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AD642 Datasheet(PDF) 6 Page - Analog Devices

Part No. AD642
Description  Precision, Low Cost Dual BiFET Op Amp
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD642 Datasheet(HTML) 6 Page - Analog Devices

   
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AD642
REV. 0
–6–
AD642
linearization of transducers having exponential outputs, and
analog computing, ranging from simple translation of natural
relationships in log form (e.g., computing absorbance as the log-
ratio of input currents), to the use of logarithms in facilitating
analog computation of terms involving arbitrary exponents and
multiterm products and ratios.
The picoamp level input current and low offset voltage of the
AD642 make it suitable for wide dynamic range log amplifiers.
Figure 28 is a schematic of a log ratio circuit employing the
AD642 that can achieve less than 1% conformance error over 5
decades of current input, 1 nA to 100
µA. For voltage inputs,
the dynamic range is typically 50 mV to 10 V for 1% error
limited on the low end by the amplifier’s input offset voltage.
Figure 28. Log-Ratio Amplifier
The conversion between current (or voltage) input and log
output is accomplished by the base emitter junctions of the dual
transistor Q1. Assuming Q1 has
β>100, which is the case for the
specified transistor, the base-emitter voltage on side 1 is to a
close approximation:
V BE A = kT /q ln I1/IS1
This circuit is arranged to take the difference of the V
BE’s of
Q1A and Q1B, thus producing an output voltage proportional
to the log of the ratio of the inputs:
V
OUT = – K (V BE A – V BE B ) = –
KkT
q
(ln I
1/IS1 –ln I2 /IS2 )
V
OUT = – KkT /q ln I1/I2
The scaling constant, K is set by R1 and R
TC to about 16, to
produce 1 V change in output voltage per decade difference in
input signals. R
TC is a special resistor with a +3500 ppm/°C
temperature coefficient, which makes K inversely proportional
to temperature, compensating for the “T” in kT/q. The log-ratio
transfer characteristic is therefore independent of temperature.
This particular log ratio circuit is free from the dynamic prob-
lems that plague many other log circuits. The –3 dB bandwidth
is 50 kHz over the top 3 decades, 100 nA to 100
µA, and
decreases smoothly at lower input levels. This circuit needs no
additional frequency compensation for stable operation from
input current sources, such as photodiodes, that may have
100 pF of shunt capacitance. For larger input capacitances a
20 pF integration capacitor around each amplifier will provide a
smoother frequency response.
The log ratio amplifier can be readily adjusted for optimum
accuracy by following this simple procedure. First, apply V1 =
V2 = –10.00 V and adjust “Balance” for V
OUT = 0.00 V. Next
apply V1 = –10.00 V, V2 = –1.00 V and adjust gain for V
OUT =
+1.00 V. Repeat this procedure until gain and balance readings
are within 2 mV of ideal values.
The low input bias current (35 pA) and low noise characteristics
of the AD642 make it suitable for electrometer applications
such as photo diode preamplifiers and picoampere current-to-
voltage converters. The use of guarding techniques in printed
circuit board layout and construction is critical in printed circuit
board layout and construction is critical for achieving the
ultimate in low leakage performance that the AD642 can
deliver. The input guarding scheme shown in Figure 29 will
minimize leakage as much as possible; the guard ring should be
applied to both sides of the board. The guard ring is connected
to a low impedance 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, they must be carried in rigid shielded cables.
Figure 29. Board Layout for Guarding Inputs
INPUT PROTECTION
The AD642 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
±0.5 volts while
maintaining the full differential input resistance of 10
12
Ω. This
makes the AD642 suitable for low speed voltage comparators
directly connected to a 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 perfor-
mance. The AD642 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
performance and give complete overload protection. Figure 30
shows proper connections.
Figure 30. AD642 Input Protection


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