This circuit offers the best of both worlds, It can be combined with a low input offset voltage and drift without degrading the overall system's dynamic performance. Compared to a standalone FETinput operational amplifier, the composite amplifier circuit exhibits a 20-fold improvement in voltage offset and drift.
In this circuit arrangement, A1 is a highspeed FET input op amp with a closed-loop gain of 100 (the source impedance was arbitrarily chosen to be 100 kΩ). A2 is a SuperBeta bipolar input op amp. It has good dc characteristics, biFET-level input bias current, and low noise. A2 monitdrs the voltage at the input of Al and injects current to Al's null pins. This forces A1 to have the input properties of a bipolar amplifier while maintaining its bandwidth and low-inputbias-current noise.
The AD705 is a low power bipolar op amp that has the low input bias current of a BiFET amplifier but which offers a significantly lower IB drift over temperature. The AD705 offers many of the advantages of BiFET and bipolar op amps without their inherent disadvantages. It utilizes superbeta bipolar input transistors to achieve the picoampere input bias current levels of FET input amplifiers (at room temperature), while its IB typically only increases 5 times vs. BiFET amplifiers which exhibit a 1000X increase over temperature. This means that, at room temperature, while a typical BiFET may have less IB than the AD705, the BiFET's input current will increase to a level of several nA at +125°C. Superbeta bipolar technology also permits the AD705 to achieve the microvolt offset voltage and low noise characteristics of a precision bipolar input amplifier.
The AD705 is a high quality replacement for the industry- standard OP07 amplifier while drawing only one sixth of its power supply current. Since it has only 1/20th the input bias current of an OP07, the AD705 can be used with much higher source impedances, while providing the same level of dc precision. In addition, since the input bias currents are at picoAmplevels, the commonly used "balancing" resistor (connected between the noninverting input of a bipolar op amp and ground) is not required. The AD705 is an excellent choice for use in low frequency active filters in 12- and 14-bit data acquisition systems, in precision instrumentation and as a high quality integrator.
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Low Frequency Active FiltersPrecision InstrumentationPrecision Integrators
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DC PERFORMANCE100 pA max Input Bias Current (AD705K)600 pA max IB Over MIL Temperature Range (AD705T)114 dB min CMRR (AD705K/T)114 dB min PSRR (AD705T)200 V/mV min Open Loop Gain800 kHz Unity Gain Crossover Frequency10,000 pF Capacitive Load Drive CapabilityLow CostAvailable in 8-Pin Plastic Mini-DlP, Hermetic Cerdip and Surface Mount (SOIC) PackagesMIL-STD-883B Processing AvailableDual Version Available: AD706Quad Version: AD704
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Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ±18 V
Internal Power Dissipation2. . . . . . . . . . . . . . . . . . . 650 mW
Input Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ±VS
Differential Input Voltage3. . . . . . . . . . . . . . . . . . . . . ±0.7 V
Output Short Circuit Duration . . . . . . . . . . . . . . . . Indefinite
Storage Temperature Range (N, R) . . . . . . . 65°C to +125°C
Storage Temperature Range (Q) . . . . . . . . . 65°C to +150°C
Operating Temperature Range
AD705J/K . . . . . . . . . . . . . . . . . . . . . . . . . . . 0°C to +70°C
AD705A/B . . . . . . . . . . . . . . . . . . . . . . . . . 40°C to +85°C
AD705T . . . . . . . . . . . . . . . . . . . . . . . . . . 55°C to +125°C
Lead Temperature Range (Soldering 60 sec) . . . . . . . . +300°C
Post a Buying Lead
Required Field 
