This article is about the ADC/DAC design classic question and answer, covering clock cycle, common mode voltage, gain error, differential phase error, intermodulation distortion and other common problems.
1. What is the small signal bandwidth (SSBW)?
Small Signal Bandwidth (SSBW) is the small signal bandwidth when the output amplitude of the specified amplitude input signal and the specified frequency is lower than the output amplitude of the low frequency. .
2. What is the common mode voltage (VCM)?
The Common Mode Voltage (VCM) is the same DC input voltage on the two pins of the differential input.
3. What is the MSB (most significant bit)?
MSB (Most Significant Bit) is the bit with the largest value or weight. Its value is half of full scale.
4. What is the sampling (aperture) delay?
Sampling (Aperture) Delay is the time it takes for the trailing edge of the clock input to turn on the sampling switch. The sample/hold circuit effectively stops the input signal capture and enters the "hold" mode to determine the sample after the clock delay.
5. What is the full scale (FS) input range?
Full Scale Input Range refers to the input range of the digitized input voltage on the analog-to-digital converter, which is neither below this range nor beyond this range. For example, V REF + = 3.5V and VREF - = 1.5V, FS = (VREF + ) - (VREF - ) = 2.0V.
6. What is the clock duty cycle?
Clock Duty Cycle is the ratio of the high time of the clock waveform to the total time of one clock cycle.
7. What is a valid number of bits (ENOB, or a valid bit)?
The effective number of bits (ENOB, or EBITcTIve Bits) is the ratio of signal-to-noise ratio to distortion, or another way of expressing SINAD. The ENOB is defined as (SINAD -1.76) / 6.02. This number of bits (ENOB) indicates that the converter is equivalent to an ideal analog to digital converter.
8. What is the gain error?
The gain error is the difference between the actual input voltage and the ideal input voltage when the first code and the last code are converted. That is, the difference is: full scale - 2 LSB.
9. Many analog-to-digital converters in the data sheet provide three capacitors on the Va, Vd, and Vref pins. Are these three capacitors necessary?
Depending on the layout of the particular device and board, one or two capacitors may be sufficient. Larger capacitors, usually 5 to 10? F's provide low-impedance, high-capacity storage that ensures voltage stability during conversion. Smaller capacitors absorb higher frequency noise spikes. If the printed circuit board has a very good layout for low noise operation and does not contain a microcontroller or other noisy digital logic, fewer capacitors may be required. However, in order to ensure the accuracy of the ADC operating level, it is generally best to follow the recommendations in the data sheet.
10. What is zero error?
The zero error of the ADC's two-stage output is the difference between the theoretical input voltage (typically centered plus 1/2 LSB) and the actual input voltage. This actual input voltage causes the output to transition from 0 to 1.
11. What is the output hold time?
The output hold time is the length of time during which valid data is output after the input clock edge.
12. What is the resolution?
The resolution is an analog increment, which is equivalent to a change in the 1 LSB converter code. The resolution is also defined as the number of converter bits (n). The number of numeric codes is equal to 2^n, where "n" is the number of bits. As an example, a 12-bit converter analog signal and a 2^12 = 4096 digitally encoded mapping relationship. The resolution of a 12-bit A/D converter is the full scale of the input voltage divided by 2^12, or 4096, without causing the output code to go out of range.
13. What is the differential phase error?
DifferentTial Phase Error is the difference in output phase of a small signal sine wave reconstructed at two different DC input levels.
14. What is the dynamic indicator of the A/D converter?
The dynamic specifications of the analog-to-digital converter (DynamicSpecificaTIons) relate to those alternating current (AC) input signals. These include signal-to-noise ratio (SNR), SINAD (signal noise + distortion), ENOB (effective number of bits), THD (total harmonic distortion), IMD (intermodulation distortion), FPBW (full power bandwidth), SSBW (small Signal bandwidth).
15. What is Intermodulation Distortion (IMD)?
Intermodulation Distortion is an additional spectral component that does not appear at the input as the frequency of two sinusoids simultaneously acts on the input of the analog-to-digital converter. It is defined as the ratio of the energy in the intermodulation product to the total energy in the original frequency. Intermodulation distortion (IMD) is usually expressed in decibels (dB).
16. What is the gain temperature coefficient (full scale temperature coefficient)?
The gain temperature coefficient (full-scale temperature coefficient) is the ratio of the amount of change in gain error to the amount of change in temperature. Usually expressed in parts per million per degree Celsius (ppm/°C).
17. What is Total Harmonic Distortion (THD)?
Total Harmonic Distortion (THD), expressed in dB or dBc, refers to the total harmonic level (National Semiconductor Analog-to-Digital Converter is 9 harmonic segments) and the multiplier of the input signal appears at the output level. The total harmonic distortion (THD) is calculated as follows: THD=“sqrtâ€[ ( f2xf2 + f3xf3 +f4xf4 + f5xf5 + f6xf6 + f7xf7 + f8xf8 + f9xf9 + f10xf10) / (f1xf1) ] f1 is the fundamental frequency of the input signal, f2 The nine harmonic frequencies to f10 are the multiples of the fundamental frequency.
18. What is the zero scale offset error?
The zero-scale offset error of a unipolar output analog-to-digital converter is the difference between the ideal input voltage (1/2 LSB) and the actual input voltage, causing the conversion of the output code from 0 to output code 1.
