OP-41_15 Datasheet, PDF (10/11 Pages) Analog Devices – LOW BIAS CURRENT HIGH STABILITY JFET OPERATIONAL AMPLIFIER

- ^{ANALOGDEVICES fAX-ON-DEMAND HOTLINE}
Page 11
OP-41
The High Q Notch Filter benefits from the low bias current and
high input impedance of the OP-41, Figure 10. These features
enable small value capacitors and large resistors to be used in
this 60Hz notch filter. The SpA bias current only develops 100pV
across R1 and R2.
FIGURE 10: HIGH a NOTCH FILTER
AI
V'N
10M
VOUT
FIGURE 12: AMPLIFIER FOR PIEZOELECTRIC TRANSDUCERS
CI
lO~F
C2
R3
O.DWF
22M
OUTPUT
R2
11M
A1
11M
D TRANSDUCER
~LOW FREQUENCY
CUTOFF
A1 CI
WIDE RANGE LOW-CURRENT AMMETER
Cl
. _270".
R3
511'
C2
270"F
", 1_-
.^{0 2.R1Cl}
R 1 A2 2R3
Cl C2 " C31Z
OLow power consumption, low bias current, and low offset
voltage make the OP-41 an ideal current-to-voltage converter,
B ^{Figure 11.}
In this application, the PM-7541 and the OP-41 provide complete
S _{12-bit digital-to-analog}
current.
conversion with less than 3mA supply
OL _{FIGURE 11: DAC CIRCUIT USING THE OP-41}
E _.15V
TE _RFEEDBACK
The circuit shown in Figure 13 can measure currents from
100pA to 1O0t-lA without the use of high value resistors
Accuracy is better than 1% over most of the range. depending
upon the accuracy of the divider resistor and the input bias
current of the op amp. Using the OP-41 as the input amplifier
allows low end measurement down to a few pA due to the 3.5pA
input bias current.
One of the requirements for a good current meter is low series
voltage drop. Since the voltage across the inputs of an op amp
is forced to virtually zero, it makes a good choiceforthe input of
a current meter. Amplifier Ai isusedasan inverting amplifierfor
the input. This ensures less than 500J.lV drop at any current
level.
Feedback around the op amp is accompHshed witha transistor,
rather than a resistor. The op amp forces the collector current of
Q1A to equal the input current. This causes the emitter-base
voltage of Q1A to be proportional to the log of the input current.
Resistors R1, R2, R3 and capacitors Cl, C2 frequency com-
BIT 2
DIGITAL
INPUT
.'oun
1
PM-7541
6 ^VOUT
pensate the log circuit since Q1 A provides gain in the feedback
loop.
The output of the log amplifier is taken from the emitter of Q1A
to drive Q18. Q1Banti-logs the output and drives the meter. The
BIT 12 !lSBI
output of Q18 is proportional to the log of the Input current
scaled by a constant, which is proportional to the voltage from
the divider,selected by 51. For transistors operating at different
current levels, the Vbedifference equals:
Figure 12shows an amplifier for high-impedance ac transducers
like a piezoelectric accelerometer. These sensors normally
require a high-Input-resistance amplifier. The OP-41 can pro-
vide input resistance in the range of 10'20, however, a dc return
for bias current is needed. To maintain a high RIN' large value
resistors above 22MO are often required. These may not be
practicable.
Using the circuit in Figure 12, input resistances that are orders
of magnitude greater than the values of the dc return resistors
can be obtained. This is accomplished by bootstrapping the
resistors to the output. With this arrangement, the lower cutoff
frequency is determined more by the RC product of Ai and C1
than it is by resistor values and the equivalent capacitance of
the transducer.
v^A
U
-
be -
kT
-In
I ^C2
-
q
IC1
solving for IC2
( ) ^..lVboeQ
IC2 '" IC1 ekT-
Where IC1 and IC2 are the collector currents of Q1A and Q1B;
Q is the charge of an electron; k is Boltzmann's constant; Tis
temperature in degrees Kelvin; and Vbeis the voltage applied to
the base of Q18. If Vbe varies as absolute temperature, the
exponent will be a constant.
-10-