AD644 Analog Devices, AD644 Datasheet - Page 5

• Manufacturer Part Number: AD644
• Description: Dual High Speed/ Implanted BiFET Op Amp
• Manufacturer: Analog Devices

REV. A
The fast settling time (3.0 s 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
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
isolation resistor
–5–
capacitance, C
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.
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
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 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.
low pass filter formed by the 100
Figure 25. Board Layout for Guarding Inputs
Figure 26. AD644 Input Protection
L
.
series resistor and the load
12
AD644
. This