LTC1050ACN8 View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LTC1050ACN8

Precision Zero-Drift Operational Amplifier with Internal Capacitors

Linear Technology 

LTC1050

APPLICATI S I FOR ATIO

Figure 2 is an example of the introduction of an unneces-

sary resistor to promote differential thermal balance.

Maintaining compensating junctions in close physical prox-

imity will keep them at the same temperature and reduce

thermal EMF errors.

NOMINALLY

UNNECESSARY

RESISTOR USED TO

THERMALLY BALANCE

OTHER INPUT RESISTOR

RESISTOR LEAD, SOLDER

COPPER TRACE JUNCTION

LEAD WIRE/SOLDER/COPPER

TRACE JUNCTION

+

LTC1050

–

OUTPUT

1050 F02

Figure 2

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

Tin Oxide

Carbon Composition

Metal Film

Wire Wound

Evenohm

Manganin

THERMAL EMF/°C GRADIENT

~ mV/°C

~ 450µV/°C

~ 20µV/°C

~ 2µV/°C

~ 2µV/°C

PACKAGE-INDUCED OFFSET VOLTAGE

Package-induced thermal EMF effects are another impor-

tant source of errors. It arises at the copper/kovar junctions

formed when wire or printed circuit traces contact a

package lead. Like all the previously mentioned thermal

EMF effects, it is outside the LTC1050’s offset nulling loop

and cannot be cancelled. The input offset voltage specifi-

cation of the LTC1050 is actually set by the package-induced

warm-up drift rather than by the circuit itself. The thermal

time constant ranges from 0.5 to 3 minutes, depending

upon package type.

OPTIONAL EXTERNAL CLOCK

An external clock is not required for the LTC1050 to

operate. The internal clock circuit of the LTC1050 sets the

nominal sampling frequency at around 2.5kHz. This fre-

quency is chosen such that it is high enough to remove the

amplifier 1/f noise, yet still low enough to allow internal

circuits to settle.The oscillator of the internal clock circuit

has a frequency 4 times the sampling frequency and its

output is brought out to Pin 5 through a 2k resistor. When

the LTC1050 operates without using an external clock,

Pin 5 should be left floating and capacitive loading on this

pin should be avoided. If the oscillator signal on Pin 5 is

used to drive other external circuits, a buffer with low

input capacitance is required to minimize loading on this

pin. Figure 3 illustrates the internal sampling frequency

versus capacitive loading at Pin 5.

3

VS = ± 5V

2

1

1

5 10

100

CAPACITANCE LOADING (pF)

1050 F03

Figure 3. Sampling Frequency vs Capacitance Loading at Pin 5

1050fb

7

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