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1、ICL7135 4 1/2 Digit, BCD Output, A/D ConverterThe Intersil ICL7135 precision A/D converter, with its multiplexed BCD output and digit drivers, combines dual-slope conversion reliability with +1 in 20,000 count accuracy and is ideally suited for the visual display DVM/DPM market. The 2.0000V full sca
2、le capability, auto-zero, and auto-polarity are combined with true ratiometric operation, almost ideal differential linearity and true differential input. All necessary active devices are contained on a single CMOS lC, with the exception of display drivers, reference, and a clock.The ICL7135 brings
3、together an unprecedented combination of high accuracy, versatility, and true economy. It features auto-zero to less than 10 V, zero drift of less than 1V/, input bias current of 10pA (Max), and rollover error of less than one count. The versatility of multiplexed BCD outputs is increased by the add
4、ition of several pins which allow it to operate in more sophisticated systems. These include STROBE , OVERRANGE , UNDERRANGE , RUN/HOLD and BUSY lines, making it possible to interface the circuit to a microprocessor or UART.Features* Accuracy Guaranteed to+1 Count Over Entire 20000 Counts (2.0000V F
5、ull Scale)* Guaranteed Zero Reading for 0V Input* 1pA Typical Input Leakage Current* True Differential Input* True Polarity at Zero Count for Precise Null Detection* Single Reference Voltage Required* Over range and Under range Signals Available for Auto-Range Capability* All Outputs TTL Compatible*
6、 Blinking Outputs Gives Visual Indication of Over range* Six Auxiliary Inputs/Outputs are Available for Interfacing to UARTs , Microprocessors, or Other Circuitry* Multiplexed BCD Outputs* Pb-Free Available (RoHS Compliant)Detailed DescriptionAnalog SectionEach measurement cycle is divided into four
7、 phases. They are (1) auto-zero (AZ), (2) signal-integrate (INT), (3) de-integrate (DE) and (4) zero-integrator (Zl).Auto-Zero PhaseDuring auto-zero, three things happen. First, input high and low are disconnected from the pins and internally shorted to analog COMMON. Second, the reference capacitor
8、 is charged to the reference voltage. Third, a feedback loop is closed around the system to charge the auto-zero capacitor CAZ to compensate for offset voltages in the buffer amplifier, integrator, and comparator. Since the comparator is included in the loop, the AZ accuracy is limited only by the n
9、oise of the system. In any case, the offset referred to the input is less than 10V.Signal Integrate PhaseDuring signal integrate , the auto-zero loop is opened, the internal short is removed, and the internal input high and low are connected to the external pins. The converter then integrates the di
10、fferential voltage between IN HI and IN LO for a fixed time. This differential voltage can be within a wide common mode range; within one volt of either supply. If, on the other hand, the input signal has no return with respect to the converter power supply, IN LO can be tied to analog COMMON to est
11、ablish the correct common-mode voltage. At the end of this phase, the polarity of the integrated signal is latched into the polarity F/F.De-Integrate PhaseThe third phase is de-integrate or reference integrate. Input low is internally connected to analog COMMON and input high is connected across the
12、 previously charged reference capacitor. Circuitry within the chip ensures that the capacitor will be connected with the correct polarity to cause the integrator output to return to zero. The time required for the out- put to return to zero is proportional to the input signal. Specifically the digit
13、al reading displayed is:Zero Integrator PhaseThe final phase is zero integrator. First, input low is shorted to analog COMMON. Second, a feedback loop is closed around the system to input high to cause the integrator output to return to zero. Under normal condition, this phase lasts from 100 to 200
14、clock pulses, but after an over range conversion, it is extended to 6200 clock pulses.Differential InputThe input can accept differential voltages anywhere within the common mode range of the input amplifier; or specifically from 0.5V below the positive supply to 1V above the negative supply. In thi
15、s range the system has a CMRR of 86dB typical. However, since the integrator also swings with the common mode voltage, care must be exercised to assure the integrator output does not saturate. A worst case condition would be a large positive common-mode voltage with a near full scale negative differ
16、ential input voltage. The negative input signal drives the integrator positive when most of its swing has been used up by the positive common mode voltage. For these critical applications the integrator swing can be reduced to less than the recommended 4V full scale swing with some loss of accuracy.
