Datasheet AD7450 (Analog Devices) - 8

AD7450 TERMINOLOGY Aperture Jitter Signal-to-(Noise + Distortion) Ratio
This is the sample-to-sample variation in the effective point in This is the measured ratio of signal-to-(noise + distortion) at time at which the actual sample is taken. the output of the ADC. The signal is the rms amplitude of the
Full Power Bandwidth
fundamental. Noise is the sum of all nonfundamental signals up The full power bandwidth of an ADC is that input frequency at to half the sampling frequency (fS/2), excluding dc. The ratio is which the amplitude of the reconstructed fundamental is reduced dependent on the number of quantization levels in the digitiza- by 0.1 dB or 3 dB for a full-scale input. tion process; the more levels, the smaller the quantization noise. The theoretical signal-to-(noise + distortion) ratio for an ideal
Common-Mode Rejection Ratio (CMRR)
N-bit converter with a sine wave input is given by: The common-mode rejection ratio is defined as the ratio of the power in the ADC output at full-scale frequency, f, to the power Signal–to–(Noise + Distortion) = (6.02 N + 1.76) dB of a 200 mV p-p sine wave applied to the common-mode volt- Thus, for a 12-bit converter, this is 74 dB. age of VIN+ and VIN– of frequency fs:
Total Harmonic Distortion
CMRR(dB) = 10 log (Pf /Pfs) Total harmonic distortion (THD) is the ratio of the rms sum of Pf is the power at the frequency f in the ADC output; Pfs is the harmonics to the fundamental. For the AD7450, it is defined as: power at frequency fs in the ADC output. 2 2 2 2 2 V +V
Integral Nonlinearity (INL)
3 +V4 +V5 +V THD (dB) = 20 2 6 log This is the maximum deviation from a straight line passing V1 through the endpoints of the ADC transfer function. where V1 is the rms amplitude of the fundamental and V2, V3,
Differential Nonlinearity (DNL)
V4, V5, and V6 are the rms amplitudes of the second to the sixth This is the difference between the measured and the ideal 1 LSB harmonics. change between any two adjacent codes in the ADC.
Peak Harmonic or Spurious Noise Zero Code Error
Peak harmonic or spurious noise is defined as the ratio of the This is the deviation of the midscale code transition (111...111 rms value of the next largest component in the ADC output to 000...000) from the ideal V spectrum (up to f IN+ – VIN– (i.e., 0 LSB). S/2 and excluding dc) to the rms value of the fundamental. Normally, the value of this specification is deter-
Positive Gain Error
mined by the largest harmonic in the spectrum, but for ADCs This is the deviation of the last code transition (011...110 to where the harmonics are buried in the noise floor, it will be a 011...111) from the ideal VIN+ – VIN– (i.e., +VREF – 1 LSB), noise peak. after the zero code error has been adjusted out.
Intermodulation Distortion Negative Gain Error
With inputs consisting of sine waves at two frequencies, fa and fb, This is the deviation of the first code transition (100...000 to any active device with nonlinearities will create distortion 100...001) from the ideal VIN+ – VIN– (i.e., –VREF + 1 LSB), after products at sum and difference frequencies of mfa ± nfb where the zero code error has been adjusted out. m and n = 0, 1, 2, or 3. Intermodulation distortion terms are those
Track and Hold Acquisition Time
for which neither m nor n are equal to zero. For example, the The track and hold acquisition time is the minimum time re- second order terms include (fa + fb) and (fa – fb), while the third quired for the track and hold amplifier to remain in track mode order terms include (2fa + fb), (2fa – fb), (fa + 2fb), and (fa –2fb). for its output to reach and settle to within 0.5 LSB of the ap- The AD7450 is tested using the CCIF standard, where two plied input signal. input frequencies near the top end of the input bandwidth are
Power Supply Rejection Ratio (PSRR)
used. In this case, the second order terms are usually distanced The power supply rejection ratio is defined as the ratio of the in frequency from the original sine waves, while the third order power in the ADC output at full-scale frequency, f, to the power terms are usually at a frequency close to the input frequencies. of a 200 mV p-p sine wave applied to the ADC VDD supply of As a result, the second and third order terms are specified frequency fS. separately. The calculation of the intermodulation distortion is PSRR (dB) = 10 log (Pf /Pfs) as per the THD specification, where it is the ratio of the rms sum of the individual distortion products to the rms amplitude of the Pf is the power at frequency f in the ADC output; Pfs is the sum of the fundamentals expressed in dBs. power at frequency fs in the ADC output.
Aperture Delay
This is the amount of time from the leading edge of the sampling clock until the ADC actually takes the sample. Rev. A –7–