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INA121UA Datasheet(PDF) 9 Page - Texas Instruments |
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INA121UA Datasheet(HTML) 9 Page - Texas Instruments |
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9 / 20 page  9 ® INA121 INA121 1M Ω 1M Ω INA121 10k Ω Thermocouple INA121 Center-tap provides bias current return. INA121 Bridge resistance provides bias current return. Crystal or Ceramic Transducer Bridge V REF The INA121 provides excellent rejection of high frequency common-mode signals. The typical performance curve, “Common-Mode Rejection vs Frequency” shows this be- havior. If the inputs are not properly balanced, however, common-mode signals can be converted to differential sig- nals. Run the V IN and VIN connections directly adjacent each other, from the source signal all the way to the input pins. If possible use a ground plane under both input traces. Avoid running other potentially noisy lines near the inputs. NOISE AND ACCURACY PERFORMANCE The INA121’s FET input circuitry provides low input bias current and high speed. It achieves lower noise and higher accuracy with high impedance sources. With source imped- ances of 2k Ω to 50kΩ the INA114, INA128, or INA129 may provide lower offset voltage and drift. For very low source impedance ( ≤1kΩ), the INA103 may provide improved accuracy and lower noise. At very high source impedances (> 1M Ω) the INA116 is recommended. OFFSET TRIMMING The INA121 is laser trimmed for low offset voltage and drift. Most applications require no external offset adjust- ment. Figure 2 shows an optional circuit for trimming the output offset voltage. The voltage applied to Ref terminal is summed at the output. The op amp buffer provides low impedance at the Ref terminal to preserve good common- mode rejection. Trim circuits with higher source impedance should be buffered with an op amp follower circuit to assure low impedance on the Ref pin. Input circuitry must provide a path for this input bias current if the INA121 is to operate properly. Figure 3 shows various provisions for an input bias current path. Without a bias current return path, the inputs will float to a potential which exceeds the common-mode range of the INA121 and the input amplifiers will saturate. If the differential source resistance is low, the bias current return path can be connected to one input (see the thermo- couple example in Figure 3). With higher source impedance, using two resistors provides a balanced input with possible advantages of lower input offset voltage due to bias current and better high-frequency common-mode rejection. + – INPUT BIAS CURRENT RETURN PATH The input impedance of the INA121 is extremely high— approximately 1012 Ω. However, a path must be provided for the input bias current of both inputs. This input bias current is typically 4pA. High input impedance means that this input bias current changes very little with varying input voltage. INA121 V IN V IN R G – + 10k Ω(1) V O OPA277 Ref ±10mV Adjustment Range 100 Ω(1) 100 Ω(1) 100µA 1/2 REF200 100µA 1/2 REF200 V+ V– NOTE: (1) For wider trim range required in high gains, scale resistor values larger INPUT COMMON-MODE RANGE The linear input voltage range of the input circuitry of the INA121 is from approximately 1.2V below the positive supply voltage to 2.1V above the negative supply. A differ- ential input voltage causes the output voltage to increase. The linear input range, however, will be limited by the output voltage swing of amplifiers A1 and A2. So the linear common-mode input range is related to the output voltage of the complete amplifier. This behavior also depends on sup- ply voltage—see typical performance curve “Input Com- mon-Mode Range vs Output Voltage”. FIGURE 3. Providing an Input Common-Mode Current Path. FIGURE 2. Optional Trimming of Output Offset Voltage. |
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