19-0805; Rev 1; 12/94 SUA XAL/VI 31/2 Digit A/D Converters with Bandgap Reference and Charge-Pump Voltage Converter General Description The MAX138/MAX139 are 314 digit A/D converters (ADCs) with on-board LCD (MAX138) and LED (MAX139) display drivers. The MAX138/MAX139 also contain a charge- pump voltage inverter. The charge-pump inverter allows the MAX138/MAX139 to measure both positive and nega- tive input voltages while operating from a single power- supply voltage from +2.5V to +7V. The operating circuits of the MAX138/MAX139 are similar to those of the ICL7136 and ICL7137 respectively, except the MAX138/MAX139 have an internal oscillator and an external charge-pump capacitor connected to pins 38 and 40. Features @ Single Supply +2.5V to +7.0V Operation @ Measures Both Positive and Negative Input Voltages @ Charge-Pump Voltage Inverter Generates a Negative Supply Voltage Internal Bandgap Reference @ On-Board Display Driver @ Low Segment Current (MAX140) Ordering Information MAX 140 is a low segment-current version of the MAX139 PART TEMP. RANGE PIN-PACKAGE intended for use with low-current LED displays. MAX138CPL OC to +70C 40 Plastic DIP Applications MAX138CMH 0C to +70C 44 MQFP MAX138CQH OC to +70C 44 PLCC +5V Powered Panel Meters MAX138C/D OCto+70C Dice" +3V Powered DMMs MAX138EPL -40C to +85C 40 Plastic DIP Instruments MAX138EQH __-40C to +85C 44 PLCC Portable Monitors MAX139CPL OC to +70C 40 Plastic DIP MAX139CMH OC to +70C 44 MQFP. Weigh Scales MAX139CQH OC to +70C 44 PLCC Digital Thermometers MAX139C/D 0C to +70C Dice* = - = MAX139EPL -40C to +85C AO Plastic DIP . Pin Configurations MAX139EQH -40C to +85C 44 PLCC TOP VIEW = MAX140CPL OC to +70C AO Plastic DIP vw a] a0] CAP MAX140CMH 0C to +70C 44 MOFP DI (2 sa) GND MAX140CQH OC to +70C 44 PLCC ct [3] Iss CAP- MAX140C/D OG to +70C Dice* "Sg fa] 7] TEST MAX140EPL -40C to +85C 40 Plastic DIP Ai S| Esa] REF Hi MAX140EQH 40C to +85C 44 PLCC Fi 6 | 35] REF LO * Consult factory for dice specifications. MAXIM | 6 Lz) maxiag [pt C+ REF Typical Operating Circuit Et fs] MAX139 a3) C- REF oz [ef MAX140 Ios) comMON 7 9 P' ne LCD/LEDDISPLAY Ss 11 30! } a A2 Fa bo AZ oan \ eT F2 fra a) Bure Anas a fee 2 1a] l27| INT - ] MAXUM _! | 0B as] rag V- MAX 138 ions 83 Lig es. G2 (TENS) Waxtag y ! F3 [17 24] C3 c a * 3 Tal 23] 3 | s 1000s-AB4 [Fo] paca eS VRE J POL [20] [21] BP (MAX 138) } 4 a, DIGITAL GND (MINUS SIGN) (MAX139/MAX140) + : DIP ,_! | Pin 21 must be GND on MAX139/MAX140. : MOFP and PLCC on last page. MAXIM Maxim Integrated Products 1 Calli toll free 1-800-998-8800 for free samples or literature. OVLXVIN/6GELXVIN/SELXVUINMAX1 38/MAX 139/MAX140 31/2 Digit A/D Converters with Bandgap Reference and Charge-Pump Voltage Converter ABSOLUTE MAXIMUM RATINGS (Note 1) Supply Voltage (V+ to GND} Supply Voltage (V+ to GND} MAX 138 MAX139/MAX140 Analog Input Voltage (either input) (Note 2) Reference Input Voltage (either input) ...... Power Dissipation (Note 3) 40-Pin Plastic DIP..............0..... 44-PinPLCC 0... ee 44-Pin MQFP. 2... ee Operating Temperature Ranges: MAX1__C oo. eee MAX1__E eee Lena +7.5V .... OC to +70C .. .-40C to +85C Storage Temperature Range .... Lead Temperature (Soldering, 10sec.) Note 1: Note 2: Note 3: -65'C to +160 C +300C V- is generated on the device and is equal to V+, but opposite in polarity Input voltages may exceed the supply voltages, provided the input current is limited to +1mA Dissipation rating assumes device is mounted with all leads soldered to printed circuit board. Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only and functional operation uf the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability ELECTRICAL CHARACTERISTICS (MAX138/MAX139/MAX140) (V+ = +5V; TA = +25C; test circuit of Figure 1, unless otherwise noted.) PARAMETERS CONDITIONS MIN TYP MAX UNITS VIN = 0.0V, Full Scale = 200mV Digital Zero Input Reading Ta = +25C (Note 4) -000.0 +000.0 +000.0 Roedig TMIN < Ta < TMAX (Note 5) 000.0 +000.0 +000.0 g VIN = VREF, VREF = 100mV Digital Ratiometric Reading Ta = +25C (Note 4) 999 999/1000 1000 | R gt Tain < Ta < Tmax (Note 5) 998 999/1000 1001 eading Rollover Error (Difference in reading for -VIN = +VIN = 200mV equal positive and negative reading near TA = +25C (Note 4) -1 +0.2 +4 * Counts full scale) TMIN S$ TA < Tmax (Note 5) +0.2 Linearity (Max deviation from best Full Scale = 200mV - straight-line fit) or Full Scale = 2.000V (Note 6) 1 +02 a | Counts an : Vom = +1V, VIN = OV Common-Mode Rejection Ratio Full Scale - bom 50 uviv Noise (peak-to-peak value not exceeded VIN = OV 15 Vv 95% of time) Full Scale = 200mV u VIN=O Input Leakage Current Ta = +25C (Note 4) 1 10 pA TMIN $ TA S TMAX 20 200 : . VIN = 0 " Zero Reading Drift TMIN < Ta < Tmax (Note 4) 0.2 nVv/"C VIN = 199mV Scale-Factor Temperature Coefficient TMINS TAS TM 1 ppm/C (Ext. Ref. opomy: 6) (Note 4) VIN=O V+ Supply Current (See Figure 4A) TA =+25C 200 500 HA TMIN < TA < TMAX 800 Analog Common Voltage (with respectto | pos. supply) 25kQ between Common & Pos. Supply 2,95 3.05 3.15 Vv Temp. Coeff. of Analog Common (with respect fo pos. supply) 25kQ between Common & Pos. Supply +20 +100 ppm/Cc MAXIM31/2 Digit A/D Converters with Bandgap Reference and Charge-Pump Voltage Converter ELECTRICAL CHARACTERISTICS (MAX138) (V+ = +5V; Ta = +25C: test circuit of Figure 1, unless otherwise noted.) PARAMETERS CONDITIONS MIN TYP MAX UNITS | Peak-to-Peak Segment Drive Voltage 4 5 6 Vv Peak-to-Peak Backplane Drive Voltage | Test-Pin Voitage With Respect to V+ 4 5 6 V ELECTRICAL CHARACTERISTICS (MAX139/MAX140) (V+ = +5V, Ta = +25C, test circuit of Figure 2, unless otherwise noted.) PARAMETER CONDITIONS MIN TYP MAX UNITS Except Pin 19 5 9 15 MAX139 mA Pin 19 10 18 30 Segment Drive Current - Except Pin 19 1.5 2.5 4 MAX140 mA Pin 19 3 5 8 Note 4: Test condition is Vin applied between pin IN HI and IN LO through a 1MQ series resistor as shown in Figures 74 and 2. Note 5: 1MQ resistor is removed in Figures 1 and 2 Note 6: Guaranteed by design. Basic Applications Figures 1 and 2 show the typical operating circuit for the MAX138/139/140 when powered by a single +5V supply. Compatibility with ICL7106, 1CL7136, and ICL7137 The MAX138/139/140 can replace the ICL7106/ICL7 136/ ICL7137 with minor circuit and component value chan- ges. The ICL7106/ICL7 136/ICL7 137 oscillator components are not used and are replaced with a 1uF capacitor CAP- and GND. There must be a 1uF filter capacitor con- nected to V-. The filter capacitor can be connected between either V- and GND or V- and V+. System Reference Point The analog block diagram of the MAX138/MAX139 is shown in Figure 3. The MAX138/MAX139 use the IN LO pin as the reference point for the integrator. The circuit configuration of the MAX138/MAX139 results in a superior 120aB rejection of common-mode voltages applied to IN Hl and INLO. The MAX138/MAX139 con- figuration does not have good rejection of AC noise on the IN LO pin during de-integration. If an AC-DC con- verter is used with a MAX138/MAX 139, it should either be a half-wave circuit or should have adequate filtering to avoid inducing additional noise. Detailed Description Conversion Method The MAX138/139/140 use the dual-slope integration method of conversion with the addition of an auto-zero phase to compensate for the offset of the buffer and MAXIM integrator and a zero integrator phase to ensure rapid recovery from an overrange conversion. Refer to the ICL7106 data sheet for a detailed description of the conversion phases and tiniiag. The conversion result is 1000 x (IN HI-IN LO)/(REF HI-REF LO) with a maximum conversion result of +1999. If the input voltage is greater than full scale, the MAX138/139/140 will blank the lower three digits and display the leading "1" digit. If the input voltage is nega- tive, the MAX138/139/1 40 will turn on the minus segment. COMMON Voltage Reference The COMMON voltage is derived from a bandgap reference, unlike earlier devices which derive the COMMON voltage from a zener. The bandgap reference eliminates the exces- sive long-term drift associated with low-current zeners, and the MAX138/139/140 can be a source of a high-quality reference