ML2264CCP View Datasheet(PDF) - Micro Linear Corporation
Part Name
Description
MFG CO.
'ML2264CCP' PDF : 17 Pages View PDF
ML2264
1.6 DYNAMIC PERFORMANCE
1.6.1 Sinusoidal Inputs
Since the ML2264 has an internal sample and hold, the
device can digitize high frequency sinusoids with little or
no signal degradations. Using the Nyquist criteria, the
highest frequency input to the converter could
theoretically be 1/2 the sampling rate (fS). Any frequency
components above fS/2 will be aliased below fS/2. In most
applications, these aliased components cause
unacceptable distortion and must be filtered out of the
input. If the input frequency is too close to fS/2, then the
requirements on the anti-alias filter become difficult to
impossible to realize with standard component and
tolerances. In most practical applications, the highest
input frequency has to be limited to 1/3 to 1/4 of fMAX in
order to relax the filtering requirements enough to make a
realizable anti-alias filter.
The maximum sampling rate (fmax) for the ML2264 in the
WR-RD mode, (tRD < tINTL) can be calculated as follows:
fmax
=
t WR
+
1
tRD +
tRI
+
tP
fmax
=
190ns
+
275ns
1
+ 235ns
+
300ns
fmax = 1.000 MHz
tWR = Write Pulse Width
tRD = Delay Time between WR and RD Pulses
tRI = RD to INT Delay
tP = Delay Time between Conversions
This permits a maximum sampling rate of 1MHz for the
ML2264. The dynamic performance specifications (SNR,
HD, IMD, and FR) for the ML2264 are all specified at
250kHz, which is approximately 1/4 of the sampling rate, fS.
In applications where aliased frequency components are
acceptable and filtering of the input signal is not needed,
or where a filter with a steep amplitude response is
available, the user can apply an input sinusoid higher than
250kHz to the device. Note, however, that as the input
frequency increases above 500kHz, dynamic performance
degradation will occur due to the finite bandwidth of the
internal sample and hold.
The Figure 11 plots are 4096 point FFT’s of the ML2264
converting a 257kHz and a 491kHz, 0 to 4.5V, low
distortion sine wave input. The ML2264 samples and
digitizes at its specified accuracy, dynamic input signals
with frequency components up to the Nyquist frequency
(one-half the sampling rate). The output spectra yields
precise measure-ments of the input signal level, harmonic
components, and signal to noise ratio up to the 8-bit level.
The near ideal signal to noise ratio is maintained
independent of increasing analog input frequencies to
500kHz.
1.6.2 Signal-To-Noise Ratio
Signal-to-noise ratio (SNR) is the measured signal to noise
at the output of the converter. The signal is the rms
magnitude of the fundamental. Noise is the rms sum of all
the nonfundamental signals up to half the sampling
frequency. SNR is dependent on the number of
quantization levels used in the digitization process; the
more the levels, the smaller the quantization noise. The
theoretical SNR for a sine wave is given by
SNR = (6.02N + 1.76) dB
where N is the number of bits. Thus for ideal 8-bit
converter, SNR = 49.92 dB.
1.6.3 HARMONIC DISTORTION
Harmonic distortion is the ratio of the rms sum of
harmonics to the fundamental. Total harmonic distortion
(THD) of the ML2264 is defined as
0
–10 SNR 48.4dB
–20
HD –62.87dB
VCC = VREF = 5.0V
–30 TA = 25 C
–40
–50
–60
–70
–80
–90
–100
–110
–120
0
200
400
FREQUENCY (kHz)
a) Output Spectrum with fIN = 257kHz, fS = 1MHz
0
–10 SNR 49.1dB
–20
HD –58.33dB
VCC = VREF = 5.0V
–30 TA = 25 C
–40
–50
–60
–70
–80
–90
–100
–110
–120
0
200
400
FREQUENCY (kHz)
b) Output Spectrum with fIN = 491kHz, fS = 1MHz
Figure 11. Dynamic Performance, Sample and Hold Mode
12
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