SC7106/7107

HANGZHOU SILAN MICROELECTRONICS CO.,LTD

REV:1.2

2006.02.27

Http: www.silan.com.cn

Page 9 of 22

The limitations of the on chip reference should also be recognized, however. With the SC7107, the internal

heating which results from the LED drivers can cause some degradation in performance. Due to their higher

thermal resistance, plastic parts are poorer in this respect than ceramic. The combination of reference

Temperature Coefficient (TC), internal chip dissipation, and package thermal resistance can increase noise near

full scale from 25

µV to 80µVp-p. Also the linearity in going from a high dissipation count such as 1000 (20

segments on) to a low dissipation count such as 1111(8 segments on) can suffer by a count or more. Devices

with a positive TC reference may require several counts to pull out of an over-range condition. This is because

over-range is a low dissipation mode, with the three least significant digits blanked. Similarly, units with a

negative TC may cycle between over-range and a non-over-range count as the die alternately heats and cools.

All these problems are of course eliminated if an external reference is used.

The SC7106, with its negligible dissipation, suffers from none of these problems. In either case, an external

reference can easily be added, as shown in Figure 4.

Analog COMMON is also used as the input low return during auto-zero and de-integrate. If IN LO is different

from analog COMMON, a common mode voltage exists in the system and is taken care of by the excellent

CMRR of the converter. However, in some applications IN LO will be set at a fixed known voltage (power supply

common for instance). In this application, analog COMMON should be tied to the same point, thus removing the

common mode voltage from the converter. The same holds true for the reference voltage. If reference can be

conveniently tied to analog COMMON, it should be since this removes the common mode voltage from the

reference system.

Within the IC, analog COMMON is tied to an N-Channel FET that can sink approximately 30mA of current to

hold the voltage 2.8V below the positive supply (when a load is trying to pull the common line positive). However,

there is only 10

µA of source current, so COMMON may easily be tied to a more negative voltage thus overriding

the internal reference.

FIGURE 4. USING AN EXTERNAL REFERENCE

8. TEST

The TEST pin serves two functions. On the SC7106 it is coupled to the internally generated digital supply

through a 500

Ω resistor. Thus it can be used as the negative supply for externally generated segment drivers

such as decimal points or any other presentation the user may want to include on the LCD display. Figures 5 and

6 show such an application. No more than a 1mA load should be applied.