voltage without the use of external bandgap refer- ence diodes. The COMMON voltage does have slightly more wideband noise than a zener-derived COMMON voitage, but a O.1uF or greater reference capacitor will reduce the bandwidth sufficiently to virtually eliminate the noise. The long-term stability of the COMMON voltage is ap- proximately 0.01% (100ppm or 1/5 count). These devices are sample tested to ensure a maximum temperature coefficient of 100ppm/"C. The COMMON voltage is buffered by an op amp that has an output impedance of 10, an output sink current of up to 2mA, and a short-circuit current of approximately 25mA max at +3.5V. COMMON has a small pull-up current of 1pA typical which can be driven to a voltage more negative than its internally generated voltage by overpowering the pull-up current source. OVEXVIW/6ELXVW/SELXVINMAX138/MAX139/MAX140 31/2 Digit A/D Converters with Bandgap Reference and Charge-Pump Voltage Converter O.1pF KE 34 33 C+REF C-REF _ Le DISPLAY iMa 31] NHI 249) seqment \| MEI fe Tm) + \AAp-e O.0ipF t maxim RIVE Wo He SNe _! 30 MAX138 2 | on INLO POL ANALOG BACKPLANE INPUT 321 common pp | 21_MINUS SIGN | DRIVE 2") purr ve |! ___, zavt047v 180K < 2 047 0.47uF 240kG2 S 29 i out ye AZ ape wi [36 VREF 5 snus Lo 2?) nt REF LO} $+ GND CAP- CAPs V-} 26 T {uF I 38 [ , 40 TO ANALOG <+ COMMON (P32) ae Configuration is 40-Pin DIP. FULL-SCALE INPUT 100.0mv VREF 200.0mVv | O1yF ae 34 a C+REF C-REF LED DISPLAY 3 - ain WHT sw INHI 2-19) SEGMENT 7 22-25] prive 7 WONT O.01uF MAXLMN wa | . 0 INLO MAX139 po, [20 _ ANALOG MAX140 INPUT 21 eommon DiGiTaL GND |- 24 1 Lua in| BUFF va lA ite 2 SV 180K u nS 21 yz i aN REF HI 3 VREEs, Oke 27) wt REF LO | 32 oot GND CAP. CAPs V- an [39] 38 7 40 = TO ANALOG = J COMMON (P32) IF Configuration is 40-Pin DIP. FULL-SCALE INPUT VREF 200.0mv 100.0mv Figure 1. MAX138 Typical Operating Circuit The COMMON voltage is trimmed to +3.05V +100mV which is Significantly more accurate than the +2.4V to +3.2V span allowed in the ICL7106. The better voltage accuracy allows the trim range of the reference voltage to be reduced, increasing resolution and adjustment ease. MAX139/MAX140 Test Voltage This internal test voltage is coupled to the TEST pin viaa 5002 resistor. When this pin is pulled high, all segments are turned on. The MAX138/139/140 oscillator circuit uses no external components. It is trimmed during production to 40kHz nominal. This results in a conversion rate of approximate- ly 2.5 conversions per second. The typical characteristics graph (Figure 4B) shows the variation with changes in supply voltage. IN LO and IN HI Differential Inputs These ADCs measure the differential voltage between IN LO and IN HI. The typical common-mode rejection ratio (CMRR) is 120dB. IN HI has a guaranteed maximum input leakage current of only 10pA and can be directly driven by high source impedances, such as pH sensors and the 10MQ input impedance attenuators normally used in digital multi- meters. Both IN HI and IN LO have protection clamp diodes to V+ and V-. If the input voltage can go above V+ or below V-, the input currents should be limited to less than 1mA to prevent damage to the ADC. The MAX138/139/140 common-mode voltage range for IN HI and IN LO is a minimum of +1V around COMMON. 4 Figure 2. MAX139/MAX 140 Typical Operating Circuit Under some circumstances, IN HI and IN LO can range from V- + 1.5V to V+ - 1.5V. See Common-Mode Voltage- Range Considerations section of the Application Notes for further information. REFHI, REFLO and CREF Pins As shown in Figure 3, REF HI and REF LO are connected to the CREF pins during auto-zero and zero-integrate phases via analog switches. This charges an external reference capacitor that is used as either a positive or a negative reference voltage during the de-integration phase. The common-mode voltage range (CMVR) of REF HI and REF LO is V+ to V-, and any voltage between V+ and V-, can be used to drive the REF HI and REF LO inputs. The differential voltage between REF HI and REF LO sets the full-scale voltage. A full-scale output of +1999 counts occurs with an input voltage of +1.999 times the differential voltage between REF HI and REF LO. If the differential reference voltage is 1.0V, the full-scale input voltage is 1.999V. With 100mV reference, the full-scale input voltage is 199.9mV. LCD Display-Driver Outputs The MAX138 LCD display-driver outputs swing from V+ to GND at a frequency of 20 times the conversion rate with an output impedance of approximately 3kQ. The LCD display-driver outputs are not directly driven in- phase with the backplane output to turn an LCD segment off and drive 180 out-of-phase with the backplane output (BP) to turn an LCD segment on. MAXIM31/2 Digit A/D Converters with Bandgap Reference and Charge-Pump Voltage Converter CREF C+REF REF Hi REF LO GH REF __ BUFF INLO AZ 1 AyZ bint | cae 35 | o la | oy _- INTEGRATOR ; | ta} | 10 | AZ ? AZ DIGITAL Zl al SECTION wa ea i so5v, AR : Se yOEG COMPARATOR | i. ANAXLAN 32 \ MAX138 common | BANDGAP MAX139 | 30 REERENCE MAX140 Sere _ RINT C CINT Configuration is 40- Pin DIP, Figure 3, Analog Section of MAX 138/139/140 The BP has an output impedance of 5002. The LCD drive waveforms are 50% duty cycle with matched rise and fall times to minimize the DC component across the LCD display. Tne MAX139/MAX140 LED display-driver outputs are N- channel current sinks with output current vs. voltage charac- teristics as shown in the Typical Characteristics graphs. Component Selection integrator Resistor, RINT The MAX138/139/140 integrator and buffer amplifiers have a class A output stage that can deliver up to 4yA with high linearity. The MAX 138/139/1 40 integrator resis- tor is normally chosen to set the maximum current to 1.1pA by setting its value to 2 x VREF/1.1yA. Fora 1V reference, the correct value is 1.8MQ. For a 100mV reference, the correct value is 180k. Since the absolute value of RINT does not affect the conversion accuracy, the type of resistor used for RINT is not critical. Integrator Capacitor The integrator capacitor is normally polypropylene, which has low dielectric absorption. Dielectric absorption will cause integral linearity errors. For example, if polyester or Mylar is used, the measured value of inputs near full scale will be approximately 0.1% lower than expected while the measured value of low input voltages will be as expected. Proper selection of the integrator capacitor value can be verified by monitoring the outout swing of the integrator with tfull-scale input voltages. In a properly operating circuit, +full-scale input voltages will cause the integrator output (INT} to swing to about +2V. INT can drive to about 0.3V from either supply while maintaining high linearity. Ifthe value of the integrator capacitor or integrator resistor is too low, +full-scale inputs will cause the integrator to MAXIMA saturate as it attempts to drive above V+ or below V-. If this occurs, operation will appear normal for low input voltages, but the conversion results will be less than full scale for higher output voltages . Very low integrator swing will increase the amount of naise or flicker of the conversions. A full-scale integrator swing of +1V is sufficient to avoid any significant degradation of the noise performance and should be used for operation with a +2.5V supply. Reference Capacitor For most circuits, a reference-capacitor value of 0. 1yF is adequate. However, a larger value is needed to prevent rollover error if there is significant stray capacitance at the reference-capacitor terminals. Minimize the stray capacitance on the reference-capacitor terminals to reduce the rollover error, and increase the reference capacitor value to 1.0uF if necessary. The printed circuit board should be carefully cleaned to minimize leakage at the CREF terminals since leakage will cause both gain and rollover errors. Due to the increased leakage of the MAX138/139/140 at +70C, a 1.0uF reference capacitor is recommended to reduce rollover and gain errors at high temperature. The reference capacitor is typically a low-leakage film capacitor. Polyester (Mylar) is acceptable in