6
LTC1050
1050fb
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A
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PP
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IC
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IFOR ATIO
ACHIEVING PICOAMPERE/MICROVOLT
PERFORMANCE
Picoamperes
In order to realize the picoampere level of accuracy of the
LTC1050, proper care must be exercised. Leakage currents
in circuitry external to the amplifier can significantly degrade
performance. High quality insulation should be used (e.g.,
Teflon, Kel-F); cleaning of all insulating surfaces to remove
fluxes and other residues will probably be necessary—
particularly for high temperature performance. Surface
coating may be necessary to provide a moisture barrier in
high humidity environments.
Board leakage can be minimized by encircling the input
connections with a guard ring operated at a potential close
to that of the inputs: in inverting configurations the guard
ring should be tied to ground; in noninverting connections
to the inverting input (see Figure 1). Guarding both sides
of the printed circuit board is required. Bulk leakage reduc-
tion depends on the guard ring width.
Microvolts
Thermocouple effect must be considered if the LTC1050’s
ultralow drift is to be fully utilized. Any connection of dis-
similar metals forms a thermoelectric junction producing
an electric potential which varies with temperature (Seebeck
effect). As temperature sensors, thermocouples exploit this
phenomenon to produce useful information. In low drift
amplifier circuits the effect is a primary source of error.
Connectors, switches, relay contacts, sockets, resistors,
solder and even copper wire are all candidates for thermal
TEST CIRCUITS
–
+
LTC1050 OUTPUT
1050 TC01
V
+
1k
1M
6
7
2
3
4
R
L
V
–
–
+
LTC1050
–
+
LT
®
1012
1050 TC02
10Ω
100k
158k 316k 475k
475k
0.015µF
FOR 1Hz NOISE BW, INCREASE ALL
THE CAPACITORS BY A FACTOR OF10
0.015µF
0.015µF
TO X-Y
RECORDER
Electrical Characteristics Test Circuit
DC-10Hz Noise Test Circuit
EMF generation. Junctions of copper wire from different
manufacturers can generate thermal EMFs of 200nV/°C—
4 times the maximum drift specification of the LTC1050.
The copper/kovar junction, formed when wire or printed
circuit traces contact a package lead, has a thermal EMF of
approximately 35µV/°C—700 times the maximum drift
specification of the LTC1050.
Minimizing thermal EMF-induced errors is possible if ju-
dicious attention is given to circuit board layout and
component selection. It is good practice to minimize the
number of junctions in the amplifier’s input signal path.
Avoid connectors, sockets, switches and relays where
possible. In instances where this is not possible, attempt
to balance the number and type of junctions so that differ-
ential cancellation occurs. Doing this may involve
deliberately introducing junctions to offset unavoidable
junctions.
8
1
7
V
+
V
–
6
OUTPUT
1050 F01
5
OPTIONAL
EXTERNAL
CLOCK
4
GUARD
2
3
INPUTS
Figure 1
