LTC2057 datasheet(21/32 Pages) LINER

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LTC2057/LTC2057HV

2057f

For more information www.linear.com/LTC2057

applicaTions inForMaTion

Figure 7b. Example Layout of Inverting Amplifier with Leakage Guard Ring

HIGH-Z SENSOR

LOW IMPEDANCE

NODE ABSORBS

LEAKAGE CURRENT

GUARD RING

LEAKAGE

CURRENT

V–

GRD

+IN

GRD

–IN

OUT

V+

VOUT

SDCOM

‡

‡ NO LEAKAGE CURRENT. V–IN = VGRD

§ AVOID DISSIPATING SIGNIFICANT AMOUNTS OF POWER IN THIS RESISTOR.

IT WILL GENERATE THERMAL GRADIENTS WITH RESPECT TO THE INPUT PINS

AND LEAD TO THERMOCOUPLE-INDUCED ERROR. THERMALLY ISOLATE OR

ALIGN WITH INPUTS IF RESISTOR WILL CAUSE HEATING.

VBIAS

RF§

2057 F07b

LTC2057

MS10

NO SOLDER

MASK OVER

GUARD RING

–

GUARD RING

LTC2057

LEAKAGE

CURRENT

LEAKAGE CURRENT IS ABSORBED BY GROUND INSTEAD OF

CAUSING A MEASUREMENT ERROR.

VOUT

V+

V–

HIGH-Z SENSOR

VBIAS

–

VIN

RIN

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 summary of techniques can be found in Figure 6.

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.

Board leakage can be minimized by encircling the input

connections with a guard ring operated at a potential very

close to that of the inputs. The ring must be tied to a low

impedance node. For inverting configurations, the guard

ring should be tied to the potential of the positive input

(+IN). For non-inverting configurations, the guard ring

shouldbetiedtothepotentialofthenegativeinput(–IN).In

order for this technique to be effective, the guard ring must

not be covered by solder mask. Ringing both sides of the

printedcircuitboardmayberequired.SeeFigures7aand7b

for examples of proper layout.

For low-leakage applications, the LTC2057 is available in

an MS10 package with a special pinout that facilitates the

layout of guard ring structures. The pins adjacent to the

inputs have no internal connection, allowing a guard ring

to be routed through them.

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