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ADL5306 Datasheet(PDF) 11 Page - Analog Devices |
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ADL5306 Datasheet(HTML) 11 Page - Analog Devices |
11 / 16 page ADL5306 Rev. 0 | Page 11 of 16 APPLICATIONS The ADL5306 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 (i.e., represented in decibel terms). Basic connections for measuring a single current input are shown in Figure 22, which includes various nonessential components, as will be explained. TEMPERATURE COMPENSATION BIAS GENERATOR 451 Ω 8k Ω 12k Ω 14.2k Ω 80k Ω 20k Ω RREF 200k Ω 6.69k Ω COMM COMM COMM VNEG VSUM VPOS INPT VREF NC IREF 0.5V +5V 2.5V 0.5V VBE1 ILOG VBE2 VOUT SCAL BFIN VLOG 0.5 log10 ( ) IPD 1nA 1k Ω 1k Ω 1nF 1nF 1nF VBIAS IPD Q2 Q1 CFLT 10nF 03727-0-022 Figure 22. Basic Connections for Fixed Intercept Use The 2 V difference in voltage between VREF and INPT, in conjunction with the external 200 kΩ resistor RREF, provides a reference current IREF of 100 µA into Pin IREF. The internal reference 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. 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, shown here 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/dec and a swing of 4 V at the buffer output would therefore 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 22 provides an overall slope of 0.5 V/dec (25 mV/dB). Thus, using IREF = 100 µA, VLOG runs from 0.2 V at IPD = 100 nA to 0.8 V at IPD = 100 µA. The buffer output runs from 0.5 V to 2.0 V, corresponding to a dynamic range of 60 dB electrical (30 dB optical) power. The optional capacitor from VLOG to ground forms a single- pole low-pass filter in combination with the 4.55 kΩ resistance at this pin. For example, using a CFLT of 10 nF, the –3 dB corner frequency is 3.2 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. For examples, see the AD8304 Data Sheet. The dynamic response of this overall input system is influenced by the external RC networks connected from the two inputs (INPT, IREF) to ground. These are required to stabilize the input systems over the full current range. The bandwidth changes with the input current due to the widely varying pole frequency. The RC network adds a zero to the input system to ensure stability over the full range of input current levels. The network values shown in Figure 22 will usually suffice, but some experimentation may be necessary when the photodiode’s capacitance is high. Although the two current inputs are similar, some care is needed to operate the reference input at extremes of current (<100 nA) and temperature (<0°C). Modifying the RC network to 4.7 nF and 2 kΩ will allow operation to –40°C at 10 nA. By inspecting the transient response to perturbations in IREF at representative current levels, the capacitor value can be adjusted to provide fast rise and fall times with acceptable settling. To fine-tune the network zero, the resistor value should be adjusted. USING A NEGATIVE SUPPLY Most applications of the ADL5306 require only a single supply of 3.0 V to 5.5 V. However, to provide further versatility, dual supplies may be employed, as illustrated in Figure 23. TEMPERATURE COMPENSATION BIAS GENERATOR 451 Ω 8k Ω 12k Ω 14.2k Ω 80k Ω 20k Ω RREF 200k Ω 6.69k Ω COMM COMM COMM VNEG VSUM – VF ≤ –0.5V VF RS ≤ Iq + Isig Isig = IPD + IREF VN VSUM VPOS INPT VREF NC IREF 0.5V +5V 2.5V 0.5V VBE1 ILOG VBE2 VOUT SCAL BFIN VLOG 0.5 log10 ( ) IPD 1nA 1k Ω 1k Ω 1nF 1nF VBIAS IPD Q2 Q1 CFLT 10nF C1 VN – VF Iq + Isigmax 03727-0-023 Figure 23. Negative Supply Application |
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