17、 The integrator output can swing within 0.3V of either supply without loss of linearity.Analog COMMONAnalog COMMON is used as the input low return during auto-zero and de-integrate. If IN LO is different from analog COMMON, a common mode voltage exists in the system and is taken care of by the excel
18、lent CMRR of the converter. However, in most applications IN LO will be set at a fixed known voltage (power supply common for instance). In this application, analog COMMON should be tied to the same point, thus removing the common mode voltage from the converter. The reference voltage is referenced
19、to analog COMMON.ReferenceThe reference input must be generated as a positive voltage with respect to COMMON,Digital SectionFigure 5 shows the Digital Section of the ICL7135. The ICL7135 includes several pins which allow it to operate conveniently in more sophisticated systems. These include:Run/HOL
20、D (Pin 25)When high (or open) the A/D will free-run with equally spaced measurement cycles every 40,002 clock pulses. If taken low, the converter will continue the full measurement cycle that it is doing and then hold this reading as long as R/H is held low. A short positive pulse (greater than 300n
21、s) will now initiate a new measurement cycle, beginning with between 1 and 10,001 counts of auto zero. If the pulse occurs before the full measurement cycle (40,002 counts) is completed, it will not be recognized and the converter will simply complete the measurement it is doing. An external indicat
22、ion that a full measurement cycle has been completed is that the first strobe pulse (see below) will occur 101 counts after the end of this cycle. Thus, if Run/HOLD is low and has been low for at least 101 counts, the converter is holding and ready to start a new measurement when pulsed high.STROBE
23、(Pin 26)This is a negative going output pulse that aids in transferring the BCD data to external latches, UARTs, or microprocessors. There are 5 negative going STROBE pulses that occur in the center of each of the digit drive pulses and occur once and only once for each measurement cycle starting 10
24、1 clock pulses after the end of the full measurement cycle. Digit 5 (MSD) goes high at the end of the measurement cycle and stays on for 201 counts. In the center of this digit pulse (to avoid race conditions between changing BCD and digit drives) the first STROBE pulse goes 1negative for 1/2 clock
25、pulse width. Similarly, after digit 5, digit 24 goes high (for 200 clock pulses) and 100 pulses later the STROBE goes negative for the second time. This continues through digit 1 (LSD) when the fifth and last STROBE pulse is sent. The digit drive will continue to scan (unless the previous signal was
26、 over range) but no additional STROBE pulses will be sent until a new measurement is available.BUSY (Pin 21)BUSY goes high at the beginning of signal integrate and stays high until the first clock pulse after zero crossing (or after end of measurement in the case of an over range). The internal latc
27、hes are enabled (i.e., loaded) during the first clock pulse after busy and are latched at the end of this clock pulse. The circuit automatically reverts to auto-zero when not BUSY, so it may also be considered a (Zl + AZ) signal. A very simple means for transmitting the data down a single wire pair
28、from a remote location would be to AND BUSY with clock and subtract 10,001 counts from the number of pulses received - as mentioned previously there is one “NO-count” pulse in each reference integrate cycle.OVERRANGE (Pin 27)This pin goes positive when the input signal exceeds the range (20,000) of
29、the converter. The output F/F is set at the end of BUSY and is reset to zero at the beginning of reference integrate in the next measurement cycle.UNDERRANGE (Pin 28)This pin goes positive when the reading is 9% of range or less. The output F/F is set at the end of BUSY (if the new reading is 1800 o
30、r less) and is reset at the beginning of signal integrate of the next reading.POLARlTY (Pin 23)This pin is positive for a positive input signal. It is valid even for a zero reading. In other words, +0000 means the signal is positive but less than the least significant bit. The converter can be used
31、as a null detector by forcing equal frequency of (+) and (-) readings. The null at this point should be less than 0.1 LSB. This output becomes valid at the beginning of reference integrate and remains correct until it is revalidated for the next measurement.Digit Drives (Pins 12, 17, 18, 19 and 20)E
32、ach digit drive is a positive going signal that lasts for 200 clock pulses. The scan sequence is D5 (MSD), D4 , D3 , D2 , and D1 (LSD). All five digits are scanned and this scan is continuous unless an over range occurs. Then all digit drives are blanked from the end of the strobe sequence until the
33、 beginning of Reference Integrate when D5 will start the scan again. This can give a blinking display as a visual indication of over range.BCD (Pins 13, 14, 15 and 16)The Binary coded Decimal bits B8 , B4 , B2 , and B1 are positive logic signals that go on simultaneously with the digit driver signal
