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AD9022 Arkusz danych(PDF) 8 Page - Analog Devices

Numer części AD9022
Szczegółowy opis  12-Bit 20 MSPS Monolithic A/D Converter
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Producent  AD [Analog Devices]
Strona internetowa  http://www.analog.com
Logo AD - Analog Devices

AD9022 Arkusz danych(HTML) 8 Page - Analog Devices

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–8–
AD9022
REV. B
This will be true only for converters in which perfect quantiza-
tion noise dominates. There may be an upper sample rate,
above which the thermal noise of the converter is the dominant
source of noise. In this case, normalization would be based on
the noise bandwidth of the ADC. For an AD9022 with a typical
SNR of 64 dB and a sample rate of 20 MSPS, the normalized
SNR is equal to 134 dB (64 + 70). Both thermal and quantiza-
tion noise contribute to this number.
The SNR of the input is assumed to be limited by the thermal
noise of the input resistance, or –174 dBm/Hz. The input signal
level is +10 dBm (2 V p-p into 50
Ω). Noise figure of the ADC
can be calculated by:
NF = SNR (in) – SNR (out) = [+10 – (174)] – 134 = 50 dB
Most ADCs detect input voltage levels, not power. Conse-
quently, the input SNR can be determined more accurately by
determining the ratio of the signal voltage to the noise voltage of
the terminating resistor. However, both the input signal and
noise voltage delivered to the ADC are also a function of the
source impedance. The dependence of NF on sample rate,
linearity, source and terminating impedances, and the number
of assumptions required, highlight the weakness of using NF as
a figure of merit for an ADC. The rather large number that
results bolsters this belief by indicating the ADC is often the
weakest link in the signal processing path.
Linearity
The Third Order intercept point for a linear device (with some
nonlinearity) is a good way to predict 3rd order spurious signals
as a function of input signal level. For an ADC, however, this in
an invalid concept except with signals near full scale. As the
input signal is reduced, the performance burden shifts from the
input track-and-hold (T/H) to the encoder. This creates a non-
linear function, as contrasted with the third order intercept
behavior, which predicts an improvement in dynamic range as
the signal level is decreased.
For signals near full scale, the intercept point is calculated the
same as any device:
Intercept Point = [Harmonic Suppression/(N –1)] + Input Power
where N = the order of the IMD (3 in this case)
AD9022 Intercept Point = 80/2 + 3 dBm (7 dBm below full scale)
= 43 dBm
For signals below this level, the spurious free dynamic range
(SFDR) curves shown in the data sheet are a more accurate
predictor of dynamic range. The SFDR curve is generated by
measuring the ratio of the signal (either tone in the two-tone
measurement) to the worst spurious signal, which is observed as
the analog input signal amplitude is swept.
The worst spurious signal is usually the second harmonic or 3rd
order IMD. Actual results are shown on several plots. The
straightline with a slope of one is constructed at the point where
the worst SFDR touches the line. This line, extrapolated to full
scale, gives the SFDR of the ADC. This value can then be used
to predict the dynamic range by simply subtracting the input
level from the SFDR.
It should be noted that all SFDR lines are constructed to be
valid only below a certain level below full scale. Above these
points, the linearity of the device is dominated by the nonlinearities
of the front end and best predicted by the intercept point.
AD9022 NOISE PERFORMANCE
High speed, wide bandwidth ADCs such as the AD9022 are
optimized for dynamic performance over a wide range of analog
input frequencies. However, there are many applications (Imag-
ing, Instrumentation, etc.) where dc precision is also important.
Due to the wide input bandwidth of the AD9022 for a given
input voltage, there will be a range of output codes which may
occur. This is caused by unavoidable circuit noise within the
wideband circuits in the ADC. If a dc signal is applied to the
ADC and several thousand outputs are recorded, a distribution
of codes such as that shown in the histogram below may result.
OUTPUT CODE
2.0
–2.0
–1.0
–1.5
x–3
0
–0.5
0.5
1.0
1.5
ONE STANDARD
DEVIATION = RMS
NOISE LEVEL
x–2
x–1
x
x+1
x+2
x+3
Figure 11. ADC Equivalent Input Noise
The correct code appears most of the time, but adjacent codes
also appear with reduced probability. If a normal probability
density curve is fitted to this Gaussian distribution of codes, the
standard deviation will be equal to the equivalent input rms
noise of the ADC. The rms noise may also be approximated by
converting the SNR, as measured by a low frequency FFT, to
an equivalent input noise. This method is accurate only if the
SNR performance is dominated by random thermal noise (the
low frequency SNR without harmonics is the best measure).
Sixty-three dB equates to 1 LSB rms for a 2 V p-p (0.707 V
rms) input signal. The AD9022 has approximately 0.5 LSB of
rms noise or a noise limited SNR of 69 dB, indicating that noise
alone does not limit the SNR performance of the device (quanti-
zation noise and linearity are also major contributors).
This thermal noise may come from several sources. The drive
source impedance should be kept low to minimize resistor
thermal noise. Some of the internal ADC noise is generated in
the wideband T/H. Sampling ADCs generally have input band-
widths which exceed the Nyquist frequency of one-half the
sampling rate. (The AD9022 has an input bandwidth of over
100 MHz, even though the sampling rate is limited to 20 MSPS.)
Wide bandwidth is required to minimize gain and phase distor-
tion and to permit adequate settling times in the internal ampli-
fiers and T/Hs. But a certain amount of unavoidable noise is
generated in the T/H and other wideband circuits within the
ADC; this causes variation in output codes for dc inputs. Good
layout, grounding and decoupling techniques are essential to
prevent external noise from coupling into the ADC and further
corrupting performance.


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