplitude to the rms value of the peak spurious spectral component. The peak spurious component may or may not be a harmonic. It also may be reported in dBc (i.e., degrades as signal level is lowered) or dBFS (i.e., always related back to converter full scale).
Temperature Drift
Temperature drift is specified as the maximum change from the ambient (25°C) value to the value at either TMIN or TMAX. For offset and gain drift, the drift is reported in ppm of full-scale range (FSR) per °C. For reference drift, the drift is reported in ppm per °C.
Total Harmonic Distortion (THD)
The ratio of the fundamental frequency to its harmonic.
Transient Response Time
Transient response time is defined as the time it takes for the ADC to reacquire the analog input after a transient from 10% above negative full scale to 10% below positive full scale.
Twos Complement
Bipolar binary code in which positive and negative codes of the same magnitude sum to all zeros plus a carry. The twos complement is obtained by complementing the MSB of the offset binary code.
Two Tone Intermodulation Distortion
The ratio of the rms value of either input tone to the rms value of the worst third order intermodulation product; reported in dBc.
Two Tone SFDR
The ratio of the rms value of either input tone to the rms value of the peak spurious component. The peak spurious component may or may not be an IMD product. It also may be reported in dBc (i.e., degrades as signal level is lowered) or in dBFS (i.e., always relates back to converter full scale).
Worst Other Spur
The ratio of the rms signal amplitude to the rms value of the worst spurious component (excluding the second and third harmonic) reported in dBc.
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