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AD644 Datasheet(PDF) 5 Page - Analog Devices |
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AD644 Datasheet(HTML) 5 Page - Analog Devices |
5 / 6 page AD644 REV. A –5– The fast settling time (3.0 µs to 0.01% for 20 V p-p step) and low offset voltage make the AD644 an excellent choice as an output amplifier for current output D/A converters such as the AD7541. The upper trace of the oscilloscope photograph of Fig- ure 23b shows the settling characteristics of the AD644. The lower trace represents the input to Figure 23a. The AD644 has been designed for fast settling to 0.01%, however, feedback components, circuit layout and circuit design must be carefully considered to obtain the optimum settling time. The circuit in Figure 24 employs a 100 Ω isolation resistor which enables the amplifier to drive capacitive loads exceeding 500 pF; the resistor effectively isolates the high frequency feed- back from the load and stabilizes the circuit. Low frequency feedback is returned to the amplifier summing junction via the low pass filter formed by the 100 Ω series resistor and the load capacitance, CL. The low input bias current (35 pA), low noise, high slew rate and high bandwidth characteristics of the AD644 make it suit- able for electrometer applications such as photodiode preampli- fiers and picoampere current-to-voltage converters. The use of guarding techniques in printed circuit board layout and con- struction is critical for achieving the ultimate in low leakage per- formance that the AD644 can deliver. The input guarding scheme shown in Figure 25 will minimize leakage as much as possible. The same layout should be used on both sides of a double side board. The guard ring is connected to a low imped- ance potential at the same level as the inputs. High impedance signal lines should not be extended for any unnecessary length on a printed circuit; to minimize noise and leakage, such con- ductors should be replaced by rigid shielded cables. Figure 25. Board Layout for Guarding Inputs INPUT PROTECTION The AD644 is guaranteed for a maximum safe input potential equal to the power supply potential. The input stage design also allows differential input voltages of up to ±1 volt while main- taining the full differential input resistance of 10 12 Ω. This makes the AD644 suitable for comparator situations employing a direct connection to high impedance source. Many instrumentation situations, such as flame detectors in gas chromatographs, involve measurement of low level currents from high voltage sources. In such applications, a sensor fault condition may apply a very high potential to the input of the current-to-voltage converting amplifier. This possibility necessi- tates some form of input protection. Many electrometer type devices, especially CMOS designs, can require elaborate Zener protection schemes which often compromise overall performance. The AD644 requires input protection only if the source is not current-limited, and as such is similar to many JFET-input designs. The failure mode would be overheating from excess current rather than voltage breakdown. If the source is not current-limited, all that is required is a resistor in series with the affected input terminal so that the maximum overload current is 1.0 mA (for example, 100 k Ω for a 100 volt overload). This simple scheme will cause no significant reduction in perfor- mance and give complete overload protection. Figure 26 shows proper connections. Figure 26. AD644 Input Protection |
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