applica- tions where the reference voltage is constant. A low- dielectric absorption capacitor such as polypropylene should be used if the reference voltage is variable since any dielectric absorption will increase the settling time in response to a change in reference voltage. Since the reference voltage varies in circuits that measure resis- tance ratiometrically, a polyproplylene reference capacitor should be used in ohmmeters. OV LXVIN/6ELXVIW/SELXUINMAX138/MAX139/MAX140 31/2 Digit A/D Converters with Bandgap Reference and Charge-Pump Voltage Converter Auto-Zero Capacitor The noise of the ADC is influenced by the auto-zero capacitor. For the best noise performance, an auto-zero capacitor value of at least 4 times the integrator capacitor value is recom- mended. For a 2V scale, a 0.047uF (47nF) capacitor is adequate. An auto-zero capacitor of 0.47yF or greater is recommended for a 200mvV full scale. All of Maxim's integrat- ing ADCs have a zero-integrator phase which allows the use of high vaiues for the auto-zero capacitor without causing hysteresis or slowing the overload recovery time. 500 450 400 350 300 250 200 150 100 50 0 SUPPLY CURRENT (1A} The auto-zero capacitor can be any low-leakage film capacitor in most applications. A low-dielectric polyproplene capacitor is recommended if there are rapid changes in 0 1 2 3 4 5 6 7 8 common-mode voltage, or if the ADC must rapidly stabilize SUPPLY VOLTAGE upon power-up. Figure 4A. MAX138/139/140 Typical Supply Current vs. Supply Charge-Pump Capacitors Voltage The charge-pump capacitors should be 1pF. Application Notes Common-ModeVoltage-Range Considerations Operation with low supply voltages or with either IN LO or IN HI near either supply requires careful evaluation of the effect of common-mode voltages. Since the MAX138/139/140 perform all conversion phases (including auto-zero and de-integration), using IN LO as the reference point results in an excellent normal- mode rejection of approximately 120d0B. OSCILLATOR FREQUENCY (kHz) There are three basic internal limitations on the allowable common-mode voltage (Figure 3): 9 1 2 3 4 5 8 7 8 1) The buffer input CMVR is (V- + 1.5V) to (V+ - 1.5V). SUPPLY VOLTAGE 2) The integrator CMVR is (V- + 1.5V) to (V+ - 1.5V). Figure 4B. MAX 138/139/140 Typical Oscillator Frequency vs 3) The integrator output swing is limited to V- to V+. Supply Voltage 4) The IN LO must not go higher than 1.0V above COMMON. Figure 3 shows the buffer input can be connected to either IN HI (IN LO +VREF) or (IN LO -VREF). The integrator MAXTSO noninverting input is always connected to IN LO. Combining both system CMVR limitations with possible = V+ = +5V i i imitati i = 6 connections results in the limitations shown in Table 1. oS = 5 Low-Battery Detector = 4 Since the voltage between COMMON and V+ is between e& 4 MAXTAO +2.95V and +3.15V until the voltage between V+ and V- 5 falls to less than +4V, a simple low-battery detector can be made using a transistor voltage detector as shown in ' Figure 6. When Q1 is off, the low-battery segmentis driven 0 Tye RP TO in phase with the backplane off. When Q1 turns on, the 8 low-battery LCD segment becomes visible. Q1 turns on QUTPUT CURRENT (VSEGMENT} when the voltage at the base of Q1 is one base-emitter Figure 5. Output Current vs. Output Voltage voltage more positive than COMMON voltage. 6 MAXIM312 Digit A/D Converters with Bandgap Reference and Charge-Pump Voltage Converter 4--- t Ve = IMQ ANAXLAA A MAX138 LDN COMMON ~ BP 2 ~10 iV LOW-BATTERY BATTERY _ GND_TEST ANNUNCIATOR CT, SEGMENT Figure 6. Low-Battery Detector and LCD Segment Drive Overload Display The least significant three digits are blanked if the input voltage exceeds full scale. The leading 1" is displayed for positive overloads, and a"-1"is displayed for negative overloads. Any of the conditions that cause erratic read- ings (as discussed above) may cause overload readings. In addition, check the differential voltage between IN HI and IN LO to be sure it is no more than twice the differential voltage between REF HI and REF LO. Also