PDF ADL5306 Data sheet ( Hoja de datos )

Número de pieza	ADL5306
Descripción	60 dB Range (100 nA to 100 UA) Low Cost Logarithmic Converter
Fabricantes	Analog Devices
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No Preview Available ! www.DataSheet4U.com FEATURES Optimized for fiber optic photodiode interfacing Measures current over 3 decades Law conformance 0.1 dB from 100 nA to 100 μA Single- or dual-supply operation (3 V to ±5.5 V total) Full log-ratio capabilities Temperature stable Nominal slope of 10 mV/dB (200 mV/decade) Nominal intercept of 1 nA (set by external resistor) Optional adjustment of slope and intercept Rapid response time for a given current level Miniature 16-lead chip scale package (LFCSP 3 mm × 3 mm) Low power: ~5 mA quiescent current APPLICATIONS Low cost optical power measurement Wide range baseband logarithmic compression Measurement of current and voltage ratios Optical absorbance measurement GENERAL DESCRIPTION The ADL5306∗ is a low cost microminiature logarithmic converter optimized for determining optical power in fiber optic systems. The ADL5306 is derived from the AD8304 and AD8305 translinear logarithmic converters. This family of devices provides wide measurement dynamic range in a versatile and easy-to-use form. A single-supply voltage between 3 V and 5.5 V is adequate; dual supplies may optionally be used. Low quiescent current (5 mA typical) permits use in battery-operated applications. IPD, the 100 nA to 100 µA input current applied to the INPT pin, is the collector current of an optimally scaled NPN transistor that converts this current to a voltage (VBE) with a precise logarithmic relationship. A second converter is used to handle the reference current, IREF, applied to IREF. These input nodes are biased slightly above ground (0.5 V). This is generally acceptable for photodiode applications where the anode does not need to be grounded. Similarly, this bias voltage is easily accounted for in generating IREF. The logarithmic front end’s output is available at VLOG. The basic logarithmic slope at this output is 200 mV/decade (10 mV/dB) nominal; a 60 dB range corresponds to a 600 mV output change. When this voltage (or the buffer output) is applied to an ADC that permits an external reference voltage to be employed, the ADL5306’s 2.5 V voltage reference output at VREF can be used to improve scaling accuracy. Rev. 0 Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective companies. DataSheet4 U .com 60 dB Range (100 nA to 100 µA) Low Cost Logarithmic Converter ADL5306 FUNCTIONAL BLOCK DIAGRAM NC VREF VPOS +5V ( )0.2 log10 IPD 1nA VOUT RREF 200kΩ 0.5V 20kΩ 2.5V BIAS 80kΩ GENERATOR COMM VBIAS IREF 1kΩ 1nF IPD INPT VBE2 14.2kΩ SCAL BFIN Q2 TEMPERATURE ILOG Q1 COMPENSATION 451Ω VLOG VBE1 6.69kΩ 1kΩ VSUM 1nF 1nF 0.5V VNEG COMM COMM 03727-0-001 Figure 1. Functional Block Diagram The logarithmic intercept (reference current) is nominally positioned at 1 nA by using the externally generated, 100 µA IREF current provided by a 200 kΩ resistor connected between VREF, at 2.5 V, and IREF, at 0.5 V. The intercept can be adjusted over a narrow range by varying this resistor. The part can also operate in a log-ratio mode, with limited accuracy, where the numerator and denominator currents are applied to INPT and IREF, respectively. A buffer amplifier is provided to drive substantial loads, raise the basic 10 mV/dB slope, serve as a precision comparator (threshold detector), or implement low-pass filters. Its rail-to-rail output stage can swing to within 100 mV of the positive and negative supply rails, and its peak current-sourcing capacity is 25 mA. A fundamental aspect of translinear logarithmic converters is that small-signal bandwidth falls as current level diminishes, and low frequency noise-spectral density increases. At the 100 nA level, the ADL5306’s bandwidth is about 100 kHz; it increases in proportion to IPD up to a maximum of about 10 MHz. The increase in noise level at low currents can be addressed by using a buffer amplifier to realize low-pass filters of up to three poles. The ADL5306 is available in a 16-lead LFCSP package and is specified for operation from–40°C to +85°C. ∗Protected by US Patents 4,604,532 and 5,519,308; other patents pending. One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.326.8703 © 2003 Analog Devices, Inc. All rights reserved. 1 page www.DataSheet4U.com ADL5306 PIN CONFIGURATION AND PIN FUNCTION DESCRIPTIONS COMM COMM COMM COMM 16 15 14 13 NC 1 12 VOUT VREF 2 IREF 3 ADL5306 11 SCAL 10 BFIN INPT 4 9 VLOG 5678 VSUM VNEG VNEG VPOS 03727-0-002 Figure 2. 16-Lead Leadframe Chip Scale Package (LFCSP) Table 3. Pin Function Descriptions Pin No. Mnemonic Function 1 NC N/A 2 VREF Reference Output Voltage of 2.5 V. 3 IREF Accepts (Sinks) Reference Current IREF. 4 INPT Accepts (Sinks) Photodiode Current IPD. Usually connected to photodiode anode such that photocurrent flows into INPT. 5 VSUM Guard Pin. Used to shield the INPT current line and for optional adjustment of the INPT and IREF node potential. 6, 7 VNEG 8 VPOS 9 VLOG Optional Negative Supply, VN. This pin is usually grounded; for details of usage, see the Applications section. Positive Supply, ( VP – VN ) ≤ 11 V. Output of the Logarithmic Front End. 10 BFIN Buffer Amplifier Noninverting Input. 11 SCAL Buffer Amplifier Inverting Input. 12 VOUT Buffer Output. 13–16 COMM Analog Ground. DataSheet4 U .com Rev. 0 \| Page 5 of 16 5 Page www.DataSheet4U.com ADL5306 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. NC VREF RREF 200kΩ 20kΩ 80kΩ 0.5V COMM VPOS +5V ( )0.5 log10 IPD 1nA 2.5V BIAS GENERATOR VOUT 12kΩ VBIAS IREF 1kΩ 1nF IPD INPT 1kΩ VSUM 1nF 1nF 0.5V 14.2kΩ SCAL BFIN VBE2 Q2 TEMPERATURE ILOG VLOG Q1 COMPENSATION 451Ω VBE1 CFLT 6.69kΩ 10nF COMM 8kΩ VNEG COMM 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. NC VREF RREF 200kΩ 20kΩ 80kΩ 0.5V COMM VPOS +5V ( )0.5 log10 IPD 1nA VOUT 2.5V BIAS GENERATOR 12kΩ VBIAS IREF 1kΩ 1nF IPD INPT 14.2kΩ SCAL BFIN VBE2 Q2 TEMPERATURE ILOG VLOG Q1 COMPENSATION 451Ω VBE1 CFLT 6.69kΩ 10nF 1kΩ 1nF VSUM COMM 0.5V VF VNEG Iq + Isig Isig = IPD + IREF RS ≤ VN – VF Iq + Isigmax VN VSUM – VF ≤ –0.5V C1 COMM 8kΩ 03727-0-023 Figure 23. Negative Supply Application Rev. 0 \| Page 11 of 16 DataSheet4 U .com 11 Page

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