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AMP02FP Datasheet(PDF) 8 Page - Analog Devices |
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AMP02FP Datasheet(HTML) 8 Page - Analog Devices |
8 / 12 page AMP02 REV. D –8– The voltage gain can range from 1 to 10,000. A gain set resistor is not required for unity-gain applications. Metal-film or wire- wound resistors are recommended for best results. The total gain accuracy of the AMP02 is determined by the tol- erance of the external gain set resistor, RG, combined with the gain equation accuracy of the AMP02. Total gain drift com- bines the mismatch of the external gain set resistor drift with that of the internal resistors (20 ppm/ °C typ). Maximum gain drift of the AMP02 independent of the external gain set resistor is 50 ppm/ °C. All instrumentation amplifiers require attention to layout so thermocouple effects are minimized. Thermocouples formed be- tween copper and dissimilar metals can easily destroy the TCVOS performance of the AMP02 which is typically 0.5 µV/°C. Resistors themselves can generate thermoelectric EMFs when mounted parallel to a thermal gradient. The AMP02 uses the triple op amp instrumentation amplifier configuration with the input stage consisting of two transimped- ance amplifiers followed by a unity-gain differential amplifier. The input stage and output buffer are laser-trimmed to increase gain accuracy. The AMP02 maintains wide bandwidth at all gains as shown in Figure 26. For voltage gains greater than 10, the bandwidth is over 200 kHz. At unity-gain, the bandwidth of the AMP02 exceeds 1 MHz. Figure 26. The AMP02 Keeps Its Bandwidth at High Gains COMMON-MODE REJECTION Ideally, an instrumentation amplifier responds only to the differ- ence between the two input signals and rejects common-mode voltages and noise. In practice, there is a small change in output voltage when both inputs experience the same common-mode voltage change; the ratio of these voltages is called the common- mode gain. Common-mode rejection (CMR) is the logarithm of the ratio of differential-mode gain to common-mode gain, ex- pressed in dB. Laser trimming is used to achieve the high CMR of the AMP02. APPLICATIONS INFORMATION INPUT AND OUTPUT OFFSET VOLTAGES Instrumentation amplifiers have independent offset voltages associated with the input and output stages. The input offset component is directly multiplied by the amplifier gain, whereas output offset is independent of gain. Therefore, at low gain, output-offset-errors dominate, while at high gain, input-offset- errors dominate. Overall offset voltage, VOS, referred to the out- put (RTO) is calculated as follows: VOS (RTO) = (VIOS G) + VOOS where VIOS and VOOS are the input and output offset voltage specifications and G is the amplifier gain. The overall offset voltage drift TCVOS, referred to the output, is a combination of input and output drift specifications. Input offset voltage drift is multiplied by the amplifier gain, G, and summed with the output offset drift: TCVOS (RTO) = (TCVIOS G) + TCVOOS where TCVIOS is the input offset voltage drift, and TCVOOS is the output offset voltage drift. Frequently, the amplifier drift is referred back to the input (RTI) which is then equivalent to an input signal change: TCVOS (RTI) =TCVIOS + TCV OOS G For example, the maximum input-referred drift of an AMP02EP set to G = 1000 becomes: TCVOS (RTI) = 2 µV/°C + 100 µV /°C 1000 = 2.1 µV/°C INPUT BIAS AND OFFSET CURRENTS Input transistor bias currents are additional error sources which can degrade the input signal. Bias currents flowing through the signal source resistance appear as an additional offset voltage. Equal source resistance on both inputs of an IA will minimize offset changes due to bias current variations with signal voltage and temperature. However, the difference between the two bias currents, the input offset current, produces an error. The mag- nitude of the error is the offset current times the source resistance. A current path must always be provided between the differential inputs and analog ground to ensure correct amplifier operation. Floating inputs, such as thermocouples, should be grounded close to the signal source for best common-mode rejection. GAIN The AMP02 only requires a single external resistor to set the voltage gain. The voltage gain, G, is: G = 50 k Ω RG +1 and RG = 50 k Ω G –1 |
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