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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING ADC chips of the ICL71X6 family at reduced supply voltage. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Radio amateur designer The ICL7106 chips are manufactured by Harris (Intersil). Maxim also produces ICs labeled ICL7106 and its micro-power variant MAX130, as well as the ICL7136 and its improved variant MAX131. The ICL1 chip mentioned in [7126] is a micropower analogue of the 7106. The Harris ICL7136 is a micropower analogue of the 7106 and replaces the ICL7126. The KR572PV5 microcircuit is manufactured by the Mikron enterprise (Zelenograd); ADC KR1175PV5 is produced by Sapphire software. There are modifications of the 7106 microcircuit with the "Hold" mode - these are 572PV8 (analogous to ICL7116), 572PV10 (manufactured by Alpha or Mikron) [1]. The microcircuits of the family are completely identical in pinout (for the DIP-40 package) and switching circuits, but they have some circuitry features that lead to differences in characteristics (supply voltage, current consumption, noise, stability). For all Maxim microcircuits (and in ICL7136 Harris) the fourth phase appeared in the timing diagram (see [11]) - integrator zero correction, which allows faster ADC recovery after overload (overrange recovery); in MAX130/131 microcircuits, the error (rollover error) is less than one of the least significant digit. A distinctive feature of MAX130/131/138 microcircuits can be called their internal reference voltage source (ION), which uses the bandgap effect for silicon (Bandgap) [9]. This gives higher temperature stability at lower noise level compared to Zener-based ION. The presence of such an ION allows you to expand the allowable supply voltage range of the MAX13x microcircuits to 4,5 .... 14 V. The MAX138 microcircuit is also distinguished by a built-in power inverter, which converts an external unipolar supply into an internal bipolar one. In typical schemes for the use of ADC microcircuits of these series, the values of the elements are somewhat different. Details can be found in the manufacturer's documentation. In [1] on p. 222-224 tables of differences in the parameters of these microcircuits and the recommended values of the elements are presented. ADCs (ICL7107 and its analogues) designed to work with LED indicators have not been studied by the author, but it is necessary to mention them. In the proprietary documentation for various types of microcircuits of this family, a specific example of powering the ICL7107 from a "unipolar" source of +5 V is considered. The conditions under which a decrease in the supply voltage is permissible are as follows:
For the ICL7107 (KR572PV2) microcircuit, the nominal power supply is ±5 V with a midpoint connected to the corresponding output of the microcircuit - GND (pin 21). As a result of this connection, the supply voltage of the digital section of the ADC is fixed, regardless of the total supply voltage. In the ICL7106 ADC, at supply voltages less than 6,8 V, the supply voltage of the digital part is not stabilized, since the internal regulator does not work. The analog sections, as well as the voltage regulators of the ICL7106 and ICL7107, are the same, which means that the power conditions of the digital section of the ADC are the only reason why manufacturers do not allow the ICL7106 to be used at reduced voltage. The reasons for the stabilization of the power supply for digital logic can be found in the frequency instability of the RC generator, which does not affect the measurement process only to a limited extent, as well as in some restrictions on the supply voltage of the LCD. The issue of frequency stability can be solved by using a quartz resonator, and modern LCDs work normally with a voltage amplitude on the segments of at least 3 V. Thus, there is no reason why you should not try to use the ICL7106 with a reduced supply voltage. Consider a variant of a voltmeter with an ADC, in which the input signal voltage does not exceed 200 mV (see Fig. 1 in [12] - a circuit of a miniature digital voltmeter). Only an external reference and a range switch distinguish this device from a conventional multimeter. With a voltmeter layout (without a divider), the ADC was tested. Such an ADC with an indicator has good repeatability and is operable with any of the listed types of microcircuits of the considered family. In the layout, 20 ADC chips of seven different types and manufacturers were tested. The test results are summarized in the table.
