Datasheet

LTC2057/LTC2057HV
19
2057f
For more information www.linear.com/LTC2057
applicaTions inForMaTion
The DC average of injection current is the specified input
bias current, but this current has a frequency component
at the chopping frequency as well. When these small
current pulses, typically about 0.7nA
RMS
, interact with
source impedances or gain setting resistors, the resulting
voltage spikes are amplified by the closed loop gain. For
high impedances, this may cause the 100kHz chopping
frequency to be visible in the output spectrum, which is
a phenomenon known as clock feed-through.
For zero-drift amplifiers, clock feed-through will be
proportional to source impedance and the magnitude of
injection current, a measure of which is I
B
at 25°C. In
order to minimize clock feed-through, keep gain-setting
resistors and source impedances as low as possible. If
high impedances are required, place a capacitor across
the feedback resistor to limit the bandwidth of the closed
loop gain. Doing so will effectively filter out the clock
feed-through signal.
Injection currents from the two inputs are of equal magni
-
tude but
opposite direction. Therefore, input bias current
effects
due to injection currents will not be canceled by
placing matched impedances at both inputs.
Above 75°C, leakage of the ESD protection diodes
begins to
dominate the input bias current and continues to increase
exponentially at elevated temperatures. Unlike injection
current, leakage currents are in the same direction for both
inputs. Therefore, the output error due to leakage currents
can be mitigated by matching the source impedances seen
by the two inputs.
Thermocouple Effects
In order to achieve accuracy on the microvolt level, ther
-
mocouple effects must be considered. Any connection
of dissimilar metals forms a thermoelectric junction and
generates a small temperature-dependent voltage. Also
known as the Seebeck Effect, these thermal EMFs can be
the dominant error source in low-drift circuits.
Connectors, switches, relay contacts, sockets, resistors,
and solder are all candidates for significant thermal EMF
generation. Even junctions of copper wire from different
manufacturers can generate thermal EMFs of 200nV/°C,
which is over 13 times the maximum drift specification of
the LTC2057. Figures 4 and 5 illustrate the potential magni
-
tude of
these voltages and their sensitivity to temperature.
In
order to minimize thermocouple-induced errors, atten-
tion must
be 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 and avoid con
-
nectors, sockets, switches, and relays whenever possible.
If such components are required, they should be selected
for low thermal EMF characteristics. Furthermore, the
number, type, and layout of junctions should be matched
for both inputs with respect to thermal gradients on the
circuit board. Doing so may involve deliberately introducing
dummy junctions to offset unavoidable junctions.
Figure 4. Thermal EMF Generated by Two Copper Wires
From Different Manufacturers
Figure 5. Solder-Copper Thermal EMFs
TEMPERATURE (°C)
25
MICROVOLTS REFERRED TO 25°C
1.8
2.4
3.0
2.8
2.6
2.0
2.2
1.4
1.6
0.800
1.0
0.200
0.400
30 40 45
2057 F04
1.2
0.600
0
35
SOLDER-COPPER JUNCTION DIFFERENTIAL TEMPERATURE
SOURCE: NEW ELECTRONICS 02-06-77
0
THERMALLY PRODUCED VOLTAGE IN MICROVOLTS
0
50
40
2057 F05
–50
–100
10
20
30
50
100
SLOPE ≈ 1.5µV/°C
BELOW 25°C
SLOPE ≈ 160nV/°C
BELOW 25°C
64% SN/36% Pb
60% Cd/40% SN