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AD22057 データシート(PDF) 6 Page - Analog Devices |
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AD22057 データシート(HTML) 6 Page - Analog Devices |
6 / 8 page AD22057 –6– REV. A To produce a full-scale output of +4 V, a gain ×40 is used, adjust- able by ±5% to absorb the tolerance in the sense resistor. There is sufficient headroom to allow at least a 10% overrange (to +4.4 V). The roughly triangular voltage across the sense resistor is aver- aged by a single-pole low-pass filter, here set with a corner fre- quency of fC = 3.6 Hz, which provides about 30 dB of attenuation at 100 Hz. A higher rate of attenuation can be obtained by a two-pole filter having fC = 20 Hz, as shown in Figure 11. Al- though this circuit uses two separate capacitors, the total capaci- tance is less than half that needed for the single-pole filter. AD22057 +IN OFS +VS OUT –IN GND A1 A2 100m SOLENOID LOAD POWER DARLINGTON CMOS DRIVER +VS (BATTERY) CHASSIS C 432k 50k +5V ANALOG OUTPUT CORNER FREQUENCY = 1Hz- F (0.05 F FOR fC= 20Hz) ANALOG COMMON FLYBACK DIODE 127k C Figure 11. Illustration of Two-Pole Low-Pass Filtering STRAIN GAGE INTERFACE: MIDSCALE OFFSET FEATURE The AD22057 can be used to interface a strain gage to a subse- quent process where only a single supply voltage is available. In this application, the midscale offset feature is valuable, since the output of the bridge may have either polarity. Figure 12 shows typical connections. AD22057 +IN OFS +VS OUT –IN GND A1 A2 RL 10k +VS ANALOG OUTPUT ANALOG COMMON 100k VOS NULL OPTIONAL LP FILTER 125k (SETS GAIN TO 100) VG R R R R Figure 12. Typical Connections for a Strain Gage Interface Using the Offset Feature The offset is obtained by connecting Pin 7 (OFS) to the supply voltage. In this way, the output of the AD22057 is centered to midway between the supply and ground. In many systems the supply will also serve as the reference voltage for a subsequent A/D converter. Alternatively, Pin 7 may be tied to the reference voltage from an independent source. The AD22057 is trimmed to guarantee an accuracy of ±2% on the 0.5 ratio between the voltage on Pin 7 and the output. An ac excitation of up to ±2 V can also be used because the common-mode range of the AD22057 extends to –1 V. Assum- ing a full-scale bridge output (VG) of ± 10 mV, a gain of ×100 might be used to provide an output of ±1 V (a full-scale range of +1.5 V to +3.5 V). This gain is achieved using the method discussed in connection with Figure 5. Note that the gain- setting resistor does not affect the accuracy of the midscale offset. (However, if the gain were lowered, using a resistor to ground, this offset would no longer be accurate.) A VOS nulling pot is included for illustrative purposes. One-, two- and three- pole filtering can also be implemented, as discussed in the Low-Pass Filtering section. Using the Midscale Offset Feature Figure 13 shows a more detailed schematic of the output am- plifier A2. Because this is a single supply device, the output stage has no pull-down transistor. Such a transistor would limit the minimum output to several hundred millivolts above ground. When using the AD22057 in unipolar mode (Pin 7 grounded), the resistors making up the feedback network also act as a pull-down for the output stage. A2 10k 95k +VS 20k OFS OUT 20k RL GND Figure 13. Detailed Schematic of Output Amplifier A2 If the output is called upon to source current (not sink), then it can swing almost completely to ground (within 20 mV). How- ever, if the offset pin is connected to some positive voltage source, this source will “pull up” the output voltage, thereby limiting the minimum output swing. With no external load the minimum output voltage possible is VOFS/2. For example, if Pin 7 is connected to +5 V, the minimum output voltage is equal to the offset voltage of 2.5 V. By adding an additional load, as shown, the output swing toward ground can be extended. The relationship is described by: VOUT > 1 2 VOFS RL RL + 20 kΩ * *This 20 k Ω resistor is internal to the AD22057 and can vary by ±30%. where RL is an externally applied load resistor. However, RL cannot be made arbitrarily small since this would require exces- sive current from the output. The output current should be limited to 5 mA total. |
同様の部品番号 - AD22057 |
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同様の説明 - AD22057 |
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