Electronic Components Datasheet Search |
|
ADL5306 Datasheet(PDF) 9 Page - Analog Devices |
|
ADL5306 Datasheet(HTML) 9 Page - Analog Devices |
9 / 16 page ADL5306 GENERAL STRUCTURE The ADL5306 addresses a wide variety of interfacing conditions to meet the needs of fiber optic supervisory systems, and is useful in many nonoptical applications. This section explains the structure of this unique style of translinear log amp. The simplified schematic in Figure 21 shows the key elements. Q2 Q1 451 Ω 14.2k Ω 80k Ω 20k Ω 6.69k Ω PHOTODIODE INPUT CURRENT BIAS GENERATOR TEMPERATURE COMPENSATION (SUBTRACT AND DIVIDE BY T°K) 2.5V COMM COMM VLOG VNEG (NORMALLY GROUNDED) VSUM INPT VREF IREF IREF VBE1 VBE2 IPD VBE1 VBE2 44 µA/dec 2.5V 0.5V 0.5V 0.5V 03727-0-021 Figure 21. Simplified Schematic The photodiode current IPD is received at Pin INPT. The voltage at this node is essentially equal to the voltage on the two adjacent guard pins, VSUM and IREF, due to the low offset voltage of the JFET op amp. Transistor Q1 converts IPD to a corresponding logarithmic voltage, as shown in Equation 1. A finite positive value of VSUM is needed to bias the collector of Q1 for the usual case of a single-supply voltage. This is internally set to 0.5 V, one fifth of the 2.5 V reference voltage appearing on Pin VREF. The resistance at the VSUM pin is nominally 16 kΩ; this voltage is not intended as a general bias source. The ADL5306 also supports the use of an optional negative supply voltage, VN , at Pin VNEG. When VN is –0.5 V or more negative, VSUM may be connected to ground; thus, INPT and IREF assume this potential. This allows operation as a voltage- input logarithmic converter by the inclusion of a series resistor at either or both inputs. Note that the resistor setting, IREF, will need to be adjusted to maintain the intercept value. It should also be noted that the collector-emitter voltages of Q1 and Q2 are now the full VN, and effects due to self-heating will cause errors at large input currents. The input-dependent VBE1 of Q1 is compared with the reference VBE2 of a second transistor, Q2, operating at IREF. This is generated externally to a recommended value of 10 µA. However, other values over a several-decade range can be used with a slight degradation in law conformance (see Figure 8). THEORY The base-emitter voltage of a BJT (bipolar junction transistor) can be expressed by the following equation, which immediately shows its basic logarithmic nature: VBE = kT/q ln(IC / IS) (1) where: IC is the collector current IS is a scaling current, typically only 10–17 A kT/q is the thermal voltage, proportional to absolute temperature (PTAT), and is 25.85 mV at 300 K. IS is never precisely defined and exhibits an even stronger temperature dependence, varying by a factor of roughly a billion between –35°C and +85°C. Thus, to make use of the BJT as an accurate logarithmic element, both of these temperature- dependencies must be eliminated. The difference between the base-emitter voltages of a matched pair of BJTs, one operating at the photodiode current IPD and the other operating at a reference current IREF, can be written as VBE1 – VBE2 = kT/q ln(IPD / IS) – kT/q ln(IREF / IS) = ln(10) kT/q log10(IPD /IREF) (2) = 59.5 mV log10(IPD /IREF) (T = 300 K) The uncertain, temperature-dependent saturation current, IS, that appears in Equation 1 has therefore been eliminated. To eliminate the temperature variation of kT/q, this difference voltage is processed by what is essentially an analog divider. Effectively, it puts a variable under Equation 2. The output of this process, which also involves a conversion from voltage mode to current mode, is an intermediate, temperature- corrected current: ILOG = IY log10(IPD / IREF) (3) where IY is an accurate, temperature-stable scaling current that determines the slope of the function (change in current per decade). For the ADL5306, IY is 44 µA, resulting in a temperature-independent slope of 44 µA/decade for all values of IPD and IREF . This current is subsequently converted back to a voltage-mode output, VLOG, scaled 200 mV/decade. Rev. 0 | Page 9 of 16 |
Similar Part No. - ADL5306 |
|
Similar Description - ADL5306 |
|
|
Link URL |
Privacy Policy |
ALLDATASHEET.NET |
Does ALLDATASHEET help your business so far? [ DONATE ] |
About Alldatasheet | Advertisement | Contact us | Privacy Policy | Link Exchange | Manufacturer List All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |