ML2252, ML2259
1.4 REFERENCE
1.5 POWER SUPPLY AND REFERENCE DECOUPLING
The voltage applied to the +V
and –V
inputs
A 10µF electrolytic capacitor is recommended to bypass
REF
REF
defines the voltage span of the analog input (the
V
to GND, using as short a lead length as possible. In
CC
difference between V
and V
) over which the
INMIN
addition, with clock frequencies above 1MHz, a 0.1µF
INMAX
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.
ceramic disc capacitor should be used to bypass V to
CC
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).
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
1.6 DYNAMIC PERFORMANCE
Signal-to-Noise Ratio
+V
pin can be tied to V and –V
tied to GND. This
REF
CC
REF
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
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.
+V
and –V
can be at any voltage between V and
SNR = (6.02N + 1.76)dB
REF
REF CC
GND. In addition, the difference between +V
and
REF
where N is the number of bits. Thus for ideal 8-bit
converter, SNR = 49.92dB.
–V
can be set to small values for conversions over
REF
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.
HarmonicDistortion
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
2
2
2
2
1/ 2
(V + V + V + V )
2
3
4
5
20log
V
1
where V is the rms amplitude of the fundamental and V ,
1
2
V , V , V are the rms amplitudes of the individual
3
4
5
harmonics.
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
37.5
75
FREQUENCY (kHz)
Figure 6. Output Spectrum
9
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
