SC7106/7107

HANGZHOU SILAN MICROELECTRONICS CO.,LTD

REV:1.2

2006.02.27

Http: www.silan.com.cn

Page 13 of 22

3.AUTO-ZERO CAPACITOR

The size of the auto-zero capacitor has some influence on the noise of the system. For 200mV full scale where

noise is very important, a 0.47

µF capacitor is recommended. On the 2V scale, a 0.047µF capacitor increases the

speed of recovery from overload and is adequate for noise on this scale.

4.REFERENCE CAPACITOR

A 0.1

µF capacitor gives good results in most applications. However, where a large common mode voltage

exists (i.e., the REF LO pin is not at analog COMMON) and a 200mV scale is used, a larger value is required to

prevent roll-over error. Generally 1

µF will hold the roll-over error to 0.5 count in this instance.

5.OSCILLATOR COMPONENTS

For all ranges of frequency a 100k

Ω resistor is recommended and the capacitor is selected from the equation:

f=0.45/RC, for 48kHz clock (3 Readings/sec), C=100pF.

6.REFERENCE VOLTAGE

The analog input required to generate full-scale output (2000 counts) is: VIN=2VREF. Thus, for the 200mV and

connected to a transducer, there will exist a scale factor other than unity between the input voltage and the digital

reading. For instance, in a weighing system, the designer might like to have a full-scale reading when the voltage

from the transducer is 0.662V. Instead of dividing the input down to 200mV, the designer should use the input

Ω

and 0.22

µF. This makes the system slightly quieter and also avoids a divider network on the input. The SC7107

with

±5V supplies can accept input signals up to ±4V. Another advantage of this system occurs when a digital

reading of zero is desired for VIN

≠0. Temperature and weighing systems with a variable fare are examples. This

offset reading can be conveniently generated by connecting the voltage transducer between IN HI and COMMON

and the variable (or fixed) offset voltage between COMMON and IN LO.

7.SC7107 POWER SUPPLIES

The SC7107 is designed to work from

±5V supplies. However, if a negative supply is not available, it can be

generated from the clock output with 2 diodes, 2 capacitors, and an inexpensive IC. Figure 10 shows this

application.

In fact, in selected applications no

negative

supply

is

required.

The

conditions to use a single +5V supply

are:

1) The input signal can be referenced

to the center of the common mode

range of the converter.

2) The signal is less than

±1.5V.

3) An external reference is used.

FIGURE 10. GENERATING NEGATIVE SUPPLY FROM +5V