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INA115BU Datasheet(PDF) 8 Page - Texas Instruments |
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INA115BU Datasheet(HTML) 8 Page - Texas Instruments |
8 / 16 page 8 ® INA115 APPLICATION INFORMATION Figure 1 shows the basic connections required for operation of the INA115. Applications with noisy or high impedance power supplies may require decoupling capacitors close to the device pins as shown. The output is referred to the output reference (Ref) terminal which is normally grounded. This must be a low-impedance connection to assure good common-mode rejection. A resis- tance of 5 Ω in series with the Ref pin will cause a typical device to degrade to approximately 80dB CMR (G=1). The INA115 has a separate output sense feedback connection (pin 12). Pin 12 must be connected (normally to the output terminal, pin 11) for proper operation. The output sense connection can be used to sense the output voltage directly at the load for best accuracy. SETTING THE GAIN Gain of the INA115 is set by connecting a single external resistor, R G: FIGURE 1. Basic Connections. DESIRED R G NEAREST 1% R G GAIN ( Ω)(Ω) 1 No Connection No Connection 2 50.00k 49.9k 5 12.50k 12.4k 10 5.556k 5.62k 20 2.632k 2.61k 50 1.02k 1.02k 100 505.1 511 200 251.3 249 500 100.2 100 1000 50.05 49.9 2000 25.01 24.9 5000 10.00 10 10000 5.001 4.99 G = 1 + 50 kΩ R G (1) Commonly used gains and resistor values are shown in Figure 1. For G=1, no resistor is required, but connect pins 2-3 and connect pins 14-15. Gain peaking in G=1 can be reduced by shorting the internal 25k Ω feedback resistors (see typical performance curve Gain vs Frequency). To do this, connect pins 1-2-3 and connect pins 8-14-15. The 50k Ω term in equation 1 comes from the sum of the two internal feedback resistors. These are on-chip metal film resistors which are laser trimmed to accurate absolute values. The accuracy and temperature coefficient of these resistors are included in the gain accuracy and drift specifications of the INA115. The stability and temperature drift of the external gain setting resistor, R G , also affects gain. R G ’s contribution to gain error and drift can be directly inferred from the gain equation (1). Low resistor values required for high gain can make wiring resistance important. The “force and sense” type connections illustrated in Figure 1 help reduce the effect of interconnection resistance. A 1 A 2 A 3 12 11 10 25k Ω 25k Ω 25k Ω 25k Ω 13 7 5 4 V IN V IN R G V+ V– INA115 – + Over-Voltage Protection 25k Ω 25k Ω Over-Voltage Protection 15 14 3 2 8 V O2 V O1 1 Load G = 1 + 50k Ω R G V O = G • (VIN – VIN) 0.1µF 0.1µF + – V O Also drawn in simplified form: Ref INA115 V IN V IN R G – + V O V O2 V O1 |
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