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WS7107 Datasheet(PDF) 6 Page - Wing Shing Computer Components |
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WS7107 Datasheet(HTML) 6 Page - Wing Shing Computer Components |
6 / 14 page 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 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 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 25 µVto80µVP-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. 7106, 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 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. FIGURE 4A. FIGURE 4B. FIGURE 4. USING AN EXTERNAL REFERENCE V REF LO REF HI V+ V- 6.8V ZENER IZ V REF HI REF LO COMMON V+ ICL8069 1.2V REFERENCE 6.8k Ω 20k Ω V+ BP TEST 21 37 TO LCD BACKPLANE TO LCD DECIMAL POINT 1M Ω FIGURE 5. SIMPLE INVERTER FOR FIXED DECIMAL POINT WS7106 / WS7107 (WS7106) or for any system WS7107, the internal heating The WS WS7106 WS7106 WS7107 WS7106 WS7107 WS7106 it is 6 |
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