be sure the voltage at REF HI is more positive than the voltage at REF LO since incorrect reference polarity causes an overload reading. Gross Nonlinearity If the results are linear for low input voltages but stops increasing as higher input voltages are applied, saturation of the integrator output is the most likely cause. With a full-scale voltage applied, look at the voltage on INT. It should not come closer than 0.3V to either supply. Increase the integrator capacitor value if the INT output swing is excessive. Alternatively, increase the oscillator frequency by changing the oscillator resistor and capacitor values. Nonlinearities of 2 to 20 Counts A polyester (Mylar) integrator capacitor will result in about 2 or 3 counts of nonlinearity at fullscale. Use polypropylene for best linearity. Leakages into the integrator capacitor, the auto-zero capacitor, or the reference capacitor will also cause linearity errors. Make sure printed circuit boards are thoroughly cleaned after soldering. Gain Error and Rollover Error A gross gain error will result if the integrator output current capabilities are exceeded. Make sure RINT 2 VREF/0.6uiA. Table 1. Common-Mode Voltage Limits Gain errors less than ten counts are generallly caused by either too much stray capacitance on the CREF terminals or excessive printed circuit board leakage. Stray capacitance and leakage can be detected by reducing the reference- capacitor by a factor of ten. If the error dramatically in- creases, either stray capacitance or leakage at the reference-capacitor terminals is the culprit. Error caused by stray capacitance tends to be a pure gain error while errors due to leakage tend to be nonlinear (typically square law). Errors due to leakage can also be detected by cleaning the board and then baking to reduce moisture content. Missing Segments on the LCD Display Missing segments on the LCD display is rarely a problem of the MAX138 and is usually caused by open circuits in the LCD connector/bezel, particularly if an elastomeric connector (zebra strip) is used. Check the voltage waveform at the pins of the MAX138. A signal in-ohase with the backplane turns off an LCD segment, and a signal 180 out-of-phase from the backplane turns on anLCD segment. Troubleshooting Noisy Readings The most common reason for noisy readings, particularly in engineering labs, is a noisy input signal. The 1MQ/10nF input filter shown in Figures 1 and 2 will significantly reduce high-frequency noise, and the capacitor value can be increased to further attenuate 50/60Hz. \f the input signal is clean, check the integrator swing since low integrator swing will increase the noise. If the integrator swing must be reduced to less than 1V for some reason, increasing the value of the auto-zero capacitor will improve the noise performance. For most circuits, the integrator swing should be approximately +2V. Avery low value for the auto-zero capacitor will also make the readings noisy. The value of the auto-zero capacitor should be at least twice the value of the integration capacitor. Increasing the auto-zero capacitor value to between 4 and 10 times that of the integrator capacitor will improve the noise performance, particularly with low- reference voltages. Stray coupling of noise signals, either digital/microproces- sor noise or 50/60Hz and 100/120Hz ripple can also be a cause of noisy readings. The auto-zero capacitor is most likely to pick up stray signals. The distance between the auto-zero capacitor and AZ should be minimized. The distance between the auto-zero capacitor and the integra- tion resistor and capacitor should also be minimized. Since DEVICE IN HI Positive Input Voltage V- + 1.5V to V+ - 1.5V Negative Input Voltage V- + 1.5V to V+ - 1.5V | INLO INTEGRATOR SWING V- + (1.5V + VREF) to V+ - 1.5V (IN LO - V-) V- + 1.5V to V+ - (1.5V + VREF) (V+ -INLO) MAXIM OVLXVIW/6GELXVIW/SELXVINMAX138/MAX139/MAX140 31/2 Digit A/D Converters with Bandgap Reference and Charge-Pump Voltage Converter BUFF and INT are the outputs of op amps, they are less sensitive to