PS081 datasheet(46/155 Pages) AMSCO

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3-23

PSØ81

Recommended values:

UCOMP1: not below 1 µF

STOP:

lower than Cucomp1/3000

Example:

UCOMP1 = 1 µF

STOP < 1 µF/3000

330 pF selected.

The noise will slightly increase by about 0.3 – 0.5 Bit.

3.5.7.5

Correction of Comparator Delay

The focus of the comparator performance is on ultra low noise, and therefore it has a delay which

cannot be neglected. This delay depends on temperature and results in a gain error which is too

high for precise weight scale applications. The two resistors Rtemp and Rref are used to measure

the delay time periodically during the operation. The PSØ81 corrects the measuring result with the

measured delay.

The delay of the comparator depends on the value of C

UCOMP1 and CSTOP. Because these values can

be changed by the user there is a possibility to adjust the correction routine by the register Mult_

PP[7:0]. A good value for the recommended C

UCOMP1 and CSTOP values (33 µF and 3.3 nF) is Mult_PP

= 1.28 (160 or 0xA3). If the capacitor values are increased, the correct Mult_PP value has to be

higher or vice versa. If the selected Mult_PP value is too low the gain will decrease with higher tem-

perature or lower voltage.

At the correct value of Mult_PP the gain of the electronic is absolutely stable over a very wide tempe-

rature and voltage range. The temperature drift of the gain is <1ppm/K. The power supply rejection

ratio (PSRR) is >130 dB. See also section 3.6.2.

3.5.8

Temperature Measurement

PS081 has the possibility to measure the temperature by means of an external, temperature depen-

dent resistor. The temperature information can be used to correct the gain drift of uncompensated

load cells. This we call rspan-by-temp compensation.

Temperature measurement is done by measuring the ratio of the discharge times of two resistors,

Rtemp and Rref, a temperature dependent one and a temperature stable one. The change of the

ratio is used by an implemented hardware algorithm to correct the gain drift of an uncompensated

load cell. Of course, the temperature coefficient of the resistor pair needs to match the load cell’s

temperature coefficient. This can be done by a factor TKGain (configreg_08), which scales the result

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