AN9609 ICL7106 ICL7107 ICL7116 ICL7117 ICL7129 ICL7136 ICL8069 HI7133 HI7131

Using Intersil Integrating Converters Application Note November 1996 AN9609.1 Authors: Paul Traynham and Juan C. Garcia

Overcoming Common Mode Range Issues When Using Intersil Integrating Converters Application Note November 1996 AN9609 AN9609.1 Authors: Paul Traynham and Juan C. Garcia Introduction Some applications which utilize Intersil's integrating analog to digital converters may require a way to overcome Common Mode Range (CMR) issues that may arise. Intersil's integrating analog to digital converters such as those listed in Table 1 operate consistently with an input which is within the CMR and the full-scale voltage set by the user. As a result, the converter's full scale range may vary from 200mV to as much as 2V. TABLE 1. INTERSIL INTEGRATING CONVERTERS ICL7106 ICL7106 ICL7107 ICL7107 ICL7116 ICL7116 ICL7117 ICL7117 ICL7129 ICL7129 ICL7136 ICL7136 31/2 Digit LCD Display ADC 31/2 Digit LED Display ADC 31/2 Digit LSD Display ADC with Display Hold 31/2 Digit LED Display ADC with Display Hold 41/2 Digit LCD Single Chip ADC 31/2 Digit LCD Low Power Display ADC with TO PIN 1 ICL7107 ICL7107 OSC 1 40 OSC 2 39 100K OSC 3 38 TEST 37 100pF SET VREF TO 100mV REF HI 36 REF LO 35 CREF 34 CREF 33 +5V 1K 10K 15K 0.1µF 1.2V (ICL8069 ICL8069) COMMON 32 + IN HI 31 A-Z 29 BUFF 28 Overrange Recovery INT 27 31/2 Digit LED Low Power Display ADC with Overrange Recovery 31/2 Digit Low Power LCD Display ADC C3 24 HI7133 HI7133 31/2 Digit Low Power LED Display ADC A3 23 0.01µF - 0.47µF 47K G2 25 HI7131 HI7131 IN 1M IN LO 30 ICL7137 ICL7137 0.22µF V- 26 TO DISPLAY G3 22 Applications GND 21 Figure 1 illustrates a typical application for the ICL7107 ICL7107, 31/2 Digit LED Display ADC, using a single 5V supply and full scale range set to 200mV. Here, the IN LO input is tied to analog COMMON, and the differential input is measured with respect to it. Analog COMMON sets a voltage which is approximately 2.8V less than the positive supply, V+, when V+ to V- 6.8V. Here, V+ to V- = 5V, so a fixed 1.2V reference is used. The CMR is defined as: (V- + 1.0V) < VIN < (V+ - 0.5V). Not only does the input need to be within the common mode range, but also the full-scale range as well. When operating these converters from a single supply and with an input voltage referenced to ground instead of COMMON, the designer can quickly run out of common mode range with which to work. In Figure 1, V+ = 5V and V- = 0V. VIN is limited to a common mode range of +1V to +4.5V, but has a full scale of 200mV. The 200mV full scale is below the common mode range of the device. Even if fullscale is adjusted to 2V, the full scale range of the device is limited to a 1V swing ranging from 1 to 2V. 1 FIGURE 1. ICL7107 ICL7107 OPERATED FROM A SINGLE +5V SUPPLY One solution is to separate COMMON and IN LO and tie IN LO to ground. The problem here is that gross errors can occur if IN LO is pulled to far from COMMON. A much better solution is to use a simple, non-inverting amplifier adder circuit as seen in Figure 2. A Intersil CA3130 CA3130 operational amplifier was chosen because it operates from a single 5V supply, has rail-to-rail output swing and an input range that includes ground. 1-888-INTERSIL 1-888-INTERSIL or 321-724-7143 | Copyright © Intersil Corporation 1999 Application Note 9609 Measuring the Supply Rail TO PIN 1 ICL7107 ICL7107 OSC 1 40 In battery applications where a designer wishes to not only power the converter from a single supply, but also monitor and display the battery voltage, common mode range issues again arise. From the CMR equation we note that: SET VREF 100K OSC 2 39 TO 100mV OSC 3 38 TEST 37 1V < VIN < (V+ - 0.5V) 100pF REF HI 36 REF LO 35 CREF 34 CREF 33 At first glance, it may seem that an easy solution would be to use a simple resistor voltage divider and reduce VIN by a factor of 10, set full-scale to 2V and supply this to the ADC. For common battery voltages of 6V to 12V, the corresponding input voltages would be 0.6 to 1.2V. However, if we go back to the above common mode range equation, it states that VIN must be greater than 1V. Figure 3 illustrates