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AD8305 Fiches technique(PDF) 10 Page - Analog Devices |
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AD8305 Fiches technique(HTML) 10 Page - Analog Devices |
10 / 20 page REV. A –10– AD8305 Managing Intercept and Slope When using a single supply, VRDZ should be directly connected to VREF to allow operation over the entire five-decade input current range. As noted previously, this introduces an accurate offset voltage of 0.8 V at the VLOG pin, equivalent to four decades, resulting in a logarithmic transfer function that can be written as: VV I I VI I LOG Y PD REF YPD INTC =¥ () = () log / log / 10 4 10 10 (6) where IINTC = IREF/10 4 Thus, the effective intercept current IINTC is only one ten- thousandth of IREF, corresponding to 1 nA when using the recommended value of IREF = 10 mA. The slope can be reduced by attaching a resistor to the VLOG pin. This is strongly discouraged, in view of the fact that the on-chip resistors will not ratio correctly to the added resistance. Also, it is rare that one would want to lower the basic slope of 10 mV/dB; if this is needed, it should be effected at the low impedance output of the buffer, which is provided to avoid such miscalibration and also allow higher slopes to be used. The AD8305 buffer is essentially an uncommitted op amp with rail-to-rail output swing, good load-driving capabilities and a unity-gain bandwidth of >12 MHz. In addition to allowing the introduction of gain, using standard feedback networks and thereby increasing the slope voltage VY, the buffer can be used to implement multipole low-pass filters, threshold detectors, and a variety of other functions. Further details of these can be found in the AD8304 data sheet. Response Time and Noise Considerations The response time and output noise of the AD8305 are funda- mentally a function of the signal current IPD. For small currents, the bandwidth is proportional to IPD, as shown in TPC 13. The output low frequency voltage-noise spectral-density is a function of IPD (TPC 15) and also increases for small values of IREF. Details of the noise and bandwidth performance of translinear log amps can be found in the AD8304 Data Sheet. APPLICATIONS The AD8305 is easy to use in optical supervisory systems and in similar situations where a wide ranging current is to be converted to its logarithmic equivalent, which is represented in decibel terms. Basic connections for measuring a single-current input are shown in Figure 2, which also includes various nonessential com- ponents, as will be explained. BIAS GENERATOR VLOG COMM VNEG VSUM IREF 0.5V 80k 0.5V TEMPERATURE COMPENSATION VBE2 6.69k Q1 COMM 20k 451 VREF VRDZ 14.2k Q2 INPT COMM 2.5V VPOS BFIN SCAL VOUT ILOG IPD 200k 0.5 log10 IPD ( ) 1nA 1nF 1nF 1k VBIAS 1nF 1k 12k +5V CFLT 10nF 8k – + VBE1 Figure 2. Basic Connections for Fixed Intercept Use The 2 V difference in voltage between the VREF and INPT pins in conjunction with the external 200 k W resistor RREF provide a reference current IREF of 10 mA into Pin IREF. Connecting pin VRDZ to VREF raises the voltage at VLOG by 0.8 V, effectively lowering the intercept current IINTC by a factor of 104 to position it at 1 nA. A wide range of other values for IREF, from under 100 nA to over 1 mA, may be used. The effect of such changes is shown in TPC 3. Any temperature variation in RREF must be taken into account when estimating the stability of the intercept. Also, the overall noise will increase when using very low values of IREF. In fixed- intercept applications, there is little benefit in using a large reference current, since this only compresses the low current end of the dynamic range when operated from a single supply, here shown as 5 V. The capacitor between VSUM and ground is recommended to minimize the noise on this node and to help provide a clean reference current. Since the basic scaling at VLOG is 0.2 V/decade, and thus a swing of 4 V at the buffer output would correspond to 20 decades, it will often be useful to raise the slope to make better use of the rail- to-rail voltage range. For illustrative purposes, the circuit in Figure 2 provides an overall slope of 0.5 V/decade (25 mV/dB). Thus, using IREF = 10 mA, VLOG runs from 0.2 V at IPD = 10 nA to 1.4 V at IPD = 1 mA while the buffer output runs from 0.5 V to 3.5 V, corresponding to a dynamic range of 120 dB (electrical, that is, 60 dB optical power). The optional capacitor from VLOG to ground forms a single-pole low-pass filter in combination with the 4.55 k W resistance at this pin. For example, using a CFLT of 10 nF, the –3 dB corner frequency is 3.5 kHz. Such filtering is useful in minimizing the output noise, particularly when IPD is small. Multipole filters are more effective in reducing the total noise; examples are provided in the AD8304 data sheet. |
Numéro de pièce similaire - AD8305 |
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Description similaire - AD8305 |
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