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ML2252CCP View Datasheet(PDF) - Micro Linear Corporation

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ML2252CCP Datasheet PDF : 13 Pages
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1.4 REFERENCE
The voltage applied to the +VREF and –VREF inputs
defines the voltage span of the analog input (the
difference between VINMAX and VINMIN) over which the
256 possible output codes apply. The devices can be used
in either ratiometric applications or in systems requiring
absolute accuracy. The reference pins must be connected
to a voltage source capable of driving the reference input
resistance, typically 20k.
In a ratiometric system, the analog input voltage is
proportional to the voltage used for the A/D reference.
This voltage is typically the system power supply, so the
+VREF pin can be tied to VCC and –VREF tied to GND. This
technique relaxes the stability requirements of the system
reference as the analog input and A/D reference move
together maintaining the same output code for a given
input condition.
For absolute accuracy, where the analog input varies
between specific voltage limits, the reference pins can be
biased with a time and temperature stable voltage source.
+VREF and –VREF can be at any voltage between VCC and
GND. In addition, the difference between +VREF and
–VREF can be set to small values for conversions over
smaller voltage ranges. Particular care must be taken with
regard to noise pickup, circuit layout ond system error
voltage sources when operating with a reduced span due
to the increased sensitivity converter.
ML2252, ML2259
1.5 POWER SUPPLY AND REFERENCE DECOUPLING
A 10µF electrolytic capacitor is recommended to bypass
VCC to GND, using as short a lead length as possible. In
addition, with clock frequencies above 1MHz, a 0.1µF
ceramic disc capacitor should be used to bypass VCC to
GND.
If REF+ and REF– inputs are driven by long lines, they
should be bypassed by 0.1µF ceramic disc capacitors at
the reference input pins (pins 12, 16).
1.6 DYNAMIC PERFORMANCE
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 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 = 49.92dB.
Harmonic Distortion
Harmonic distortion is the ratio of the rms sum of
harmonics to the fundamental. Total harmonic distortion
(THD) of the ML2252 and ML2259 are defined as
20log (V22 + V32 + V42 + V52)1/ 2
V1
where V1 is the rms amplitude of the fundamental and V2,
V3, V4, V5 are the rms amplitudes of the individual
harmonics.
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
37.5
75
FREQUENCY (kHz)
Figure 6. Output Spectrum
9
 

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