IN-

IN+

N/C

V

-

VOUT

N/C

V

+

N/C

GUARD

GUARD

LMP7721

www.ti.com

SNOSAW6E – JANUARY 2008 – REVISED DECEMBER 2014

9 Power Supply Recommendations

For high-sensitivity applications, the power supply rails should be as clean as possible.

Noise on the power supply lines can modulate the tiny capacitance (about 0.5 pF) of the ESD structure on each

input. While this is not a major concern for most applications, charge-sensitive or high-gain, high-impedance

applications can be affected. Common results are power line "hum" or high-frequency switcher "hash" imposed

on the signal.

TI recommends using a very low noise linear regulator and add a dedicated filter network to the LMP7721 power

supply pins consisting of a series resistor of about 100

Ω, and a bypass capacitor of 100 uF or larger. Series

inductors or ferrite beads may be required if high frequency switcher noise is present.

10 Layout

10.1 Layout Guidelines

In order to capitalize on the LMP7721’s ultra-low input bias current, careful circuit layout and assembly are

required. Guarding techniques are highly recommended to reduce parasitic leakage current by isolating the

LMP7721’s input from large voltage gradients across the PC board. A guard is a low-impedance conductor that

surrounds an input line and its potential is raised to the input line’s voltage. The input pins should be fully

guarded as shown in Figure 54. The guard traces should completely encircle the input connections. In addition,

they should be located on both sides of the PCB and be connected together.

To further guard the inputs from the supply pins, the two N/C pins may be connected to the guard trace which

will provide guarding down to the leadframe level.

Solder mask should not cover the input and the guard area including guard traces on either side of the PCB.

Keep switching power supplies and other noise-producing devices away from the input area.

10.2 Layout Example

Figure 54. Layout Example Showing Guard Trace

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