PS081 datasheet(45/155 Pages) AMSCO

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3-22

PSØ81

3.5.7.3

When should an external comparator be used?

There are three reasons for the choice of an external comparator:

a) Very high resolution

The user is looking for the best possible resolution in his application, e.g. in counting scales, high end

legal-for-trade scales.

b) Lowest current

The user is looking for the lowest possible current consumption in his application, e.g. in solar sca-

les. Because of the lower noise, the AVRate can be reduced at a given resolution and therefore the

operating current is reduced. With the bipolar comparator the operating current can be more than

halved compared to the internal comparator.

c) Ultra low voltage

In case the user wants to run his application down to < 2.1 V Vcc, e.g. with 1.55 V silver oxide bat-

teries. Then the bipolar comparator shows significantly better results.

3.5.7.4

Comparator Control

The comparator can be switched on for only the duration of the measurement for current saving

reasons or continuously (con_comp[1:0]). Further, the working resistance of the internal comparator

can be changed (sel_compr[1:0]).

We recommend the following settings:

CON_COMP

= ‘b10 g on during measurement

SEL_COMPR

= ‘b10 g 7k resistor selected

If CON_COMP is set to ‘b11 (on) the comparator needs approx. 130 µA @ 3.0 V of constant cur-

rent.

Capacitors at UCOMP1 and STOP:

The capacitors at UCOMP1 and STOP are important for the low noise figure. For best performance

we recommend 33 µF for C

UCOMP1 and 3.3 nF for CSTOP. For Cucomp1 an ordinary electrolytic capaci-

tor can be used. As C

STOP a X7R capacitor can be used.

In case the internal comparator is used C

UCOMP1 and CSTOP have to be connected as well as the 4.7k

Ohm resistor. Nevertheless, smaller values are possible, too.

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