 |
Zakładka z wyszukiwarką danych komponentów |
|
|
Zakładka z wyszukiwarką danych komponentów |
|
ADC0841BCN Arkusz danych(PDF) 7 Page - National Semiconductor (TI) |
|
|
||||||||||||||||||||||||||||||
ADC0841BCN Arkusz danych(HTML) 7 Page - National Semiconductor (TI) |
|
|
7 / 16 page  Functional Description A conversion is initiated via the CS and WR lines. If the data from a previous conversion is not read, the INTR line will be low. The falling edge of WR will reset the INTR line high and ready the A/D for a conversion cycle. The rising edge of WR starts a conversion. After the conversion cycle (t C ≤ 60 µsec), which is set by the internal clock frequency, the digital data is transferred to the output latch and the INTR is asserted low. Taking CS and RD low resets INTR output high and transfers the conversion result on the output data lines (DB0–DB7). Applications Information 1.0 REFERENCE CONSIDERATIONS The voltage applied to the reference input of this converter defines the voltage span of the analog input (the difference between V IN(MAX) and VIN(MIN)) over which the 256 possible output codes apply. The device can be used in either ratio- metric applications or in systems requiring absolute accu- racy. The reference pin must be connected to a voltage source capable of driving the minimum reference input resis- tance of 1.1 k Ω. This pin is the top of a resistor divider string used for the successive approximation conversion. In a ratiometric system ( Figure 1a), the analog input voltage is proportional to the voltage used for the A/D reference. This voltage is typically the system power supply, so the V REF pin can be tied to V CC. This technique relaxes the stability re- quirements of the system reference as the analog input and A/D reference move together maintaining the same output code for a given input condition. For absolute accuracy ( Figure 1b), where the analog input varies between very specific voltage limits, the reference pin can be biased with a time and temperature stable voltage source. The LM385 and LM336 reference diodes are good low current devices to use with this converter. The maximum value of the reference is limited to the V CC supply voltage. The minimum value, however, can be quite small (see Typical Performance Characteristics) to allow di- rect conversions of transducer outputs providing less than a 5V output span. Particular care must be taken with regard to noise pickup, circuit layout and system error voltage sources when operating with a reduced span due to the increased sensitivity of the converter (1 LSB equals V REF/256). 2.0 THE ANALOG INPUTS 2.1 Analog Differential Voltage Inputs and Common-Mode Rejection The differential inputs of this converter actually reduce the effects of common-mode input noise, a signal common to both selected “+” and “−” inputs for a conversion (60 Hz is most typical). The time interval between sampling the “+” in- put and then the “−” input is 1⁄2 of a clock period. The change in the common-mode voltage during this short time interval can cause conversion errors. For a sinusoidal common-mode signal this error is: where f CM is the frequency of the common-mode signal, Vpeak is its peak voltage value and t C is the conversion time. For a 60 Hz common-mode signal to generate a 1⁄4 LSB error ( ≈ 5 mV) with the converter running at 40 µS, its peak value would have to be 5.43V. This large common-mode signal is much greater than that generally found in a well designed data acquisition system. 2.2 Input Current Due to the sampling nature of the analog inputs, short dura- tion spikes of current enter the “+” input and exit the “−” input at the clock edges during the actual conversion. These cur- rents decay rapidly and do not cause errors as the internal comparator is strobed at the end of a clock period. Bypass capacitors at the inputs will average these currents and cause an effective DC current to flow through the output re- sistance of the analog signal source. Bypass capacitors should not be used if the source resistance is greater than 1k Ω. An op amp RC active low pass filter can provide both impedance buffering and noise filtering should a high imped- ance signal source be required. 3.0 OPTIONAL ADJUSTMENTS 3.1 Zero Error The zero of the A/D does not require adjustment. If the mini- mum analog input voltage value, V IN(MIN), is not ground, a zero offset can be done. The converter can be made to out- put 0000 0000 digital code for this minimum input voltage by biasing the V IN (−) input at this VIN(MIN) value. The zero error of the A/D converter relates to the location of the first riser of the transfer function and can be measured by grounding the V − input and applying a small magnitude posi- tive voltage to the V + input. Zero error is the difference be- tween actual DC input voltage which is necessary to just cause an output digital code transition from 0000 0000 to 0000 0001 and the ideal 1⁄2 LSB value (1⁄2 LSB=9.8 mV for V REF=5.000 VDC). 3.2 Full-Scale The full-scale adjustment can be made by applying a differ- ential input voltage which is 1 1⁄2 LSB down from the desired analog full-scale voltage range and then adjusting the mag- nitude of the V REF input for a digital output code changing from 1111 1110 to 1111 1111. 3.3 Adjusting for an Arbitrary Analog Input Voltage Range If the analog zero voltage of the A/D is shifted away from ground (for example, to accommodate an analog input signal which does not go to ground), this new zero reference should be properly adjusted first. A voltage which equals this desired zero reference plus 1⁄2 LSB (where the LSB is calcu- lated for the desired analog span, 1 LSB = analog span/256) is applied to the “+” input (V IN (+)) and the zero reference volt- age at the “−” input (V IN (−)) should then be adjusted to just obtain the 00 HEX to 01HEX code transition. www.national.com 7 |
Podobny numer części - ADC0841BCN |
|
Podobny opis - ADC0841BCN |
|
|
|
Link URL |
| Polityka prywatności |
| ALLDATASHEET.PL |
| Czy Alldatasheet okazała się pomocna? [ DONATE ] |
O Alldatasheet | Reklama | Kontakt | Polityka prywatności | Linki | Lista producentów All Rights Reserved©Alldatasheet.com |
| Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
|
Family Site : ic2ic.com |
icmetro.com |
Polish ▼
English
Chinese
German
Japanese
Russian
Korean
Spanish
French
Italian
Portuguese
Polish
Vietnamese
Indian
Mexican
British
New Zealand
