Datasheet AD8304 (Analog Devices) - 8

AD8304 BASIC CONCEPTS Optical Measurements
The AD8304 uses an advanced circuit implementation that When interpreting the current IPD in terms of optical power inci- exploits the well known logarithmic relationship between the dent on a photodetector, it is necessary to be very clear about the base-to-emitter voltage, VBE, and collector current, IC, in a transducer properties of a biased photodiode. The units of this bipolar transistor, which is the basis of the important class of transduction process are expressed as amps per watt. The param- translinear circuits*: eter ␳, called the photodiode responsivity, is often used for this purpose. For a typical InGaAs p-i-n photodiode, the responsivity V = V log(I /I ) (1) BE T C S is about 0.9 A/W. There are two scaling quantities in this fundamental equation, namely It is also important to note that amps and watts are not usually the thermal voltage VT = kT/q and the saturation current IS. These related in this proportional manner. In purely electrical circuits, are of key importance in determining the slope and intercept for this a current IPD applied to a resistive load RL results in a power class of log amp. VT has a process-invariant value of 25.69 mV proportional to the square of the current (that is, I 2 PD RL). The at T = 25°C and varies in direct proportion to absolute temperature, reason for the difference in scaling for a photodiode interface is while IS is very much a process- and device-dependent parameter, that the current IPD flows in a diode biased to a fixed voltage, and is typically 10–16 A at T = 25°C but exhibits a huge variation VPDB. In this case, the power dissipated within the detector over the temperature range, by a factor of about a billion. diode is simply proportional to the current IPD (that is, IPDVPDB) While these variations pose challenges to the use of a transistor as and the proportionality of IPD to the optical power, POPT, is an accurate measurement device, the remarkable matching and preserved. isothermal properties of the components in a monolithic process I = P ρ (4) can be applied to reduce them to insignificant proportions, as will PD OPT be shown. Logarithmic amplifiers based on this unique property Accordingly, a reciprocal correspondence can be stated between the of the bipolar transistor are called translinear log amps to distin- intercept current, IZ, and an equivalent “intercept power,” PZ, thus: guish them from other Analog Devices products designed for RF I = P ρ (5) applications that use quite different principles. Z Z The very strong temperature variation of the saturation current and Equation 2 may then be written as: IS is readily corrected using a second reference transistor, having V = V log (P /P ) (6) LOG Y 10 OPT Z an identical variation, to stabilize the intercept. Similarly, propri- etary techniques are used to ensure that the logarithmic slope is For the AD8304 operating in its default mode, its IZ of 100 pA temperature-stable. Using these principles in a carefully scaled corresponds to a PZ of 110 picowatts, for a diode having a design, the now accurate relationship between the input current, responsivity of 0.9 A/W. Thus, an optical power of 3 mW would I generate: PD, applied to Pin INPT, and the voltage appearing at the inter- mediate output Pin VLOG is: V = 0 2 . V log (3 mW 110 / pW ) . (7) 10 = 1 487V LOG V = V log (I /I ) (2) LOG Y 10 PD Z Note that when using the AD8304 in optical applications, the V interpretation of V Y is called the slope voltage (in the case of base-10 logarithms, LOG is in terms of the equivalent optical it is also the “volts per decade”). The fixed current I power, the logarithmic slope remains 10 mV/dB at this output. Z is called the intercept. The scaling is chosen so that V This can be a little confusing since a decibel change on the Y is trimmed to 200 mV/decade (10 mV/dB). The intercept is positioned at optical side has a different meaning than on the electrical side. 100 pA; the output voltage V In either case, the logarithmic slope can always be expressed in LOG would cross zero when IPD is of this value. However, when using a single supply the actual units of mV per decade to help eliminate any confusion. VLOG must always be slightly above ground. On the other hand,
Decibel Scaling
by using a negative supply, this voltage can actually cross zero at In cases where the power levels are already expressed as so many the intercept value. decibels above a reference level (in dBm, for a reference of 1 mW), Using Equation 2, one can calculate the output for any value of I the logarithmic conversion has already been performed, and the PD. Thus, for an input current of 25 nA, “log ratio” in the above expressions becomes a simple differ- ence. One needs to be careful in assigning variable names here, V = 0 2 . V log (25 nA100 / pA) . (3) 10 = 0 4796 V LOG because “P” is often used to denote actual power as well as this In practice, both the slope and intercept may be altered, to either same power expressed in decibels, while clearly these are numeri- higher or lower values, without any significant loss of calibration cally different quantities. accuracy, by using one or two external resistors, often in conjunc- Such potential misunderstandings can be avoided by using “D” tion with the trimmed 2 V voltage reference at Pin VREF. 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: V = 20 D ( − D mV d / B ) (8) LOG OPT Z where DOPT is the optical power in decibels above a reference level, and D *For a basic discussion of the topic, see Translinear Circuits: An Historical Overview, Z is the equivalent intercept power relative to the same level. B. Gilbert, Analog Integrated Circuits and Signal Processing, 9, pp. 95–118, 1996. This convention will be used throughout this data sheet. –8– REV. A Document Outline FEATURES APPLICATIONS PRODUCT DESCRIPTION FUNCTIONAL BLOCK DIAGRAM SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION PIN FUNCTION DESCRIPTIONS ORDERING GUIDE Typical Performance Characteristics BASIC CONCEPTS Optical Measurements Decibel Scaling GENERAL STRUCTURE Bandwidth and Noise Considerations Chip Enable USING THE AD8304 Slope and Intercept Adjustments Low Supply Slope and Intercept Adjustment Using the Adaptive Bias Changing the Voltage at the Summing Node Implementing Low-Pass Filters Operation in Comparator Modes Using a Negative Supply APPLICATIONS Summing Node at Ground and Voltage Inputs Providing Negative Outputs and Rescaling Inverting the Slope Programmable Level Comparator with Hysteresis Programmable Multidecade Current Source Characterization Setups and Methods Evaluation Board OUTLINE DIMENSIONS Revision History