Datasheet

LTC2057/LTC2057HV
21
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
–
V
–
GRD
+IN
GRD
–IN
OUT
NC
V
+
V
+
V
OUT
SD
SDCOM
‡
‡
NO LEAKAGE CURRENT. V
–IN
= V
GRD
§
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.
V
BIAS
R
F
§
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.
V
OUT
V
+
V
–
HIGH-Z SENSOR
R
F
V
BIAS
+
–
V
IN
R
IN
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
should be tied to the potential of the negative input (–IN). In
order for this technique to be effective, the guard ring must
not be covered by solder mask. Ringing both sides of the
printed circuit board
may be r
equired. See Figures 7a and 7b
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.