34、.Component Value SelectionFor optimum performance of the analog section, care must be taken in the selection of values for the integrator capacitor and resistor, auto-zero capacitor, reference voltage, and conversion rate. These values must be chosen to suit the particular application.Integrating Re
35、sistorThe integrating resistor is determined by the full scale input voltage and the output current of the buffer used to charge the integrator capacitor. Both the buffer amplifier and the integrator have a class A output stage with 100 A of quiescent current. They can supply 20 A of drive current w
36、ith negligible non-linearity. Values of 5 A to 40 A give good results, with a nominal of 20 A, and the exact value of integrating resistor may be chosen by:Integrating CapacitorThe product of integrating resistor and capacitor should be selected to give the maximum voltage swing which ensures that t
37、he tolerance built-up will not saturate the integrator swing (approx. 0.3V from either supply). For +5V supplies and analog COMMON tied to supply ground, a +3.5V to +4V full scale integrator swing is fine, and 0.47 F is nominal. In general, the value of CINT is given by:A very important characterist
38、ic of the integrating capacitor is that it has low dielectric absorption to prevent roll-over or ratiometric errors. A good test for dielectric absorption is to use the capacitor with the input tied to the reference.This ratiometric condition should read half scale 0.9999, and any deviation is proba
39、bly due to dielectric absorption. Polypropylene capacitors give undetectable errors at reasonable cost. Polystyrene and polycarbonate capacitors may also be used in less critical applications.Auto-Zero and Reference CapacitorThe physical size of the auto-zero capacitor has an influence on the noise
40、of the system. A larger capacitor value reduces system noise. A larger physical size increases system noise. The reference capacitor should be large enough such that stray capacitance to ground from its nodes is negligible .The dielectric absorption of the reference cap and auto-zero cap are only im
41、portant at power-on or when the circuit is recovering from an overload. Thus, smaller or cheaper caps can be used here if accurate readings are not required for the first few seconds of recovery.Reference VoltageThe analog input required to generate a full scale output is The stability of the refere
42、nce voltage is a major factor in the overall absolute accuracy of the converter. For this reason, it is recommended that a high quality reference be used where high-accuracy absolute measurements are being made.Rollover Resistor and DiodeA small rollover error occurs in the ICL7135, but this can be
43、easily corrected by adding a diode and resistor in series between the INTegrator OUTput and analog COMMON or ground. The value shown in the schematics is optimum for the recommended conditions, but if integrator swing or clock frequency is modified, adjustment may be needed. The diode can be any sil
44、icon diode such as 1N914. These components can be eliminated if rollover error is not important and may be altered in value to correct other (small) sources of rollover as needed.Max Clock FrequencyThe maximum conversion rate of most dual-slope A/D converters is limited by the frequency response of
45、the comparator. The comparator in this circuit follows the integrator ramp with a 3s delay, and at a clock frequency of 160kHz (6s period) half of the first reference integrate clock period is lost in delay. This means that the meter reading will change from 0 to 1 with a 50 V input, 1 to 2 with a 1
46、50 V input, 2 to 3 with a 250 V input, etc. This transition at mid-point is considered desirable by most users; however, if the clock frequency is increased appreciably above 160kHz, the instrument will flash “1” on noise peaks even when the input is shorted.For many dedicated applications where the
47、 input signal is always of one polarity, the delay of the comparator need not be a limitation. Since the non-linearity and noise do not increase substantially with frequency, clock rates of up to 1MHz may be used. For a fixed clock frequency, the extra count or counts caused by comparator delay will
48、 be constant and can be subtracted out digitally.The clock frequency may be extended above 160kHz without this error, however, by using a low value resistor in serieswith the integrating capacitor. The effect of the resistor is to introduce a small pedestal voltage on to the integrator output at the
49、 beginning of the reference integrate phase. By careful selection of the ratio between this resistor and the integrating resistor (a few tens of ohms in the recommended circuit), the comparator delay can be compensated and the maximum clock frequency extended by approximately a factor of 3. At higher frequencies, ringing and second order breaks will cause significant non-linearities in the first few counts of
