LT6003 datasheet(16/30 Pages) AD

16 / 30 page

LTC2066/LTC2067/LTC2068

Rev. B

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APPLICATIONS INFORMATION

to minimize power consumption and/or require a sensor

which is intrinsically high impedance. In these cases, a

capacitor can be used, either at the input or across the

feedback resistor, to limit the bandwidth of the closed-

loop system. Doing so will effectively filter out the clock

feedthrough signal.

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 significantly exceeds the maximum drift specifica-

tion of the LTC2066/LTC2067/LTC2068. Figures 2 and 3

illustrate the potential magnitude 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 num-

ber, type, and layout of junctions should be matched for

both inputs with respect to thermal gradients on the cir-

cuit board. Doing so may involve deliberately introducing

dummy junctions to offset unavoidable junctions.

Air currents can also lead to thermal gradients and cause

significant noise in measurement systems. It is important

to prevent airflow across sensitive circuits. Doing so will

often reduce thermocouple noise substantially. A sum-

mary of techniques can be found in Figure 4.

Leakage Effects

Leakage currents into high impedance signal nodes can

easily degrade measurement accuracy of sub-nanoamp

signals. High voltage and high temperature applications

are especially susceptible to these issues. Quality insula-

tion materials should be used, and insulating surfaces

should be cleaned to remove fluxes and other residues.

For humid environments, surface coating may be neces-

sary to provide a moisture barrier.

TEMPERATURE (°C)

1.8

2.4

3.0

2.8

2.6

2.0

2.2

1.4

1.6

0.8

1.0

0.2

0.4

2066 F02

1.2

0.6

Figure 2. Thermal EMF Generated by Two Copper

Wires from Different Manufacturers

SOLDER-COPPER JUNCTION DIFFERENTIAL TEMPERATURE

SOURCE: NEW ELECTRONICS 02-06-77

2066 F03

–50

–100

100

SLOPE ≈ 1.5µV/°C

BELOW 25°C

SLOPE ≈ 160nV/°C

BELOW 25°C

64% SN/36% Pb

60% Cd/40% SN

Figure 3. Solder-Copper Thermal EMFs

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