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ADN2870 Datasheet(PDF) 15 Page - Analog Devices |
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ADN2870 Datasheet(HTML) 15 Page - Analog Devices |
15 / 20 page ADN2870 Rev. 0 | Page 15 of 20 VCC PHOTODIODE ADN2870 PAVSET R µC ADC INPUT Figure 31. Single Measurement of IMPD Across a Sense Resistor in Resistor Setpoint IMPD Monitoring LOOP BANDWIDTH SELECTION To ensure that the ADN2870 control loops have sufficient bandwidth, the average power loop capacitor (PAVCAP) and the extinction ratio loop capacitor (ERCAP) are calculated using the lasers slope efficiency (watts/amps) and the average power required. For resistor point control: ) ( 6 2 . 3 Farad PAV LI E PAVCAP × − = ) ( 2 Farad PAVCAP ERCAP = For voltage setpoint control: ) ( 6 28 . 1 Farad PAV LI E PAVCAP × − = ) ( 2 Farad PAVCAP ERCAP = where PAV is the average power required and LI (mW/mA) is the typical slope efficiency at 25°C of a batch of lasers that are used in a design. The capacitor value equation is used to get a centered value for the particular type of laser that is used in a design and average power setting. The laser LI can vary by a factor of 7 between different physical lasers of the same type and across temperature without the need to recalculate the PAVCAP and ERCAP values. In ac coupling configuration the LI can be calculated as follows: Imod P0 P1 LI − = (mW/mA) where P1 is the optical power (mW) at the one level, and P0 is the optical power (mW) at the zero level. These capacitors are placed between the PAVCAP and ERCAP pins and ground. It is important that these capacitors are low leakage multilayer ceramics with an insulation resistance greater than 100 GΩ or a time constant of 1000 sec, whichever is less. The capacitor tolerance may be ±30% from the calculated value to the available off the shelf value including the capacitors own tolerance. POWER CONSUMPTION The ADN2870 die temperature must be kept below 125°C. The LFCSP package has an exposed paddle, which should be con- nected such that is at the same potential as the ADN2870 ground pins. Power consumption can be calculated as follows: ICC = ICC min + 0.3 IMOD P = VCC × ICC + (IBIAS × VBIAS_PIN) + IMOD (VMODP_PIN + VMODN_PIN)/2 TDIE = TAMBIENT + θJA × P Thus, the maximum combination of IBIAS + IMOD must be calculated. where: ICC min = 30 mA, the typical value of ICC provided in the Specifications with IBIAS = IMOD = 0. TDIE is the die temperature. TAMBIENT is the ambient temperature. VBIAS_PIN is the voltage at the IBIAS pin. VMODP_PIN is the voltage at the IMODP pin. VMODN_PIN is the voltage at the IMODN pin. AUTOMATIC LASER SHUTDOWN (TX_DISABLE) ALS (TX disable) is an input that is used to shut down the transmitter optical output. The ALS pin is pulled up internally with a 6 kΩ resistor, and conforms to SFP MSA specification. When ALS is logic high or when open, both the bias and modulation currents are turned off. BIAS AND MODULATION MONITOR CURRENTS IBMON and IMMON are current-controlled current sources that mirror a ratio of the bias and modulation current. The monitor bias current, IBMON, and the monitor modulation current, IMMON, should both be connected to ground through a resistor to provide a voltage proportional to the bias current and modulation current, respectively. When using a micro- controller, the voltage developed across these resistors can be connected to two of the ADC channels, making available a digital representation of the bias and modulation current. DATA INPUTS Data inputs should be ac-coupled (10 nF capacitors are recommended) and are terminated via a 100 Ω internal resistor between the DATAP and DATAN pins. A high impedance circuit sets the common-mode voltage and is designed to allow maximum input voltage headroom over temperature. It is necessary to use ac coupling to eliminate the need for matching between common-mode voltages. |
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Similar Description - ADN2870 |
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