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AD7418ARUZ-REEL Datasheet(PDF) 15 Page - Analog Devices |
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AD7418ARUZ-REEL Datasheet(HTML) 15 Page - Analog Devices |
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15 / 20 page  REV. G AD7416/AD7417/AD7418 –15– With a VDD = 3 V, for each 400 µs cycle, the AD7416 spends 40 µs (or 10% of the time) in conversion mode. It spends 360 µs (or 90% of time) in partial power-down mode. Thus, the aver- age power dissipated by the AD7416/AD7417/AD7418 is 30 1 1 09 12 mW ×+ × = .. . mW mW Mode 2 For applications where temperature measurements are required at a slower rate, e.g., every second, power consumption of the part can be reduced by writing to the part to go to a full power-down between reads. The current consumption in full power-down is typically 0.2 µA and full power-down is initiated when D0 = 1 in the Configuration Register. When a measurement is required, a write operation can be performed to power-up the part. The part then performs a conversion and is returned to power-down. The temperature value can be read in the full power-down because the I 2C bus is continuously active. The power dissipation in this mode depends on the rate at which reads take place. Taking the requirements for a temperature measurement every 100 ms as an example, the optimum power dissipation is achieved by placing the part in full power-down, waking it up every 100 ms, letting it operate for 400 µs and putting it into full power-down again. In this case, the average power consumption is calculated as follows. The part spends 40 µs (or 0.04% of time) converting with 3 mW dissipation and a 99.96 ms (99.96% of time) in full shutdown with 60 nW dissipa- tion. Thus, the average power dissipation is 30 004 60 0 9996 1 2 mW nW W ×+ × = .. . µ The fastest throughput rate at which the AD7416/AD7417/ AD7418 can be operated is 2.5 kHz (i.e., a read every 400 µs conversion period). Since TOTI and THYST are 2-byte reads, the read time with the I 2C operating at 100 kbit/s would be 270 µs. If temperature reads are called too often, reads will overlap with conversions, aborting them continuously, which results in invalid readings. CONVERT START MODE The AD7417/AD7418 has an extra mode, set by writing to the MSB of the Config2 Register. CONVST Pin Mode By setting the CONVST Mode Bit to 1, conversions are initiated only by using the CONVST pin. In this method of operation, CONVST is normally low. The rising edge of CONVST starts the power-up time. This power-up time is 4 µs. If the CONVST high time is longer than 4 µs, a conversion is initiated on the falling edge of CONVST and the track-and-hold also enters its hold mode at this time. If the CONVST high time is less than 4 µs, an internal timer, initiated by the rising edge of CONVST holds off the track-and- hold and the initiation of conversion until the timer times out (4 µs after the rising edge of CONVST, which corresponds with the power-up time). CONVST input remains low at the end of conversion, thus causing the part to enter its power-down mode. In this method of operation, CONVST is normally low with a high going pulse controlling the power-up and conversion starts. The CONVST pin should not be pulsed when reading from or writing to the port. Figure 17 shows the recommended minimum times for the CONVST pulse when the temperature channel is selected. Figure 18 shows the minimum times an analog input channel is selected. APPLICATIONS INFORMATION SUPPLY DECOUPLING The AD7416/AD7417/AD7418 should be decoupled with a 0.1 µF ceramic capacitor between V DD and GND. This is particularly important if the part is mounted remote from the power supply. POWER-ON-RESET To ensure proper power-on-reset, make sure that the supply voltage on the VDD pin is at 0 V. Refer to application note AN-588 for more information. A failed power-on-reset can prevent the default values from being loaded into the AD7416/ AD7417/AD7418 registers. If the correct values are not loaded into the registers, then the device will not start operating. The output from the value registers will be a constant value. To get the device operating again, the registers will have to be loaded with their default values via the I 2C bus. Therefore, in the event of an inadequate power-on-reset and for all three devices, the following registers should be loaded with their default values: Configuration Register 1—Default Value = 00h Configuration Register 2—Default Value = 00h THYST Setpoint Register—Default Value = 4B00h TOTI Setpoint Register—Default Value = 5500h MOUNTING THE AD7416 The AD7416/AD7417/AD7418 can be used for surface or air temperature sensing applications. If the device is cemented to a surface with thermally conductive adhesive, the die temperature will be within about 0.2 °C of the surface temperature, thanks to the device’s low power consumption. Care should be taken to insulate the back and leads of the device from the air, if the ambient air temperature is different from the surface tempera- ture being measured. The ground pin provides the best thermal path to the die, so the temperature of the die will be close to that of the printed circuit ground track. Care should be taken to ensure that this is in good thermal contact with the surface being measured. As with any IC, the AD7416/AD7417/AD7418 and its associ- ated wiring and circuits must be kept free from moisture to prevent leakage and corrosion, particularly in cold conditions where condensation is more likely to occur. Water resistant varnishes and conformal coatings can be used for protection. The small size of the AD7416 package allows it to be mounted inside sealed metal probes that provide a safe environment for the device. |
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