ML2258

8

valid on DB0–DB7 until the next conversion updates the

data word on the next rising edge of EOC.

A conversion can be interrupted and restarted at any time by

a new START pulse.

1.3 ANALOG INPUTS AND SAMPLE/HOLD

The ML2258 has a true sample and hold circuit which

samples both the selected input and ground

simultaneously. This simultaneous sampling with a true

S/H will give common mode rejection and AC linearity

performance that is superior to devices where the two

input terminals are not sampled at the same instant and

where true sample and hold capability does not exist.

Thus, the ML2258 can reject AC common mode signals

from DC–50kHz as well as maintain linearity for signals

from DC–50kHz.

The plot below (figure 6) shows a 2048 point FFT of the

ML2258 converting a 50kHz, 0 to 5V, low distortion sine

wave input. The ML2258 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

measurements of 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 50kHz.

The signal at the analog input is sampled during the

interval when the sampling switch is open prior to

conversion start. The sampling window (S/H acquisition

time) is 4 CLK periods long and occurs 4 CLK periods after

START goes low. When the sampling switch closes at the

start of the S/H acquisition time, 8pF of capacitance is

thrown onto the analog input. 4 CLK periods later, the

sampling switch opens, the signal present at analog input

is stored and conversion starts. Since any error on the

analog input at the end of the S/H acquisition time will

cause additional conversion error, care should be taken to

insure adequate settling and charging time from the

source. If more charging or settling time is needed to

reduce these analog input errors, a longer CLK period can

be used.

The ML2258 has improved latchup immunity. Each analog

input has dual diodes to the supply rails, and a minimum

of ±25mA (±100mA typically) can be injected into each

analog input without causing latchup.

1.4 REFERENCE

the voltage span of the analog input (the difference

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

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.

In contrast to the ADC0808 and ADC0809, the ML2258

set to small values for conversions over smaller voltage

ranges. Particular care must be taken with regard to noise

pickup, circuit layout and system error voltage sources

when operating with a reduced span due to the increased

sensitivity of the converter.

Figure 6. Output Spectrum

37.5

75

0

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

FREQUENCY (kHz)