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AD8225AR-REEL Datenblatt(PDF) 11 Page - Analog Devices |
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AD8225AR-REEL Datenblatt(HTML) 11 Page - Analog Devices |
11 / 16 page REV. A AD8225 –11– VB +VS A1 A2 C2 R2 –IN Q2 C1 R1 +IN Q1 3k 3k 15k 15k VREF A3 +VS –VS +VS –VS +VS –VS OUT +VS –VS Figure 3. Simplified Schematic THEORY OF OPERATION The AD8225 is a monolithic, three op amp instrumentation amplifier. Laser wafer trimming and proprietary circuit tech- niques enable the AD8225 to boast the lowest output offset voltage and drift of any currently available in amp (150 µV RTI), as well as a higher common-mode voltage range. Referring to Figure 3, the input buffers consist of super-beta NPN transistors Q1 and Q2, and op amps A1 and A2. The transistors are compensated so that the bias currents are extremely low, typically 100 pA or less. As a result, current noise is also low, at 50 fA/ √Hz. The unity gain input buffers drive a gain-of-five difference amplifier. Because the 3 k Ω and 15 kΩ resistors are ratio matched, gain stability is better than 5 ppm/ °C over the rated temperature range. The AD8225 also has five times the gain bandwidth of a typical in amp. This wider GBW results from compensation at a fixed gain of 5, which can be one fifth of that required if the amplifier were compensated for unity gain. High frequency performance is also enhanced by the innovative pinout of the AD8225. Since Pins 1 and 8 are uncommitted, Pin 1 may be connected to Pin 4. Since Pin 4 is also ac com- mon, the stray capacitance at Pins 2 and 3 is balanced. 8 7 6 5 NC +VS VOUT REF AD8225 +IN –IN AC GROUND AC GROUND PIN 1 HAS NO INTERNAL CONNECTION Figure 4. Pinout for Symmetrical Input Stray Capacitance APPLICATIONS Precision V-to-I Converter When small analog voltages are transmitted across significant distances, errors may develop due to ambient electrical noise, stray capacitance, or series impedance effects. If the desired voltage is converted to a current, however, the effects of ambient noise are mitigated. All that is required is a voltage to current conversion at the source, and an I-to-V conversion at the other end to reverse the process. Figure 5 illustrates how the AD8225 may be used as the trans- mitter and receiver in a current loop system. The full-scale output is 5 mA. AD8225 2 3 5 6 47pF 9k RSH 20 VSH IOUT AD8225 2 3 5 6 8 GND OR REF V 1k FULL SCALE CURRENT = 5mA eOUT 200mV pk FS eIN 200mV pk FS IOUT = VSH RSH 0.5 eIN RSH = OP27 Figure 5. Precision Voltage-to-Current Converter As noted in Figure 5, an additional op amp and four resistors are required to complete the converter. The precision gain of 5 in the AD8225s, used in the transmit and receive sections, preserves the integrity of the desired signal, while the high frequency common-mode performance at the receiver rejects noise on the transmission line. The reference of the receiver may be connected to local ground or the reference pin of an A/D converter (ADC). Figure 6 shows bench measurements of the input and output voltages, and output current of the circuit of Figure 5. The transmission media is 10 feet of insulated hook-up wire for the current drive and return lines. e IN e OUT IOUT CH 1 = 100mV, CH 2 = 100mV, CH 3 = 10mA, H = 200 s e IN = 398mV p-p, eOUT = 398mV p-p, IOUT = 10.3mA p-p 1 2 3 Figure 6. V-to-I Converter Waveforms (CH1: VIN, CH2: VOUT, CH3: IOUT) |
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