Zakładka z wyszukiwarką danych komponentów |
|
AD8307AN Arkusz danych(PDF) 10 Page - Analog Devices |
|
AD8307AN Arkusz danych(HTML) 10 Page - Analog Devices |
10 / 24 page AD8307 Rev. C | Page 10 of 24 The most widely used reference in RF systems is decibels above 1 mW in 50 Ω, written dBm. Note that the quantity (PIN – P0) is just dB. The logarithmic function disappears from the formula because the conversion has already been implicitly performed in stating the input in decibels. This is strictly a concession to popular convention; log amps manifestly do not respond to power (tacitly, power absorbed at the input), but rather to input voltage. The use of dBV (decibels with respect to 1 V rms) is more precise, though still incomplete, since waveform is involved, too. Since most users think about and specify RF signals in terms of power, more specifically, in dBm re: 50 Ω, this convention is used in specifying the performance of the AD8307. PROGRESSIVE COMPRESSION Most high speed, high dynamic range log amps use a cascade of nonlinear amplifier cells (Figure 22) to generate the logarithmic function from a series of contiguous segments, a type of piecewise linear technique. This basic topology immediately opens up the possibility of enormous gain bandwidth products. For example, the AD8307 employs six cells in its main signal path, each having a small signal gain of 14.3 dB (×5.2) and a −3 dB bandwidth of about 900 MHz. The overall gain is about 20,000 (86 dB) and the overall bandwidth of the chain is some 500 MHz, resulting in the incredible gain bandwidth product (GBW) of 10,000 GHz, about a million times that of a typical op amp. This very high GBW is an essential prerequisite for accurate operation under small signal conditions and at high frequencies. In Equation 2, however, the incremental gain decreases rapidly as VIN increases. The AD8307 continues to exhibit an essentially logarithmic response down to inputs as small as 50 μV at 500 MHz. VX VW STAGE 1 STAGE 2 STAGE N–1 STAGE N A A A A Figure 22. Cascade of Nonlinear Gain Cells To develop the theory, first consider a scheme slightly different from that employed in the AD8307, but simpler to explain and mathematically more straightforward to analyze. This approach is based on a nonlinear amplifier unit, called an A/1 cell, with the transfer characteristic shown in Figure 23. The local small signal gain δVOUT/δVIN is A, maintained for all inputs up to the knee voltage EK, above which the incremental gain drops to unity. The function is symmetrical: the same drop in gain occurs for instantaneous values of VIN less than –EK. The large signal gain has a value of A for inputs in the range −EK ≤ VIN ≤ +EK, but falls asymptotically toward unity for very large inputs. In logarithmic amplifiers based on this amplifier function, both the slope voltage and the intercept voltage must be traceable to the one reference voltage, EK. Therefore, in this fundamental analysis, the calibration accuracy of the log amp is dependent solely on this voltage. In practice, it is possible to separate the basic references used to determine VY and VX and in the case of the AD8307, VY is traceable to an on-chip band gap reference, while VX is derived from the thermal voltage kT/q and is later temperature corrected. SLOPE = A SLOPE = 1 AEK EK 0 INPUT A/1 Figure 23. A/1 Amplifier Function Let the input of an N-cell cascade be VIN, and the final output VOUT. For small signals, the overall gain is simply AN. A six stage system in which A = 5 (14 dB) has an overall gain of 15,625 (84 dB). The importance of a very high small signal gain in implementing the logarithmic function has been noted; however, this parameter is only of incidental interest in the design of log amps. From here onward, rather than considering gain, analyze the overall nonlinear behavior of the cascade in response to a simple dc input, corresponding to the VIN of Equation 1. For very small inputs, the output from the first cell is V1 = AVIN. The output from the second cell is V2 = A2 VIN, and so on, up to VN = AN VIN. At a certain value of VIN, the input to the Nth cell, VN–1, is exactly equal to the knee voltage EK. Thus, VOUT = AEK and since there are N–1 cells of gain A ahead of this node, calculate VIN = EK /AN–1. This unique situation corresponds to the lin-log transition, (labeled 1 in Figure 24). Below this input, the cascade of gain cells acts as a simple linear amplifier, while for higher values of VIN, it enters into a series of segments that lie on a logarithmic approximation (dotted line). RATIO OF A 2 1 3 3 2 EK/AN–1 EK/AN–2 EK/AN–3 EK/AN–4 LOG VIN (4A–3) EK VOUT (3A–2) EK (2A–1) EK AEK 0 (A–1) EK Figure 24. First Three Transitions Continuing this analysis, the next transition occurs when the input to the (N–1) stage just reaches EK; that is, when VIN = EK /AN–2. The output of this stage is then exactly AEK, and it is easily demonstrated (from the function shown in Figure 23) that the output of the final stage is (2A–1) EK (labeled 2 in Figure 24). Thus, the output has changed by an amount (A–1)EK for a change in VIN from EK /AN–1to EK/AN–2, that is, a ratio change |
Podobny numer części - AD8307AN |
|
Podobny opis - AD8307AN |
|
|
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 |