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AD644

Dual High Speed, Implanted BiFET Op Amp

Analog Devices 

AD644

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.

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.

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 1012 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 kΩ 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.

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

REV. A

–5–

Figure 26. AD644 Input Protection

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