OP-41_15 Datasheet, PDF (9/11 Pages) Analog Devices – LOW BIAS CURRENT HIGH STABILITY JFET OPERATIONAL AMPLIFIER

- ^{ANALOGDEVICES fAX-ON-DEHAND HOTLINE}
Page 10
OP-41
I
FIGURE 6: CIRCUIT USED TO MEASURE CMR FROM 10Hz
TO 20kHz
9kH
1':1
Ik;!
AMP-OI
AV.'1
HP 3S82A
SPECTRUM
ANALYZER
transmitters. Loss of accuracy can also occurfrom surface and
bulk leakages in printed circuit boards. Both of these conditions
can be avoided by the following methods.
Hum and RF pickup are eliminated or reduced by keeping all
high impedance leads, including feedback resistor leads, inside
shielded enclosures. In addition to shielding, power supply
lines should be bypassed where they pass through the shield-
ing. This will prevent noise from being retransmitted from the
power supply lines inside the shielded enclosure.
Noise can also be created by the flexing of coax cable, These
signals can be caused by mechanical vibrations inside or
outside the shielding. Prevention consists of securely support-
ing all high-impedance shielded lines to prevent motion.
Printed circuit board leakage currents can easily exceed the
OP-41 bias currents or the incoming signal. Leakage currents
OBSOLETE An alternate circuit may be used to make high-frequency
measurements from 2kHz to 500kHz, Figure 7. The 2kHz to
20kHz data overlap can be used to verify the accuracy of the
respective test methods.
This method drives the input stage with the test signal and
requires an accurate ratio of resistors, R4/R3 '" R1/R2. To
measure CMR to 100dB requires ratio matching to better than
10ppm. For this reason, it is not practical to use the second
method at low frequencies where CMR is greater than 80-100dB.
The OUT output is normally connected directly to R4 which
may cause problems. If the OUT is not buffered with a broad-
band low-output-impedance amplifier, the frequency-depen-
dent output impedance of the OUT, in series with R4, rapidly
unbalances the resistor ratios. This causes frequency depen-
dent errors. The OP-27 provides good performance over the
range of frequencies used.
can be minimized by using Teflon insulators to support wires
instead of using PC traces. An alternate method is guarding the
high impedance traces. When the OP-41 is in the inverting
mode, the signal traces should have grounded guard traces on
both sides, Figure 8. The opposite side of the board should be
used as a ground plane and shield. if not otherwise used, A
ground plane is implemented by leaving copper on all areas
that are not being used for signal or power conduction. Ground
connection should be made to all areas of isolated copper. In
the noninverting configuration, the OP-41's output signal or a
portion of it should be used to drive the guard traces, Figure 9.
When the guard drive voltage is equal to the input signal.
leakage currents will be effectively eliminated,
FIGURE 8: CURRENT-TO-VOLTAGE CONVERTER
FIGURE 7: CIRCUIT USED TO MEASURE CMR FROM 2kHz
TO 500kHz
SHIELD
6
OUT
TO GAIN/PHASE METER
R4
9k!J
R3
1.;1
RZ
lk!J
'",'
Rl
9k;1
TO
GAIN/PHASE
METER
FIGURE 9: VERY HIGH IMPEDANCE NONINVERTING
AMPLIFIER
GUARDING AND SHIELDING
In applications where the input is at high impedance, careful
shielding is required to prevent hum pickup from power line
sources or detection of R F from rad io stations and nearby radar
OUT
-9-
--