ML2264CCP datasheet(12/17 Pages) MICRO-LINEAR

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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:

max

+++

190

275

235

300

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

SNR 49.1dB

HD –58.33dB

VCC = VREF = 5.0V

TA = 25 C

400

200

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

FREQUENCY (kHz)

SNR 48.4dB

HD –62.87dB

VCC = VREF = 5.0V

TA = 25 C

400

200

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

FREQUENCY (kHz)

a) Output Spectrum with fIN = 257kHz, fS = 1MHz

b) Output Spectrum with fIN = 491kHz, fS = 1MHz

Figure 11. Dynamic Performance, Sample and Hold Mode

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