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7 page 
7 Analog COMMON This pin is included primarily to set the common mode voltage for battery operation (ICL7116) or for any system where the input signals are floating with respect to the power supply. The COMMON pin sets a voltage that is approxi- mately 2.8V less than the positive supply. This is selected to give a minimum end-of-life battery voltage of about 6.8V. However, analog COMMON has some of the attributes of a reference voltage. When the total supply voltage is large enough to cause the zener to regulate (>6.8V), the COM- MON voltage will have a low voltage coefficient (0.001%/V), low output impedance ( ≅15Ω), and a temperature coefficient typically less than 80ppm/oC. The limitations of the on chip reference should also be recognized, however. With the ICL7117, the internal heat- ing 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 con- dition. This is because over-range is a low dissipation mode, with the three least significant digits blanked. Simi- larly, 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 ICL7116, 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 lC, 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. TEST The TEST pin serves two functions. On the ICL7116 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 annunciator 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. The second function is a “lamp test”. When TEST is pulled high (to V+) all segments will be turned on and the display should read “-1888”. The TEST pin will sink about 5mA under these conditions. CAUTION: On the ICL7116, in the lamp test mode, the segments have a constant DC voltage (no square-wave) and may burn the LCD display if left in this mode for several minutes. FIGURE 4A. FIGURE 4B. FIGURE 4. USING AN EXTERNAL REFERENCE ICL7116 V COMMON ICL7117 REF HI V+ V- 6.8V ZENER IZ ICL7116 V REF HI COMMON V+ ICL8069 1.2V REFERENCE 6.8k Ω 20k Ω ICL7117 ICL7116 V+ BP TEST 21 37 TO LCD BACKPLANE TO LCD DECIMAL POINT 1M Ω FIGURE 5. SIMPLE INVERTER FOR FIXED DECIMAL POINT ICL7116, ICL7117 |
