поискавой системы для электроныых деталей |
|
AD7740 датащи(PDF) 9 Page - Analog Devices |
|
AD7740 датащи(HTML) 9 Page - Analog Devices |
9 / 11 page AD7740 REV. A –9– A/D Conversion Techniques Using the AD7740 One method of using a VFC in an A/D system is to count the output pulses of FOUT for a fixed gate interval (see Figure 9). This fixed gate interval should be generated by dividing down the clock input frequency. This ensures that any errors due to clock jitter or clock frequency drift are eliminated. The ratio of the FOUT frequency to the clock frequency is what is important here, not the absolute value of FOUT. The frequency divi- sion can be done by a binary counter where CLKIN is the counter input. FREQUENCY DIVIDER GATE SIGNAL FOUT VIN TO P AD7740 COUNTER CLOCK GENERATOR CLKIN Figure 9. A/D Conversion Using the AD7740 VFC Figure 10 shows the waveforms of CLKIN, FOUT, and the Gate signal. A counter counts the rising edges of FOUT while the Gate signal is high. Since the gate interval is not synchronized with FOUT, there is a possibility of a counting inaccuracy. Depending on FOUT, an error of one count may occur. GATE tGATE FOUT CLKIN Figure 10. Waveforms in an A/D Converter Using a VFC The clock frequency and the gate time determine the resolution of such an ADC. If 12-bit resolution is required and CLKIN is 1 MHz (therefore, FOUTMAX is 0.9 MHz), the minimum gate time required is calculated as follows: N counts at Full Scale (0.9 MHz) will take (N/0.9 × 106) seconds = minimum gate time N is the total number of codes for a given resolution; 4096 for 12 bits. minimum gate time = (4096/0.9 × 106) seconds = 4.551 ms Since TGATE × FOUT MAX = number of counts at full scale, the fastest conversion for a given resolution can be performed with the highest CLKIN frequency. If the output frequency is measured by counting pulses gated to a signal derived from the clock, the clock stability is unimportant and the device simply performs as a voltage-controlled frequency divider, producing a high-resolution ADC. The inherent mono- tonicity of the transfer function and wide range of input clock frequencies allows the conversion time and resolution to be optimized for specific applications. Another parameter is taken into account when choosing the length of the gate interval. Because the integration period of the VFC is equal to the gate interval, any interfering signal can be rejected by counting for an integer number of periods of the interfering signal. For example, a gate interval of 100 ms will give normal-mode rejection of 50 Hz and 60 Hz signals. Isolation Applications The AD7740 can also be used in isolated analog signal trans- mission applications. Due to noise, safety requirements or distance, it may be necessary to isolate the AD7740 from any controlling circuitry. This can easily be achieved by using opto-isolators. This is extremely useful in overcoming ground loops between equipment. The analog voltage to be transmitted is converted to a pulse train using the VFC. An opto-isolator circuit is used to couple this pulse train across an isolation barrier using light as the connecting medium. The input LED of the isolator is driven from the output of the AD7740. At the receiver side, the output transistor is operated in the photo-transistor mode. The pulse train can be reconverted to an analog voltage using a frequency- to-voltage converter; alternatively, the pulse train can be fed into a counter to generate a digital signal. The analog and digital sections of the AD7740 have been designed to allow operation from a single-ended power source, simplify- ing its use with isolated power supplies. Figure 11 shows a general purpose VFC circuit using a low cost opto-isolator. A 5 V power supply is assumed for both the iso- lated (VDD) and local (VCC) supplies. VIN FOUT GND1 VCC GND2 ISOLATION BARRIER R 0.1 F 10 F AD7740 VDD OPTOCOUPLER Figure 11. Opto-Isolated Application |
Аналогичный номер детали - AD7740_15 |
|
Аналогичное описание - AD7740_15 |
|
|
ссылки URL |
Конфиденциальность |
ALLDATASHEETRU.COM |
Вашему бизинису помогли Аллдатащит? [ DONATE ] |
Что такое Аллдатащит | реклама | контакт | Конфиденциальность | обмен ссыками | поиск по производителю 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 |