another operational amplifier configuration to solve this problem. Again a CA3130 CA3130 was chosen because of its characteristics. +5V 1K 15K 10K 0.1µF 1.2V (ICL8069 ICL8069) COMMON 32 IN HI 31 1M IN LO 30 INT 27 0.47µF CA3130 CA3130 R - BUFF 28 Rf 0.01µF 47K VOUT 0.22µF + A-Z 29 VCOM V- 26 TO PIN 1 VIN ICL7106 ICL7106 VIN G2 25 OSC 1 40 OSC 2 39 100K A3 23 OSC 3 38 TEST 37 100pF G3 22 REF HI 36 GND 21 REF LO 35 CREF 34 CREF 33 TO DISPLAY FIGURE 2. SIMPLE SUMMING AMP TO RESOLVE CMR PROBLEM By analyzing the circuit in Figure 2, we see that: Rf V OUT = 1 + - × V IN R 25K IN HI 31 BUFF 28 INT 27 where 0.01µF 0.47µF 0.22µF C3 24 A3 23 VOUT VIN VIN R1 R2 TO DISPLAY G3 22 to define the operational amplifier output. Substituting equation 2 into equation 1, and setting Rf = R yields: (EQ. 3) Using this circuit, the operational amplifier's output signal (VOUT) is now the sum of the incoming signal (VIN) and COMMON. In other words, VIN is now referenced with respect to COMMON and not ground. With a single 5V supply, an external voltage reference must be used because the internal reference is only active when V+ to V- 6.8V, and VCOM = COMMON V+ - 1.2V = 3.8V. The signal input to the ICL7107 ICL7107 will now be offset by 3.8V, which is well within the common mode range of the device. Even if V+ changes, as in battery applications, COMMON will keep the signal within the common mode range of the converter. 2 R CA3130 CA3130 VCOM G2 25 (EQ. 2) Rf 47K V- 26 1 V OUT = 2 - ( V IN + V COM ) = V IN + V COM 2 9V - IN LO 30 (EQ. 1) + 25K 0.1µF COMMON 32 A-Z 29 1 V IN = - ( V IN + V COM ) 2 SET VREF TO 1V + R1 - R1 C3 24 BP 21 TO BACKPLANE FIGURE 3. SIMPLE OP AMP CIRCUIT TO MEASURE SUPPLY RAIL (V+) Here an ICL7106 ICL7106, 31/2 Digit LCD Display ADC with full scale set to 2V is shown. In this circuit, the operational amplifier is set in an inverting gain configuration, because gains of 0.1 are desirable, and because we are only concerned with positive input voltages. Application Note 9609 Again analyzing the operational amplifier configuration, we note that: ( V IN V IN ) ( V OUT V IN ) - = -R RF For more information on Intersil integrating analog to digital converters, please refer to the AnswerFAX document numbers outlined in Table 2. (EQ. 4) TABLE 2. AnswerFAX INFORMATION and that: (EQ. 5) Setting R = 10RF, RF = R1 and R2 = R1, the equations can be reduced to: V IN V IN = 10 ( V OUT V IN ) (EQ. 6) DESCRIPTION AnswerFAX DOCUMENT NUMBER ICL7106 ICL7106 R2 V IN = V COM - R 1 + R 2 31/2 Digit LCD/LED Display ADC 3082 ICL7107 ICL7107 31/2 Digit LCD/LED Display ADC 3082 ICL7116 ICL7116 31/2 Digit LSD/LED Display ADC 3083 DEVICE with Display Hold and ICL7117 ICL7117 ICL7129 ICL7129 41/2 Digit LCD Single Chip ADC 3085 ICL7136 ICL7136 31/2 Digit LCD/LED Low Power Display ADC with Overrange Recovery 3086 31/2 Digit LCD/LED Low Power Display ADC with Overrange Recovery 3086 (EQ. 7) Combining equations 6 and 7 reveals that: V IN V OUT = V COM -10 3083 ICL7137 ICL7137 10V COM ( 10 × V COM × R 1 ) V IN = - = -11R 1 11 31/2 Digit LSD/LED Display ADC with Display Hold (EQ. 8) The output of the operational amplifier (VOUT), which is the input of the ADC, is now -0.1VIN with respect to COMMON, and since VIN = V+, the ADC will measure -10% of the supply. Simply driving the proper decimal point and not driving the sign indicator will display the proper voltage. For example, a battery voltage of 11.6V will be measured as -1.16V and can easily be displayed as 11.60V on a 31/2 Digit Display. All Intersil semiconductor products are manufactured, assembled and tested under ISO9000 ISO9000 quality systems certification. Intersil semiconductor products are sold by description only. Intersil Corporation reserves the right to make changes in circuit design and/or specifications at any time without notice. Accordingly, the reader is cautioned to verify that data sheets are current before placing orders. Information furnished by Intersil is believed to be accurate and reliable. However, no responsibility is assumed by Intersil or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may result from its use. 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