ML2252, ML2259

9

1.4 REFERENCE

defines the voltage span of the analog input (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

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.

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.

1.5 POWER SUPPLY AND REFERENCE DECOUPLING

A 10µF electrolytic capacitor is recommended to bypass

addition, with clock frequencies above 1MHz, a 0.1µF

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

20

2

2

3

2

4

2

5

2 12

1

log

()

/

VVV

V

V

++

+

harmonics.

Figure 6. Output Spectrum

37.5

75

0

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

FREQUENCY (kHz)