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INA111AUE4 Datasheet(PDF) 8 Page - Burr-Brown (TI) |
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INA111AUE4 Datasheet(HTML) 8 Page - Burr-Brown (TI) |
8 / 14 page ® INA111 8 The INA111 exhibits approximately 6dB rise in gain at 2MHz in unity gain. This is a result of its current-feedback topology and is not an indication of instability. Unlike an op amp with poor phase margin, the rise in response is a predictable +6dB/octave due to a response zero. A simple pole at 700kHz or lower will produce a flat passband response (see Input Filtering). The INA111 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 VIN 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 INA111’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 may provide lower offset voltage and drift. For very low source impedance ( ≤1kΩ), the INA103 may provide improved accuracy and lower noise. OFFSET TRIMMING The INA111 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. Low impedance must be maintained at this node to assure good common-mode rejection. The op amp shown maintains low output impedance at high fre- quency. Trim circuits with higher source impedance should be buffered with an op amp follower circuit to assure low impedance on the Ref pin. INPUT BIAS CURRENT RETURN PATH The input impedance of the INA111 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 less than 10pA. High input impedance means that this input bias current changes very little with varying input voltage. Input circuitry must provide a path for this input bias current if the INA111 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 INA111 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. INA111 1M Ω 1M Ω INA111 10k Ω Thermocouple INA111 Center-tap provides bias current return. Crystal or Ceramic Transducer FIGURE 3. Providing an Input Common-Mode Current Path. FIGURE 2. Optional Trimming of Output Offset Voltage. INPUT COMMON-MODE RANGE The linear common-mode range of the input op amps of the INA111 is approximately ±12V (or 3V from the power supplies). As the output voltage increases, however, the linear input range will be limited by the output voltage swing of the input amplifiers, A1 and A2. The common-mode range is related to the output voltage of the complete amplifier— see performance curve “Input Common-Mode Range vs Output Voltage”. INA111 V IN V IN R G – + 10k Ω(1) V O OPA177 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 + – |
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