|
楼主 |
发表于 2008-2-12 17:03:34
|
显示全部楼层
Gain MatchingGain matching indicates how well the gains of all channels in a multichannel ADC are matched to each other. To calculate gain matching, apply the same input signal to all channels, and report the maximum deviation in gain, typically in dB.
Glitch ImpulseGlitch impulse is the voltage transient that appears at the DAC output when a major-carry transition occurs. Typically measured as nV•s, it equals the area under the curve on a voltage-vs-time graph.
HarmonicA harmonic of a periodic signal is a sinewave multiple of the signal's fundamental frequency.
Integral Nonlinearity (INL) ErrorFor data converters, INL is the deviation of an actual transfer function from a straight line. After nullifying offset and gain errors, the straight line is either a best-fit straight line or a line drawn between the end points of the transfer function. INL is often called 'relative accuracy.'
See also application note INL/DNL Measurements for High-Speed Analog-to-Digital Converters (ADCs).
INL for an ADC and a DAC.
Intermodulation Distortion (IMD)IMD is a phenomenon in which nonlinearity in a circuit or device creates new frequency components not in the original signal. IMD includes the effects of harmonic distortion and two-tone distortion. It is measured as the total power of those selected intermodulation products (i.e., IM2 through IM5) to the total power of the two input signals, f1 and f2. The signals f1 and f2 are of equal amplitude and very close to one another in frequency. The 2nd- to 5th-order intermodulation products are as follows: - 2nd-order intermodulation products (IM2): f1 + f2, f2 - f1
- 3rd-order intermodulation products (IM3): 2 x f1 - f2, 2 x f2 - f1, 2 x f1 + f2, 2 x f2 + f1
- 4th-order intermodulation products (IM4): 3 x f1 - f2, 3 x f2 - f1, 3 x f1 + f2, 3 x f2 + f1
- 5th-order intermodulation products (IM5): 3 x f1 - 2 x f2, 3 x f2 - 2 x f1, 3 x f1 + 2 x f2, 3 x f2 + 2 x f1.
Least Significant Bit (LSB)In a binary number, the LSB is the least weighted bit in the group. Typically, the LSB is the furthest right bit. For an ADC or DAC, the weight of an LSB equals the full-scale voltage range of the converter divided by 2N, where N is the converter's resolution. For a 12-bit ADC with a unipolar full-scale voltage of 2.5V, 1LSB = (2.5V/212) = 610µV
Major-Carry TransitionAt the major-carry transition (around mid-scale), either the MSB changes from low to high and all other bits change from high to low, or the MSB changes from high to low and all other bits change from low to high. For example, 01111111 to 10000000 is a major-carry transition. Major-carry transitions often produce the worst switching noise. (See Glitch Impulse.)
MonotonicA sequence increases monotonically if for every n, Pn + 1 is greater than or equal to Pn. Similarly, a sequence decreases monotonically if for every n, Pn + 1 is less than or equal to Pn. A DAC is monotonic if the analog output always increases as the DAC-code input increases. An ADC is monotonic if the digital output code always increases as the ADC analog input increases. A converter is guaranteed monotonic if the DNL error is no greater than ±1LSB
Most Significant Bit (MSB)In a binary number, the MSB is the most weighted bit in the number. Typically, the MSB is the left-most bit.
Multiplying DAC (MDAC)A multiplying DAC allows an AC signal to be applied to the reference input. By feeding the signal of interest into the reference input and by using the DAC codes to scale the signal, the DAC can be used as a digital attenuator.
No Missing CodesAn ADC has no missing codes if it produces all possible digital codes in response to a ramp signal applied to the analog input.
Nyquist FrequencyThe Nyquist principle states that, to allow an analog signal to be completely represented with no aliasing effects, the ADC's sampling rate must be at least twice the maximum bandwidth of the signal. This maximum bandwidth is called the Nyquist frequency.
Offset Binary CodingOffset binary is a coding scheme often used for bipolar signals. In offset binary coding, the most negative value (negative full scale) is represented by all zeros (00...000) and the most positive value (positive full scale) is represented by all ones (11...111). Zero-scale is represented by a one (MSB) followed by all zeros (10...000). This scheme is similar to straight binary coding, which is typically used for unipolar signals. (See Binary Coding, Unipolar.)
Offset Error (Bipolar)The measurement of offset error in bipolar converters is similar to that of offset error in unipolar converters. However, the error measured at zero-scale is at the midpoint of the bipolar transfer functions. (See Offset Error (Unipolar).)
Offset Error (Unipolar)Offset error, often called 'zero-scale' error, indicates how well the actual transfer function matches the ideal transfer function at a single point. For an ideal data converter, the first transition occurs at 0.5LSB above zero. For an ADC, the zero-scale voltage is applied to the analog input and is increased until the first transition occurs. For a DAC, offset error is the analog output response to an input code of all zeros.
Offset error for an ADC and a DAC.
Offset Error DriftOffset-error drift is the variation in offset error due a change in ambient temperature, typically expressed in ppm/°C.
OversamplingFor an ADC, sampling the analog input at a rate much higher than the Nyquist frequency is called oversampling. Oversampling improves the ADC's dynamic performance by effectively reducing its noise floor. Improved dynamic performance leads, in turn, to higher resolution. Oversampling is the basis of sigma-delta ADCs.
See also application note Demystifying Sigma-Delta ADCs
Phase-MatchingPhase matching indicates how well matched are the phases of identical signals applied to all channels in a multichannel ADC. Phase matching is the maximum deviation in phase among all the channels, and is typically reported in degrees.
Power-Supply Rejection (PSR)Power Supply Rejection Ratio (PSRR) is the ratio of the change in DC power supply voltage to the resulting change in full-scale error, expressed in dB.
Quantization ErrorFor an ADC, quantization error is defined as the difference between the actual analog input and the digital representation of that value. (See 'Quantization.')
Ratiometric MeasurementInstead of a constant-valued reference voltage, a fraction of the signal applied to the transducer (i.e., the load cell or bridge) is applied to the ADC's voltage reference input. This type of measurement, called ratiometric, eliminates any errors introduced by changes in the reference voltage. An example of ratiometric measurement using a resistive bridge is shown in the figure below.
Ratiometric measurement using resistive bridge network. |
|