LTC1051/LTC1053

6

10513fa

ACHIEVING PICOAMPERE/MICROVOLT PERFORMANCE

Picoamperes

In order to realize the picoampere level of accuracy of the

LTC1051/LTC1053, proper care must be exercised. Leak-

age 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 insulat-

ing 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. Guarding both sides of the printed

circuit board is required. Bulk leakage reduction depends

on the guard ring width.

Microvolts

Thermocouple effects must be considered if the LTC1051/

LTC1053’s ultra low drift op amps are to be fully utilized.

Any connection of dissimilar metals forms a thermoelec-

tric junction producing an electric potential which varies

with temperature (Seebeck effect.) As temperature sen-

sors, thermocouples exploit this phenomenon to produce

useful information. In low drift amplifier circuits, this effect

is a primary source of error.

Connectors, switches, relay contacts, sockets, resistors,

solder, and even copper wire are all candidates for thermal

EMF generation. Junctions of copper wire from different

manufacturers can generate thermal EMFs of 200nV/°C—

4 times the maximum drift specification of the LTC1051/

LTC1053. 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 LTC1051/LTC1053.

Minimizing thermal EMF-induced errors is possible if

judicious 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

differential cancellation occurs. Doing this may involve

deliberately introducing junctions to offset unavoidable

junctions.

When connectors, switches, relays and/or sockets are

necessary, they should be selected for low thermal EMF

activity. The same techniques of thermally balancing and

coupling the matching junctions are effective in reducing

the thermal EMF errors of these components.

Resistors are another source of thermal EMF errors.

Table 1 shows the thermal EMF generated for different

resistors. The temperature gradient across the resistor is

important, not the ambient temperature. There are two

junctions formed at each end of the resistor and if these

junctions are at the same temperature, their thermal EMFs

will cancel each other. The thermal EMF numbers are

approximate and vary with resistor value. High values give

higher thermal EMF.

Table 1. Resistor Thermal EMF

RESISTOR TYPE

THERMAL EMF/°C GRADIENT

Tin Oxide

~mV/°C

Carbon Composition

~450µV/°C

Metal Film

~20µV/°C

Wire Wound

Evenohm

~2µV/°C

Manganin

~2µV/°C

Input Bias Current, Clock Feedthrough

At ambient temperatures below 60°C, the input bias cur-

rent of the LTC1051/LTC1053 op amps’ is dominated by

the small amount of charge injection occurring during the

sampling and holding of the op amps’ input offset voltage.

The average value of the resulting current pulses is 10pA

to 15pA with sign convention shown in Figure 1.

Figure 1. LTC1051 Bias Current

–

+

–

+

1/2

LTC1051

1/2

LTC1051

+

–

+

–

1051/53 F01

(a)

(b)

APPLICATIO S I FOR ATIO