поискавой системы для электроныых деталей |
|
AD8307AN датащи(PDF) 10 Page - Analog Devices |
|
AD8307AN датащи(HTML) 10 Page - Analog Devices |
10 / 20 page AD8307 –10– REV. A We can model these detectors as being essentially linear gm cells, but producing an output current independent of the sign of the voltage applied to the input of each cell. That is, they imple- ment the absolute-value function. Since the output from the later A/0 stages closely approximates an amplitude-symmetric square wave for even moderate input levels (most stages of the amplifier chain operate in a limiting mode), the current output from each detector is almost constant over each period of the input. Somewhat earlier detectors stages produce a waveform having only very brief dropouts, while the detectors nearest the input produce a low level almost-sinusoidal waveform at twice the input frequency. These aspects of the detector system result in a signal that is easily filtered, resulting in low residual ripple on the output. Intercept Calibration All monolithic log amps from Analog Devices include accurate means to position the intercept voltage VX (or equivalent power for a demodulating log amp). Using the scheme shown in Figure 24, the basic value of the intercept level departs considerably from that predicted by the simpler analyses given earlier. How- ever, the intrinsic intercept voltage is still proportional to EK, which is PTAT (Equation 5). Recalling that the addition of an offset to the output produces an effect which is indistinguishable from a change in the position of the intercept, we can cancel the left-right motion of VX resulting from the temperature variation of EK by adding an offset having the required temperature behavior. The precise temperature-shaping of the intercept-positioning offset results in a log amp having stable scaling parameters, making it a true measurement device, for example, as a cali- brated Received Signal Strength Indicator (RSSI). In this appli- cation, one is more interested in the value of the output for an input waveform which is invariably sinusoidal. The input level may alternatively be stated as an equivalent power, in dBm, but here we must step carefully. It is essential to know the load impedance in which this power is presumed to be measured. In RF practice, it is generally safe to assume a reference imped- ance of 50 Ω, in which 0 dBm (1 mW) corresponds to a sinusoi- dal amplitude of 316.2 mV (223.6 mV rms). The intercept may likewise be specified in dBm. For the AD8307, it is positioned at –84 dBm, corresponding to a sine amplitude of 20 µV. It is important to bear in mind that log amps do not respond to power, but to the voltage applied to their input. The AD8307 presents a nominal input impedance much higher than 50 Ω (typically 1.1 kΩ at low frequencies). A simple input matching network can considerably improve the sensitivity of this type of log amp. This will increase the voltage applied to the input and thus alter the intercept. For a 50 Ω match, the voltage gain is 4.8 and the whole dynamic range moves down by 13.6 dB (see Figure 33). Note that the effective intercept is a function of waveform. For example, a square-wave input will read 6 dB higher than a sine wave of the same amplitude, and a Gaussian noise input 0.5 dB higher than a sine wave of the same rms value. Offset Control In a monolithic log amp, direct-coupling between the stages is used for several reasons. First, this avoids the use of coupling capacitors, which may typically have a chip area equal to that of a basic gain cell, thus considerably increasing die size. Second, the capacitor values predetermine the lowest frequency at which the log amp can operate; for moderate values, this may be as high as 30 MHz, limiting the application range. Third, the para- sitic (back-plate) capacitance lowers the bandwidth of the cell, further limiting the applications. But the very high dc gain of a direct-coupled amplifier raises a practical issue. An offset voltage in the early stages of the chain is indistinguishable from a ‘real’ signal. If it were as high as, say, 400 µV, it would be 18 dB larger than the smallest ac signal (50 µV), potentially reducing the dynamic range by this amount. This problem is averted by using a global feedback path from the last stage to the first, which corrects this offset in a similar fashion to the dc negative feedback applied around an op amp. The high frequency components of the signal must, of course, be removed, to prevent a reduction of the HF gain in the for- ward path. In the AD8307, this is achieved by an on-chip filter, providing sufficient suppression of HF feedback to allow operation above 1 MHz. To extend the range below this frequency, an external capacitor may be added. This permits the high pass corner to be lowered to audio frequencies using a capacitor of modest value. Note that this capacitor has no effect on the minimum signal frequency for input levels above the offset voltage: this extends down to dc (for a signal applied directly to the input pins). The offset voltage will vary from part to part; some will exhibit essen- tially stable offsets of under 100 µV, without the benefit of an offset adjustment. Extension of Range The theoretical dynamic range for the basic log amp shown in Figure 24 is A N. For A = 5.2 (14.3 dB) and N = 6, it is 20,000 or 86 dB. The actual lower end of the dynamic range is largely determined by the thermal noise floor, measured at the input of the chain of amplifiers. The upper end of the range is extended upward by the addition of top-end detectors. The input signal is applied to a tapped attenuator, and progressively smaller signals are applied to three passive rectifying gm cells whose outputs are summed with those of the main detectors. With care in design, the extension to the dynamic range can be seamless over the full frequency range. For the AD8307 it amounts to a further 27 dB. The total dynamic range is thus theoretically 113 dB. The speci- fied range of 90 dB (–74 dBm to +16 dBm) is that for high accuracy, calibrated operation, and includes the low end degra- dation due to thermal noise, and the top end reduction due to voltage limitations. The additional stages are not, however, redundant, but are needed to maintain accurate logarithmic conformance over the central region of the dynamic range, and in extending the usable range considerably beyond the specified range. In applications where log-conformance is less demand- ing, the AD8307 can provide over 95 dB of range. |
Аналогичный номер детали - AD8307AN |
|
Аналогичное описание - AD8307AN |
|
|
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 |