19. What is full power bandwidth (FPBW)?
Full Power Bandwidth (FPBW) is the frequency at which the full-scale input is below its low frequency value when the reconstructed output fundamental frequency drops to 3 dB.
20. What LSB (least significant digit)?
LSB (Least Significant Bit) is the smallest value or weight of all bits. This value is m*VREF/2 n, where: "m" is the primary and is the reference range factor (this is the most common unit), where "n" is the resolution of the analog-to-digital converter.
21. How can I eliminate overshoot and/or undershoot on the analog-to-digital converter clock and control lines?
Overshoot and/or undershoot are caused by the mixing of high speed signal edges and mismatched signal terminals. Add a 47 to 100Ω resistor in series to the input with the resistor as close as possible to the clock source. The purpose is to match the signal impedance on the clock line, and the transmission line should also be considered this way. A small additional power supply is used in series with the terminal, which is usually sufficient to reduce the swing effect.
22. What is the output delay?
The output delay is the delay from the trailing edge of the input clock to the data update that appears on the output pin.
23. What is aperture jitter?
Aperture jitter is the change in aperture delay between sampled values. Aperture jitter appears as input noise.
24. What is the signal noise and distortion ratio (S/(N+D) or SINAD)?
Signal noise and distortion ratio (S/(N+D) or SINAD), expressed in decibels (dB), the ratio of the effective value of the output signal present at the output to the rms value of all other spectral components, the spectral components including the clock below The frequency is half the harmonics of the spectrum, but does not include DC.
25. What is a full-scale step response?
The full-scale step response, defined as the time required for VIN to change from VREF- to VREF+, or from VREF+ to VREF-, sets the converter to adequate recovery and meets its rated accuracy.
26. What is the channel delay (latency)?
The channel delay (latency) is the number of clock cycles between when the conversion is initiated and when the converted data appears at the output driver stage. Data is sampled for any given sample using the channel delay plus the output delay. Every other clock cycle, new data is available, but the data lags behind the channel delay plus the output delay.
27. What is the static indicator of the analog/digital converter?
The static indicator is an indicator of the direct current (DC) signal input of the analog to digital converter. These include gain error, offset error, and differential and integral linearity errors.
28. What is Total Unadjusted Error (TUE)?
Total Unadjusted Error (TUE) is the maximum deviation of the voltage associated with the input voltage range at the center of the digital code. Total unadjusted errors include offset error, gain error, and differential and integral nonlinearity errors.
29. What is the full scale error?
The full-scale error is the final code conversion from the ideal 1 1/2 LSB to the VREF + to the wide range of measurements, and is defined as: VFSE = Vmax + 1.5 LSB- VREF + , where Vmax is converted to the maximum code Voltage, which can be expressed in volts, the least significant bit or the percentage of the full-scale range.
30. What is the conversion time?
The conversion time is the time required for the analog-to-digital converter to completely convert. The conversion time does not include the sampling time, the multiplexer sets the time, or completes other parts of a conversion cycle, and the conversion time may be less than the throughput time.
31. What is the power supply rejection ratio (PSRR)?
Power Supply Rejection Ratio can be divided into two specifications. The DC power supply rejection ratio (DC PSRR ) is the ratio of the amount of change in a particular parameter (eg, full-scale error) to a specified change in supply voltage. The AC Power Supply Rejection Ratio (AC PSRR) is a measure of the specific frequency and amplitude of a signal superimposed on a power supply, the ratio of the output amplitude of this signal on the output, and its amplitude on the power supply pin. The power supply rejection ratio (PSRR) is usually expressed in decibels.
32. What is a missing code?
Missing codes are those whose output codes are ignored or will never appear in the analog-to-digital converter output. These codes cannot pass any input values.
33. What is the throughput rate?
The throughput rate is the highest continuous conversion rate of the analog-to-digital converter.
34. What is the signal to noise ratio (SNR)?
The signal-to-noise ratio (SNR) is a ratio expressed in decibels (dB) that is the sum of the effective value of the output signal present at the output and all other spectral components (less than half the sampling frequency, except for the harmonic and DC components). The ratio of valid values. The signal-to-noise ratio (SNR) is the number of decibels of the ratio of the effective value of the signal level to the effective value of various noises (including quantization noise, thermal noise, white noise, etc.). The signal refers to the effective value of the fundamental component, and the noise refers to the effective value of all non-fundamental components below the Nyquist frequency (except for the harmonic component and the DC component).
35. What is the throughput time?
The throughput time is the time it takes for the converter to complete a conversion. The throughput time includes the setup time, sampling time, conversion time, and output display time of any multiplexer.
36. What is DC common mode error?
DC Common-Mode Error is a specification for the differential input of an analog-to-digital converter. The amount of change in the output code occurs when the analog voltages on the two inputs are changed by the same value. It is usually represented by LSBs.
37. What is the offset error?
The offset error of the ADC is defined as the difference between the input voltage of the ADC when the lowest bit is set to the "1" state and the input voltage when the lowest bit is theoretically set to the "1" state.
38. Should the power ground of the printed circuit board be used as a common ground for digital and analog?
Yes, they should be the same. But it's important to carefully route all power and signals to separate the ground current from the power and signal.
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