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INA126U Datasheet(PDF) 8 Page - Burr-Brown (TI) |
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INA126U Datasheet(HTML) 8 Page - Burr-Brown (TI) |
8 / 19 page INA126, INA2126 8 SBOS062A www.ti.com INPUT BIAS CURRENT RETURN The input impedance of the INA126/2126 is extremely high—approximately 109 Ω. However, a path must be pro- vided for the input bias current of both inputs. This input bias current is typically –10nA (current flows out of the input terminals). 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 for proper operation. Figure 3 shows various provisions for an input bias current path. Without a bias current path, the inputs will float to a potential which exceeds the common- mode range 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 equal resistors provides a balanced input with advantages of lower input offset voltage due to bias current and better high-frequency common-mode rejection. FIGURE 3. Providing an Input Common-Mode Current Path. INPUT COMMON-MODE RANGE The input common-mode range of the INA126/2126 is shown in the typical characteristic curves. The common- mode range is limited on the negative side by the output voltage swing of A2, an internal circuit node that cannot be measured on an external pin. The output voltage of A2 can be expressed as: VO2 = 1.25 VIN – (VIN – VIN) (10kΩ/RG) (2) (Voltages referred to Ref terminal, pin 5) – + – The internal op amp A2 is identical to A1 and its output swing is limited to typically 0.7V from the supply rails. When the input common-mode range is exceeded (A2’s output is saturated), A1 can still be in linear operation and respond to changes in the non-inverting input voltage. The output voltage, however, will be invalid. LOW VOLTAGE OPERATION The INA126/2126 can be operated on power supplies as low as ±1.35V. Performance remains excellent with power sup- plies ranging from ±1.35V to ±18V. Most parameters vary only slightly throughout this supply voltage range—see typical characteristic curves. Operation at very low supply voltage requires careful attention to ensure that the common- mode voltage remains within its linear range. See “Input Common-Mode Voltage Range.” The INA126/2126 can be operated from a single power supply with careful attention to input common-mode range, output voltage swing of both op amps and the voltage applied to the Ref terminal. Figure 4 shows a bridge ampli- fier circuit operated from a single +5V power supply. The bridge provides an input common-mode voltage near 2.5V, with a relatively small differential voltage. INPUT PROTECTION The inputs are protected with internal diodes connected to the power supply rails. These diodes will clamp the applied signal to prevent it from exceeding the power supplies by more than approximately 0.7V. If the signal source voltage can exceed the power supplies, the source current should be limited to less than 10mA. This can generally be done with a series resistor. Some signal sources are inherently current- limited and do not require limiting resistors. CHANNEL CROSSTALK—DUAL VERSION The two channels of the INA2126 are completely indepen- dent, including all bias circuitry. At DC and low frequency there is virtually no signal coupling between channels. Crosstalk increases with frequency and is dependent on circuit gain, source impedance and signal characteristics. As source impedance increases, careful circuit layout will help achieve lowest channel crosstalk. Most crosstalk is produced by capacitive coupling of signals from one channel to the input section of the other channel. To minimize coupling, separate the input traces as far as practical from any signals associated with the opposite channel. A grounded guard trace surrounding the inputs helps reduce stray cou- pling between channels. Carefully balance the stray capaci- tance of each input to ground, and run the differential inputs of each channel parallel to each other, or directly adjacent on top and bottom side of a circuit board. Stray coupling then tends to produce a common-mode signal that is rejected by the IA’s input. 47k Ω 47k Ω 10k Ω Microphone, Hydrophone etc. Thermocouple Center-tap provides bias current return. INA126 INA126 INA126 |
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