ML2281CCP datasheet(15/26 Pages) MICRO-LINEAR

15 / 26 page

ML2281, ML2282, ML2284, ML2288

ZERO ERROR ADJUSTMENT

The zero of the A/D does not require adjustment. If the

minimum analog input voltage value, VIN MIN is not ground,

a zero offset can be done. The converter can be made to

output 00000000 digital code for this minimum input

voltage by biasing any VIN– input at this VIN MIN value.

This utilizes the differential mode operation of the A/D.

The zero error of the A/D converter relates to the location

of the first riser of the transfer function and can be

measured by grounding the VIN– input and applying a

small magnitude positive voltage to the VIN+ input. Zero

error is the difference between the actual DC input

voltage which is necessary to just cause an output digital

code transition from 00000000 to 00000001 and the ideal

1/2 LSB value (1/2 LSB = 9.8mV for VREF = 5.000VDC).

FULL-SCALE ADJUSTMENT

The full-scale adjustment can be made by applying a

differential input voltage which is 1-1/2 LSB down from

the desired analog full-scale voltage range and then

adjusting the magnitude of the VREF input or VCC for a

digital output code which is just changing from 11111110

to 11111111.

ADJUSTMENT FOR AN ARBITRARY ANALOG

INPUT VOLTAGE RANGE

If the analog zero voltage of the A/D is shifted away from

ground (for example, to accommodate an analog input

signal which does not go to ground), this new zero

reference should be properly adjusted first. A VIN+ voltage

which equals this desired zero reference plus 1/2 LSB

CURRENT LIMITING

RESISTOR, I+

≤15mA

12V

VCC

V+

GND

I +

→

28.8k

3.2k

Figure 8. Shunt Regulator

I+

15mA

V+

5.5V

SLOPE =

6.9V

35k

Figure 9. I-V Characteristic of the Shunt Regulator

(where the LSB is calculated for the desired analog span,

1 LSB = analog span/256) is applied to selected “+” input

and the zero reference voltage at the corresponding “–”

input should then be adjusted to just obtain the 00000000

to 00000001 code transition.

The full-scale adjustment should be made by forcing a

voltage to the VIN+ input which is given be:

fs adjust

MAX

MIN

−









256

()

where

VMAX = high end of the analog input range

VMIN = low end (offset zero) of the analog range

The VREF or VCC voltage is then adjusted to provide a

code change from 11111110 to 11111111.

SHUNT REGULATOR

A unique feature of ML2288 and ML2284 is the inclusion

of a shunt regulator connected from V+ terminal to

ground which also connects to the VCC terminal (which is

the actual converter supply) through a silicon diode as

shown in Figure 8. When the regulator is turned on, the

V+ voltage is clamped at 11VBE set by the internal resistor

ratio. The typical I-V of the shunt regulator is shown in

Figure 9. It should be noted that before V+ voltage is high

enough to turn on the shunt regulator (which occurs at

about 5.5V), 35k

W resistance is observed between V+ and

GND. When the shunt regulator is not used, V+ pin

should be either left floating or tied to GND. The

temperature coefficient of the regulator is –22mV/°C.

ML2281CCP 数据表(PDF) 15 Page - Micro Linear Corporation

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