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INA121UA Datasheet(PDF) 8 Page - Texas Instruments |
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INA121UA Datasheet(HTML) 8 Page - Texas Instruments |
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8 / 20 page  8 ® INA121 APPLICATION INFORMATION Figure 1 shows the basic connections required for operation of the INA121. 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 8 Ω in series with the Ref pin will cause a typical device to degrade to approximately 80dB CMR (G = 1). SETTING THE GAIN Gain of the INA121 is set by connecting a single external resistor, R G, connected between pins 1 and 8: Commonly used gains and resistor values are shown in Figure 1. (1) G = 1 + 50k Ω R G The 50k Ω term in Equation 1 comes from the sum of the two internal feedback resistors of A 1 and A2. These on-chip metal film resistors are laser trimmed to accurate absolute values. The accuracy and temperature coefficient of these resistors are included in the gain accuracy and drift specifi- cations of the INA121. The stability and temperature drift of the external gain setting resistor, RG, also affects gain. RG’s contribution to gain accuracy and drift can be directly inferred from the gain equation (1). Low resistor values required for high gain can make wiring resistance important. Sockets add to the wiring resistance which will contribute additional gain error (possi- bly an unstable gain error) in gains of approximately 100 or greater. DYNAMIC PERFORMANCE The typical performance curve “Gain vs Frequency” shows that, despite its low quiescent current, the INA121 achieves wide bandwidth, even at high gain. This is due to the current-feedback topology of the INA121. Settling time also remains excellent at high gain. A 1 A 2 A 3 6 40k Ω 40k Ω 40k Ω 40k Ω 7 4 3 8 1 2 V IN V IN R G V+ V– INA121 G = 1 + 50k Ω R G – + 5 Over-Voltage Protection 25k Ω 25k Ω Over-Voltage Protection Load V O = G • (VIN – VIN) + – 0.1µF 0.1µF + – V O R G Also drawn in simplified form: INA121 Ref V O V IN – V IN + Ref DESIRED RG NEAREST 1% RG GAIN ( Ω)(Ω) 1NC NC 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 NC: No Connection. FIGURE 1. Basic Connections. |
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