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AD8304ARU датащи(PDF) 8 Page - Analog Devices |
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AD8304ARU датащи(HTML) 8 Page - Analog Devices |
8 / 20 page REV. A –8– AD8304 BASIC CONCEPTS The AD8304 uses an advanced circuit implementation that exploits the well known logarithmic relationship between the base-to-emitter voltage, VBE, and collector current, IC, in a bipolar transistor, which is the basis of the important class of translinear circuits *: VV I I BE T C S = log( / ) (1) There are two scaling quantities in this fundamental equation, namely the thermal voltage VT = kT/q and the saturation current IS. These are of key importance in determining the slope and intercept for this class of log amp. VT has a process-invariant value of 25.69 mV at T = 25 °C and varies in direct proportion to absolute temperature, while IS is very much a process- and device-dependent parameter, and is typically 10 –16 A at T = 25 °C but exhibits a huge variation over the temperature range, by a factor of about a billion. While these variations pose challenges to the use of a transistor as an accurate measurement device, the remarkable matching and isothermal properties of the components in a monolithic process can be applied to reduce them to insignificant proportions, as will be shown. Logarithmic amplifiers based on this unique property of the bipolar transistor are called translinear log amps to distin- guish them from other Analog Devices products designed for RF applications that use quite different principles. The very strong temperature variation of the saturation current IS is readily corrected using a second reference transistor, having an identical variation, to stabilize the intercept. Similarly, propri- etary techniques are used to ensure that the logarithmic slope is temperature-stable. Using these principles in a carefully scaled design, the now accurate relationship between the input current, IPD, applied to Pin INPT, and the voltage appearing at the inter- mediate output Pin VLOG is: VV I I LOG Y PD Z = log ( / ) 10 (2) VY is called the slope voltage (in the case of base-10 logarithms, it is also the “volts per decade”). The fixed current IZ is called the intercept. The scaling is chosen so that VY is trimmed to 200 mV/decade (10 mV/dB). The intercept is positioned at 100 pA; the output voltage VLOG would cross zero when IPD is of this value. However, when using a single supply the actual VLOG must always be slightly above ground. On the other hand, by using a negative supply, this voltage can actually cross zero at the intercept value. Using Equation 2, one can calculate the output for any value of IPD. Thus, for an input current of 25 nA, VV nA pA V LOG == 02 25 100 0 4796 10 . log ( / ) . (3) In practice, both the slope and intercept may be altered, to either higher or lower values, without any significant loss of calibration accuracy, by using one or two external resistors, often in conjunc- tion with the trimmed 2 V voltage reference at Pin VREF. Optical Measurements When interpreting the current IPD in terms of optical power inci- dent on a photodetector, it is necessary to be very clear about the transducer properties of a biased photodiode. The units of this transduction process are expressed as amps per watt. The param- eter , called the photodiode responsivity, is often used for this purpose. For a typical InGaAs p-i-n photodiode, the responsivity is about 0.9 A/W. It is also important to note that amps and watts are not usually related in this proportional manner. In purely electrical circuits, a current IPD applied to a resistive load RL results in a power proportional to the square of the current (that is, IPD 2 R L). The reason for the difference in scaling for a photodiode interface is that the current IPD flows in a diode biased to a fixed voltage, VPDB. In this case, the power dissipated within the detector diode is simply proportional to the current IPD (that is, IPDVPDB) and the proportionality of IPD to the optical power, POPT, is preserved. IP PD OPT =ρ (4) Accordingly, a reciprocal correspondence can be stated between the intercept current, IZ, and an equivalent “intercept power,” PZ, thus: IP ZZ =ρ (5) and Equation 2 may then be written as: VV P P LOG Y OPT Z = log ( / ) 10 (6) For the AD8304 operating in its default mode, its IZ of 100 pA corresponds to a PZ of 110 picowatts, for a diode having a responsivity of 0.9 A/W. Thus, an optical power of 3 mW would generate: VV mW pW V LOG == 02 3 110 1 487 10 . log ( / ) . (7) Note that when using the AD8304 in optical applications, the interpretation of VLOG is in terms of the equivalent optical power, the logarithmic slope remains 10 mV/dB at this output. This can be a little confusing since a decibel change on the optical side has a different meaning than on the electrical side. In either case, the logarithmic slope can always be expressed in units of mV per decade to help eliminate any confusion. Decibel Scaling In cases where the power levels are already expressed as so many decibels above a reference level (in dBm, for a reference of 1 mW), the logarithmic conversion has already been performed, and the “log ratio” in the above expressions becomes a simple differ- ence. One needs to be careful in assigning variable names here, because “P” is often used to denote actual power as well as this same power expressed in decibels, while clearly these are numeri- cally different quantities. Such potential misunderstandings can be avoided by using “D” to denote decibel powers. The quantity VY (“volts per decade”) must now be converted to its decibel value, VY´ = VY/10, because there are 10 dB per decade in the context of a power measurement. Then it can be stated that: VD D mV dB LOG OPT Z =− () 20 / (8) where DOPT is the optical power in decibels above a reference level, and DZ is the equivalent intercept power relative to the same level. This convention will be used throughout this data sheet. *For a basic discussion of the topic, see Translinear Circuits: An Historical Overview, B. Gilbert, Analog Integrated Circuits and Signal Processing, 9, pp. 95–118, 1996. |
Аналогичный номер детали - AD8304ARU |
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Аналогичное описание - AD8304ARU |
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