AD647
The picoamp level input current and low offset voltage of the
AD647 make it suitable for wide dynamic range log amplifiers.
Figure 25 is a schematic of a log ratio circuit employing the
AD647 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 amplifiers’ input offset voltage.
cuits. DC error sources such as offset voltage and bias currents
represent the largest individual contributors to output error.
Offset voltages will be passed by the filtering network and may,
depending on the design of the filter circuit, be amplified and
generate unacceptable output offset voltages. In filter circuits for
low frequency ranges large value resistors are used to generate
the low-pass filter function. Input bias currents passing through
these resistors will generate an additional offset voltage that will
also be passed to the output of the filter.
The conversion between current (or voltage) input and log out-
put 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
The use of the AD647 will minimize these error sources and,
therefore, maximize filter accuracy. The wide variety of perfor-
mance levels of the AD647 allows for just the amount of accu-
racy required for any given application.
V
BE A = kT/q ln I1/IS1
This circuit is arranged to take the difference of the VBEs of Q1A
and Q1B, thus producing an output voltage proportional to the
log of the ratio of the inputs
AD647 AS AN INSTRUMENTATION AMPLIFIER
The circuit shown in Figure 26 uses the AD647 to construct an
ultra high precision instrumentation amplifier. In this type of
application the matching characteristics of a monolithic dual
amplifier are crucial to ensure high performance.
KkT
V
OUT = –K (VBE A – VBE B) =
(ln I1/IS1 –ln I2/IS2)
q
V
OUT = –K kT/q ln Il /I2
The scaling constant, K is set by R1 and RTC to about 16, to
produce a 1 V change in output voltage per decade difference in
input signals. RTC 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 de-
creases smoothly at lower input levels. This circuit needs no
additional frequency compensation for stable operation from
input current sources, such as photodiodes, which 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.
Figure 26. Precision FET Input Instrumentation Amplifier
The use of an AD647L as the input amplifier A1, guarantees
maximum offset voltage of 250 µV, drift of 2.5 µV/°C and bias
currents of 35 pA. A2 serves two less critical functions in the
amplifier and, therefore can be an AD647J. Amplifier A is an ac-
tive data guard which increases ac CMRR and minimizes extra-
neous signal pickup and leakage. Amplifier B is the output
amplifier of the instrumentation amplifier. To attain the preci-
sion available from this configuration, a great deal of care
should be taken when selecting the external components.
CMRR will depend on the matching of resistors R1, R2, R3,
and R4. The gain drift performance of this circuit will be af-
fected by the matching TC of the resistors used.
This 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 VOUT = 0.00 V. Next
apply V1 = –10.00 V, V2 = –100 V and adjust gain for VOUT
=
+1.00 V. Repeat this procedure until gain and balance readings
are within 2 mV of ideal values.
ACTIVE FILTERS
In active low-pass filtering applications the dc accuracy of the
amplifiers used is critical to the performance of the filter cir-
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
TO-99
E-20A
–6–
REV. A
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