Some comments on the results of measurements of microchip parameters are given below. The supply voltage U min corresponds to the value at which the indicator readings change by no more than one least significant digit (e.m.r.). The value of Uref (internal) is the exemplary voltage between power pin 1 and pin 32 (COMMON) when the power supply voltage is greater than Ucr min (analog), i.e. with internal microcircuit stabilizers. In this case, the internal ION is loaded with a supply current of the external ION of about 105 μA. Parameter Ust min (analogue, digital) - the minimum supply voltage of the microcircuit, at which the internal voltage stabilizers of the analog and digital sections of the ADC are turned on, respectively. Rint min - minimum resistance Rint = R9, at which the ADC maintains linearity at the same time at the minimum supply voltage (Upit min) and the maximum voltage at the ADC input of any polarity. The practical use of the given values can be as follows: for the selected type of ADC, you can find the corresponding resistance Rint min in the table and, increasing it by 20...30%, use the resulting value in a specific design. In this case, the generator frequency must be at least 32,768 kHz, and the capacitance Sint \u6d C0,22 \u5d XNUMX μF must have a tolerance of no more than XNUMX%. The "Error" column indicates the difference in readings at the end points of the scale for positive and negative voltage at the input. For all types of ADC (according to passport data), the parameter must be less than one of the least significant digit. The last column shows the experimental data on the indicator readings when +UBX ADC is connected to the +Uref point (the left terminal of the resistor R8 according to the diagram must be connected to the upper terminal R5). This parameter is a very important generalized indicator of the correct functioning and quality of the ADC. In accordance with the internal structure of the microcircuit, the current readings of the ADC are expressed as a number equal to 1000 Uin / Uion - It would seem that if these voltages are equal, the indicator should always accurately and stably show 1000. However, even the documentation states that readings of 1000 or 999 are considered acceptable. To stabilize the frequency of the built-in ADC oscillator, a conventional clock quartz resonator at a frequency of 32,768 kHz was used. An attempt to connect a quartz resonator for a clock according to a typical circuit (to terminals 39 and 40 of the ADC) was unsuccessful. Some combinations of microcircuit-quartz pairs do not work even at a nominal supply voltage of 9 V. As a result of experiments, a non-standard connection option appeared. In fact, this is a typical RC generator, in which the jumper between pins 39 and 40 is replaced by a quartz resonator. The frequency-setting resistance Rgen (in Fig. 1 in [12] is R2 - 30 kOhm) is significantly less than recommended in the documentation [7, 8] - 100 kOhm for ICL7106 or 180 kOhm for ICL7136. It has been experimentally established that such an oscillator starts up and operates stably at the quartz frequency only if the original RC oscillator (with closed quartz) at the lower limit of the supply voltage has its own frequency higher than the frequency of the quartz resonator. With a decrease in the supply voltage of the microcircuit and a corresponding decrease in the supply voltage of the RC generator, its frequency decreases. The behavior of the RC generator is different for different types of ADC. The tested KR572PV5 microcircuit with the indicated values of the elements worked stably at a supply voltage above 4,2 V: the generator turned off at a voltage of about 3,3 ... 3,5 V, and with quartz the generator started even at 4 V. switching off of the RC generator, respectively, were in the intervals of 130 ... 3,2 and 3,5 ... 2 V. The RC generator of the ICL2,3 chip continued to work even at a supply voltage of 7136. 1,5...1,8 kHz)! In the layout, almost all clock quartzes at the disposal of the author worked normally when the supply voltage changed in the range of 4 ... 9,5 V for the ADC microcircuits listed in the table. To suppress interference with frequencies that are multiples of 50 Hz, the oscillator frequency (Fgen) must be such that during the integration time (4000 periods T of the clock generator) an integer number of K periods (20 ms) of the mains voltage fit [2]. In other words, Fgen \u1d 200 / T \u200d 100 / K, kHz, i.e. 67, 32,768, 200 kHz, etc. For better suppression of interference with the mains frequency, the selected frequency value of 6 kHz is not ideal, but not very different either from the nearest calculated frequency: 33,333/XNUMX = XNUMX kHz. In company documentation [7, 11] and articles on the use of ADC 1C1_71xx, it is recommended to use capacitors with a low absorption coefficient in the dielectric. Usually there are no additional comments; only specific values are indicated: if Sint is a capacitor with a ceramic dielectric, the conversion linearity error is of the order of 0,1%, and with a polystyrene and polypropylene dielectric, 0,01 and 0,001%, respectively. Capacitors K73-17 (0,22 uF at 63 V, dimensions 12x10x6 mm) can be considered a compromise solution when choosing between accuracy and minimal design dimensions. Therefore, the integrator capacitor (on the breadboard and in the mini-voltmeter) was chosen of the K73-17 type, the zero auto-correction capacitor was K73-30 (the dimensions of K73-30, K73-39, K73-24V are smaller than those of K73-17), and C2 -K73 -17. For ADC with low-voltage power supply, an external reference voltage source REF1004-1.2 (Burr-Broun/TI) in a SOIC-8 package was used. Its nominal voltage is 1,235 V, the minimum operating current is 10 µA. You can use LM285 / LM385Z-1.2 microcircuits (NSC, LT, Motorola, Telcom) in the TO-92 package with a nominal voltage of 1,235 V and a minimum operating current of 10 μA, as well as LM4041-1.2 or AO1580 (50 μA, 1,225 V) [13] . As a power control element, a voltage drop detector - KR1171SP42 in the TO-92 package [14] was used. Using the indicative information from the table on the minimum supply voltage +Uup min, you can choose a detector with the desired response voltage for a particular type of ADC. Precise selection of the threshold voltage increases the efficiency of battery use. In such a design, you can use any supply voltage detector with an output type - open collector (open drain) or CMOS push-pull (CMOS) and an active low logic level. Here are some of the common types (most in SOT-23 packages): MCP120, MCP809(M), TCM809, TC54VN, TC12xx (Microchip), ADM809(L,M) (ADI), MC34xxx (Motorola), MAX809M (MAXIM) etc. If it is decided that a stabilized power supply is not required for the digital section of the ADC, the next, quite logical step is to exclude the internal regulator by installing a jumper on XP2 (see Fig. 1 in [12]). This increases the voltage between positive power pin 1 and pin 37 (TEST) by approximately 1V for the ICL7136 and 1,5V for the other types. The installation of the jumper does not have any effect on the operation of the analog part, which was verified in the layout on tested microcircuits. No jumper was used during characterization. It may be needed in the case of a "failed" quartz resonator, if the internal oscillator does not start well, or with an indicator that requires a large supply voltage. So, if in an amateur or industrial design it is necessary to use a microcircuit of the ICL71x6 family with a supply voltage of 5 ... 6 V, then, given the supply voltage margin, you can use an ADC without polarity converters. Literature
Author: O. Fedorov, Moscow
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