cL7106,cL7107,cL7106s,|cL7107S Differential Reference should be since this removes the common mode voltage he reference voltage can be generated anywhere within the om the reference system power supply voltage of the converter. The main source of com- Within the IC, analog CoMMON is tied to an N-Channel FET mon mode error is a roll-over voltage caused by the reference that can sink approximately 30mA of current to hold the capacitor losing or gaining charge to stray capacity on its voltage 2.8v below the positive supply(when a load is trying nodes. If there is a large common mode voltage, the reference to pull the common line positive). However, there is only capacitor can gain charge(increase voltage) when called up to 10uA of source current, so CoMMon may easily be tied to a de-integrate a positive signal but lose charge(decrease volt- more negative voltage thus overriding the internal reference. age)when called up to de- integrate a negative input signal This difference in reference for positive or negative input voltage will give a roll-over eror. However, by selecting the reference capacitor such that it is large enough in comparison to the stray capacitance, this emor can be held to less than 0.5 count worst REF HI case(See Component Value Selection. REF LO Analog COMMON This pin is included primarily to set the common mode cL7106 voltage for battery operation(ICL7106) or for any system where the input signals are fioating with respect to the power supply. The COMMON pin sets a voltage that is approxi mately 2.8V more negative than the positive supply. This is elected to give a minimum end-of-life battery voltage of FIGURE 4A about 6V. 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(7V), the COMMON voltage will have a low voltage coefficient (0.001%/), low output impedance =1592), and a 6.8kQ temperature coefficient typically less than 80ppm/oc cL7106 The limitations of the on chip reference should also be REF HI L8069 recognized, however. With the ICL7107, the internal heating which results from the led drivers can cause some REF LO degradation in performance Due to their higher thermal resis- tance, plastic parts are poorer in this respect than ceramic. The combination of reference Temperature Coefficient (TC) internal chip dissipation, and package thermal resistance can FIGURE 4B increase noise near full scale from 25uV to 80uVp-P FIGURE 4. USING AN EXTERNAL REFERENCE inearity in going from a high dissipation count such as 1000 20 segments on)to a low dissipation count such as 1111(8 teST segments on) can suffer by a count or more. Devices with a positive TC reference may require several counts to pull out of The TEST pin serves two functions. On the ICL7106 it is an over-range condition This is because over-range is a low coupled to the internally generated digital supply through dissipation mode, with the three least significant digits 5002 resistor. Thus it can be used as the negative supply for blanked.Similarly, units with a negative Tc may cycle externally generated segment drivers such as decimal points between over-range and a non-over-range count as the die or any other presentation the user may want to include on alternately heats and cools. All these problems are of course the LCd display. Figures 5 and 6 show such an application eliminated if an external reference is used No more than a 1mA load should be applie The ICL7106, 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 1MQ Analog COMMON is also used as the input low return duri auto-zero and de-integrate. If IN LO is different from anal COMMON, a common mode voltage exists in the cL710 nd is taken care of by the excellent CmRR of the cor However, in some applications IN LO will be set at known voltage(power supply common for instance). In this application, analog COMMON should be tied to the same TEST point, thus removing from th converter. The same holds true for the reference voltage. If eference can be conveniently tied to analog COMMON, it FIGURE 5. SIMPLE INVERTER FOR FIXED DECIMAL POINT7 Differential Reference The reference voltage can be generated anywhere within the power supply voltage of the converter. The main source of com￾mon mode error is a roll-over voltage caused by the reference capacitor losing or gaining charge to stray capacity on its nodes. If there is a large common mode voltage, the reference capacitor can gain charge (increase voltage) when called up to de-integrate a positive signal but lose charge (decrease volt￾age) when called up to de-integrate a negative input signal. This difference in reference for positive or negative input voltage will give a roll-over error. However, by selecting the reference capacitor such that it is large enough in comparison to the stray capacitance, this error can be held to less than 0.5 count worst case. (See Component Value Selection.) Analog COMMON This pin is included primarily to set the common mode voltage for battery operation (ICL7106) 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 more negative than the positive supply. This is selected to give a minimum end-of-life battery voltage of about 6V. 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 (>7V), the COMMON 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 ICL7107, the internal heating which results from the LED drivers can cause some degradation in performance. Due to their higher thermal resis￾tance, 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 25µV 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 ICL7106, 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 10µA 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 ICL7106 it is coupled to the internally generated digital supply through a 500Ω 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. FIGURE 4A. FIGURE 4B. FIGURE 4. USING AN EXTERNAL REFERENCE ICL7106 V REF LO ICL7107 REF HI V+ V- 6.8V ZENER IZ ICL7106 V REF HI REF LO COMMON V+ ICL8069 1.2V REFERENCE 6.8kΩ 20kΩ ICL7107 ICL7106 V+ BP TEST 21 37 TO LCD BACKPLANE TO LCD DECIMAL POINT 1MΩ FIGURE 5. SIMPLE INVERTER FOR FIXED DECIMAL POINT ICL7106, ICL7107, ICL7106S, ICL7107S