[ /Title (AN96 09) /Subje ct (Overc oming Comm on Mode Range Issues When Using Intersil Integr ating Conve rters) /Autho r () /Keyw ords (Inters il Corpo ration, semic onduc tor) /Creat or () /DOCI NFO pdfma rk Overcoming Common Mode Range Issues When Using Intersil Integrating Converters ® Application Note August 30, 2006 AN9609.3 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 ICL7107 ICL7136 TO PIN 1 ICL7107 OSC 1 40 OSC 2 39 100K OSC 3 38 TEST 37 100pF REF HI 36 REF LO 35 CREF 34 CREF 33 31/2 Digit LCD Display ADC 31/2 Digit LED Display ADC 31/2 Digit LCD Low Power Display ADC with SET VREF TO 100mV +5V 1K 10K 15K 0.1µF 1.2V (ICL8069) COMMON 32 + IN HI 31 Overrange Recovery A-Z 29 Applications BUFF 28 Figure 1 illustrates a typical application for the 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. INT 27 IN 1M IN LO 30 0.01µF - 0.47µF 47K 0.22µF V- 26 G2 25 C3 24 A3 23 TO DISPLAY G3 22 GND 21 FIGURE 1. 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 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. [ /Page 1 CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures. 1-888-INTERSIL or 1-888-468-3774 | Intersil (and design) is a registered trademark of Intersil Americas Inc. Copyright Intersil Americas Inc. 2001, 2006. All Rights Reserved All other trademarks mentioned are the property of their respective owners. Application Note 9609 Measuring the Supply Rail TO PIN 1 ICL7107 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 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 was chosen because of its characteristics. +5V 1K 15K 10K 0.1µF 1.2V (ICL8069) COMMON 32 IN HI 31 1M IN LO 30 INT 27 0.47µF CA3130 R - BUFF 28 Rf 0.01µF 47K VOUT 0.22µF + A-Z 29 VCOM V- 26 TO PIN 1 VIN′ ICL7106 VIN G2 25 R1 R1 C3 24 TO DISPLAY OSC 1 40 OSC 2 39 A3 23 OSC 3 38 TEST 37 G3 22 REF HI 36 GND 21 REF LO 35 CREF 34 CREF 33 100pF 25K - 9V IN HI 31 IN LO 30 By analyzing the circuit in Figure 2, we see that: A-Z 29 (EQ. 1) BUFF 28 INT 27 0.01µF 0.47µF 0.22µF C3 24 A3 23 R CA3130 VOUT VIN VIN′ VCOM G2 25 (EQ. 2) Rf 470K V- 26 where 1 V IN ′ = --- ( V IN + V COM ) 2 + 25K 0.1µF COMMON 32 FIGURE 2. SIMPLE SUMMING AMP TO RESOLVE CMR PROBLEM R V OUT = ⎛ 1 + ------f⎞ × V IN ′ ⎝ R⎠ SET VREF TO 1V 100K + TEST 37 - OSC 1 40 R1 R2 TO DISPLAY G3 22 to define the operational amplifier output. BP 21 TO BACKPLANE Substituting equation 2 into equation 1, and setting Rf = R yields: 1 V OUT = 2 ⎛ --- ( V IN + V COM )⎞ = V IN + V COM ⎝2 ⎠ (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 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 FIGURE 3. SIMPLE OP AMP CIRCUIT TO MEASURE SUPPLY RAIL (V+) Here an 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. AN9609.3 August 30, 2006 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 following data sheets: (EQ. 4) TABLE 2 DEVICE and that: ⎛ R2 ⎞ V IN ′ = V COM ⎜ ---------------------⎟ ⎝ R 1 + R 2⎠ ICL7106 31/2 Digit LCD/LED Display ADC ICL7107 31/2 Digit LCD/LED Display ADC ICL7136 31/2 Digit LCD/LED Low Power Display ADC with Overrange Recovery (EQ. 5) Setting R = 10RF, RF = R1 and R2 = 10R1, the equations can be reduced to: V IN ′ – V IN = 10 ( V OUT – V IN ′ ) DESCRIPTION (EQ. 6) and 10V COM ⎛ ( 10 × V COM × R 1 )⎞ V IN ′ = ⎜ ------------------------------------------------⎟ = ----------------------11R 11 ⎝ ⎠ 1 (EQ. 7) Combining equations 6 and 7 reveals that: V IN V OUT = V COM – --------10 (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 U.S. products are manufactured, assembled and tested utilizing ISO9000 quality systems. Intersil Corporation’s quality certifications can be viewed at www.intersil.com/design/quality Intersil products are sold by description only. Intersil Corporation reserves the right to make changes in circuit design, software 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. No license is granted by implication or otherwise under any patent or patent rights of Intersil or its subsidiaries. For information regarding Intersil Corporation and its products, see www.intersil.com 3 AN9609.3 August 30, 2006