noise pickup than AZ, the input of an op amp. The MAX138/139/140 are sensitive to AC noise at IN LO during the de-integrate phase. Full-wave AC-DC con- verters should be used only if both outputs of the AC-DC converter output are well filtered. The common output on the MAX138/139/140 is derived from a bandgap reference that results in noisier common outputs than the ICL7106 and ICL7136 which are derived from zeners. This could cause an increase in conversion noise, but only if the CREF is less than 0.1uF and there is no bypassing at the reference inputs. Poor bypassing of the supply voltage may cause a couple of counts of noise in the readings, particularly if the power supply also powers digital logic since high-frequency spikes on the power supply might cause the comparator to falsely indicate zero crossing one or two clock cycles early. Ordinary 0.1uF byass capacitors are adequate in most cases. Since the MAX138/139/140 draw very little current, a simple RC filter can be used to provide greater spike and ripple attenuation in those cases where the power supply is exceptionally noisy. Since the oscillator frequency is slightly affected by the supply voltage, large changes in the supply voltage during a conversion may cause a few counts of error. A typical case where the effect must be considered is in a battery-powered circuit where the battery is also being used to drive high-current loads, such as motors or lamps. For extreme cases where high-current loads momentarily change the battery voltage a volt or more, use a series diode and a capacitor of 10uF or greater. Application Hints 1. See the ICL7136 and ICL7106 data sheets for a variety of application circuits that can also be used with the MAX138/139/140. 2. In some applications it may be useful to apply a fixed reference voltage between IN HI and IN LO and to apply the signal to REF HI and REF LO. In this mode of operation, the displayed reading is inversely proproportional to the input voltage. In other words, the displayed reading is the result of dividingthe fixed reference voltage by the signal voltage. A typical application where this function is useful is in a RPM meter where a voltage proportional to the period of a signal is divided into a fixed voltage to convert period into RPM (frequency). Another example is in a con- ductance meter where the conversion between ohms and Siemens is performed by swapping the positions of the unknown and reference resistors. 3. Aserial-outout pulse stream can be obtained from the MAX138/139/140 by monitoring the voltage at the CREF terminals (Figure 23 in the ICL7106 data sheet). Use an AND gate to combine the resulting end-of-conversion signal with the oscillator output from OSC3. 4. If the input-signal polarity is reversed from the desired polarity, use the minus segment to drive the vertical bar of a plus sign and (using one of the decimal-point driver circuits of Figure 6) to permanently turn on the horizontal bar of the plus sign. When the MAX138/139/140 measures a negative polarity, a "+" will be displayed. When the MAX138/139 measures a positive polarity, a-" will be displayed. (Normal opera- tion of the MAX138/139/140 is no polarity indication for a positive input and a -" sign for a negative input.) ____ Pin Configurations (continued) + t = ane ee ORSEGE TAMOOF Zesor gs 65) al gl la [a laa lal ac! [ar] [aol e Fitz] [30] REF LO Gite] [ss] CREF E13] [a7' CREF D2 49] ax [36 COMMON C2 Ga] [5s] IN HI NC. [rel MAX138 Isa] NC B2 [7a] MAX139 las] INLO A2 wa] MAX140 lao A/Z F215] [a1 BUFF 2 [6] [30} INT D3 G7] [2a] V TERE SeEEE S au aS <x MQFP/PLCC ato *Note: BP (MAX138) N.C. = No Connect DIGITAL GND (MAX139/140) Chip Topography Ag C2 og a AB, POL BP/DGND 0.137 (MAX138/139) _ (3.48mm) INT an BUFF INLO COMMON 0.149" (3.78mm) aFa ' D af : on . et cn Lo Cree: Note: Substrate connected to V+ Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time. 8 Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600 1994 Maxim Integrated Products Printed USA ANAXLM js a registered trademark of Maxim Integrated Products.