link to page 10 link to page 10 ICL7106, ICL7107, ICL7107S Component Value Selection Reference Voltage The analog input required to generate full scale output (2000 Integrating Resistor counts) is: VlN = 2VREF. Thus, for the 200mV and 2V scale, VREF Both the buffer amplifier and the integrator have a class A output should equal 100mV and 1V, respectively. However, in many stage with 100µA of quiescent current. They can supply 4µA of applications where the A/D is connected to a transducer, there drive current with negligible nonlinearity. The integrating resistor will exist a scale factor other than unity between the input should be large enough to remain in this very linear region over voltage and the digital reading. For instance, in a weighing the input voltage range, but small enough that undue leakage system, the designer might like to have a full scale reading when requirements are not placed on the PC board. For 2V full scale, the voltage from the transducer is 0.662V. Instead of dividing the 470kΩ is near optimum and similarly a 47kΩ for a 200mV scale. input down to 200mV, the designer should use the input voltage directly and select VREF = 0.341V. Suitable values for integrating Integrating Capacitor resistor and capacitor would be 120kΩ and 0.22µF. This makes The integrating capacitor should be selected to give the maximum the system slightly quieter and also avoids a divider network on voltage swing that ensures tolerance buildup will not saturate the the input. The ICL7107 with ±5V supplies can accept input integrator swing (approximately. 0.3V from either supply). In the signals up to ±4V. Another advantage of this system occurs when ICL7106 or the ICL7107, when the analog COMMON is used as a a digital reading of zero is desired for VIN 0. Temperature and reference, a nominal +2V full scale integrator swing is fine. For weighing systems with a variable fare are examples. This offset the ICL7107 with +5V supplies and analog COMMON tied to reading can be conveniently generated by connecting the voltage supply ground, a ±3.5V to +4V swing is nominal. For three transducer between IN HI and COMMON and the variable (or readings/second (48kHz clock) nominal values for C fixed) offset voltage between COMMON and IN LO. lNT are 0.22µF and 0.10µF, respectively. Of course, if different oscillator ICL7107 Power Supplies frequencies are used, these values should be changed in inverse proportion to maintain the same output swing. The ICL7107 is designed to work from ±5V supplies. However, if a negative supply is not available, it can be generated from the An additional requirement of the integrating capacitor is that it clock output with 2 diodes, 2 capacitors, and an inexpensive lC. must have a low dielectric absorption to prevent roll-over errors. Figure 10 shows this application. See the ICL7660 datasheet for While other types of capacitors are adequate for this application, an alternative. polypropylene capacitors give undetectable errors at reasonable cost. In fact, in selected applications no negative supply is required. The conditions to use a single +5V supply are: Auto-Zero Capacitor 1. The input signal can be referenced to the center of the The size of the auto-zero capacitor has some influence on the common mode range of the converter. noise of the system. For 200mV full scale where noise is very 2. The signal is less than ±1.5V. important, a 0.47µF capacitor is recommended. On the 2V scale, a 0.047µF capacitor increases the speed of recovery from 3. An external reference is used. overload and is adequate for noise on this scale. V+ Reference Capacitor CD4009 A 0.1µF capacitor gives good results in most applications. However, where a large common mode voltage exists (i.e., the V+OSC 1 REF LO pin is not at analog COMMON) and a 200mV scale is used, a larger value is required to prevent rollover error. Generally 1N914+OSC 2 1µF will hold the rollover error to 0.5 count in this instance. 10µFOSC 30.047- Oscillator Components µFICL7107 For all ranges of frequency a 100kΩ resistor is recommended and GND1N914 the capacitor is selected as shown in Equation 2: V- 0.45 f = ------ For 48kHz Clock (3 Readings/sec), RC (EQ. 2) V- = 3.3V C = 100pF. FIGURE 10. GENERATING NEGATIVE SUPPLY FROM +5V FN3082 Rev 9.00 Page 10 of 17 October 24, 2014
