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INA117SM Datasheet(PDF) 9 Page - Texas Instruments |
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INA117SM Datasheet(HTML) 9 Page - Texas Instruments |
9 / 17 page INA117 SBOS154A 9 In all cases, the sense resistor imbalances the input resistor matching of the INA117, degrading its CMR. Also, the input impedance of the INA117 loads RS, causing gain error in the voltage-to-current conversion. Both of these errors can be easily corrected. The CMR error can be corrected with the addition of a compensation resistor, RC, equal in value to RS as shown in Figures 4, 5, and 6. If RS is less than 20Ω, the degradation in CMR is negligible and RC can be omitted. If RS is larger than approximately 2k Ω, trimming R C may be required to achieve greater than 86dB CMR. This is because the actual INA117 input impedances have 1% typical mismatch. If RS is more than approximately 100Ω, the gain error will be greater than the 0.02% specification of the INA117. This gain error can be corrected by slightly increasing the value of RS. The corrected value, RS', can be calculated by: R Rk kR S S S ' – = •Ω Ω 380 380 Example: For a 1V/mA transfer function, the nominal, uncorrected value for RS would be 1kΩ. A slightly larger value, RS' = 1002.6Ω, compensates for the gain error due to loading. The 380k Ω term in the equation for R S' has a tolerance of ±25%, so sense resistors above approximately 400Ω may require trimming to achieve gain accuracy better than 0.02%. Of course, if a buffer amplifier is added as shown in Figure 7, both inputs see a low source impedance, and the sense resistor is not loaded. As a result, there is no gain error or CMR degradation. The buffer amplifier can operate as a unity gain buffer or as an amplifier with non-inverting gain. Gain added ahead of the INA117 improves both CMR and signal-to-noise. Added gain also allows a lower voltage drop across the sense resistor. The OPA1013 is a good choice for the buffer amplifier since both its input and output can swing close to its negative power supply. FIGURE 7. Current Sensing with Input Buffer. VX V1 –21V to +10V +15V –5V to –36V Ground –20V to –51V –15V 380k Ω 380k Ω 380k Ω 21.1k Ω 20k Ω 47 2 3 81 5 V O = I • RS • (1 + ) 6 I –15V +15V R 2 R 1 R 2* 1/2 OPA1013 V 1 R * 1 R S –V X 380k Ω 380k Ω 380k Ω 21.1k Ω 20k Ω 47 2 3 81 5 V O = I • RS 6 I V– V+ 1/2 OPA1013 R S 0.1µF IN4702 MPS-A42 180k Ω 0.01µF V Z or –V X –V X Op amp power can be derived with voltage- dropping zener diode if –V X power is relatively constant. |V X| = (5V to 36V) + VZ e.g., If V Z is 50V then VX = –55V to –86V. Regulated power for op amp allows –V X power to vary over wide range. V X = –30V to –200V *Or connect as buffer (R 2 = 0, omit R1). |
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