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AD627AR-REEL7 bảng dữ liệu(PDF) 10 Page - Analog Devices |
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AD627AR-REEL7 bảng dữ liệu(HTML) 10 Page - Analog Devices |
10 / 16 page AD627 –10– REV. A THEORY OF OPERATION The AD627 is a true “instrumentation amplifier” built using two feedback loops. Its general properties are similar to those of the classic “two op amp” instrumentation amplifier configura- tion, and can be regarded as such, but internally the details are somewhat different. The AD627 uses a modified “current feed- back” scheme which, coupled with interstage feedforward frequency compensation, results in a much better CMRR (Common-Mode Rejection Ratio) at frequencies above dc (no- tably the line frequency of 50 Hz–60 Hz) than might otherwise be expected of a low power instrumentation amplifier. Referring to the diagram, (Figure 32), A1 completes a feedback loop which, in conjunction with V1 and R5, forces a constant collector current in Q1. Assume that the gain-setting resistor (RG) is not present for the moment. Resistors R2 and R1 com- plete the loop and force the output of A1 to be equal to the voltage on the inverting terminal with a gain of (almost exactly) 1.25. A nearly identical feedback loop completed by A2 forces a current in Q2 which is substantially identical to that in Q1, and A2 also provides the output voltage. When both loops are bal- anced, the gain from the noninverting terminal to VOUT is equal to 5, whereas the gain from the output of A1 to VOUT is equal to –4. The inverting terminal gain of A1, (1.25) times the gain of A2, (–4) makes the gain from the inverting and noninverting terminals equal. R5 200k –VS OUTPUT +VS –VS 2k +IN Q2 +VS –VS Q1 2k –IN R1 100k REF EXTERNAL GAIN RESISTOR R4 100k R3 25k R2 25k RG V1 R6 200k A2 A1 Figure 32. Simplified Schematic The differential mode gain is equal to 1 + R4/R3, nominally five and is factory trimmed to 0.01% final accuracy. Adding an external gain setting resistor (RG) increases the gain by an amount equal to (R4 + R1)/RG. The output voltage of the AD627 is given by the following equation. VOUT = [VIN(+) – VIN(–)] × (5 + 200 kΩ/R G) + VREF Laser trims are performed on R1 through R4 to ensure that their values are as close as possible to the absolute values in the gain equation. This ensures low gain error and high common- mode rejection at all practical gains. USING THE AD627 Basic Connections Figure 33 shows the basic connection circuit for the AD627. The +VS and –VS terminals are connected to the power supply. The supply can either be bipolar (VS = ±1.1 V to ±18 V) or single supply (–VS = 0 V, +VS = +2.2 V to +36 V). The power supplies should be capacitively decoupled close to the devices power pins. For best results, use surface mount 0.1 µF ceramic chip capacitors. The input voltage, which can be either single ended (tie either –IN or +IN to ground) or differential. The difference between the voltage on the inverting and noninverting pins is amplified by the programmed gain. The programmed gain is set by the gain resistor (see below). The output signal appears as the volt- age difference between the output pin and the externally applied voltage on the REF pin (see below). Setting the Gain The AD627s gain is resistor programmed by RG, or more pre- cisely, by whatever impedance appears between Pins 1 and 8. The gain is set according to the equation: Gain = 5 + (200 k Ω/R G) or RG = 200 k Ω/(Gain – 5) It follows that the minimum achievable gain is 5 (for RG = ∞). With an internal gain accuracy of between 0.05% and 0.7% depending on gain and grade, a 0.1% external gain resistor would seem appropriate to prevent significant degradation of the overall gain error. However, 0.1% resistors are not available in a wide range of values and are quite expensive. Table I shows recommended gain resistor values using 1% resistors. For all gains, the size of the gain resistor is conservatively chosen as the closest value from the standard resistor table that is higher than the ideal value. This results in a gain that is always slightly less than the desired gain. This prevents clipping of the signal at the output due to resistor tolerance. The internal resistors on the AD627 have a negative tempera- ture coefficient of –75 ppm/ °C max for gains > 5. Using a gain resistor that also has a negative temperature coefficient of –75 ppm/ °C or less will tend to reduce the overall circuit’s gain drift. 0.1 F –1.1V TO –18V 0.1 F +VS –VS +1.1V TO +18V RG +IN –IN REF (INPUT) VOUT VIN RG RG OUTPUT REF +VS +2.2V TO +36V RG +IN –IN REF (INPUT) VOUT VIN RG RG OUTPUT REF GAIN = 5 + (200k /RG) 0.1 F Figure 33. Basic Connections for Single and Dual Supplies |
Số phần tương tự - AD627AR-REEL7 |
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Mô tả tương tự - AD627AR-REEL7 |
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