LMP7721MA National Semiconductor, LMP7721MA Datasheet - Page 14

• Manufacturer Part Number: LMP7721MA
• Description: IC, OP AMP, PRECISION, POWERWISE
• Manufacturer: National Semiconductor

Specifications of LMP7721MA

• Op Amp Type: Precision
• No. Of Amplifiers: 1
• Bandwidth: 17MHz
• Slew Rate: 12.76V/µs
• Supply Voltage Range: 1.8V To 5.5V
• Amplifier Case Style: SOIC
• No. Of Pins: 8
• Operating Temperature Range: -40°C To
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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FIGURE 4. Error Caused by Amplifier’s Input Bias Current
Figure 5 is an example of the LMP7721 used as a pH sensor
amplifier.
The output voltage from the pH electrode is fed to the signal
conductor of the triax and then sent to the non-inverting input
of the LMP7721. In this application, the inverting input is a low
impedance node and hence is used to drive the LMP7715
FIGURE 5. LMP7721 as pH Electrode Amplifier
and Sensor Source Impedance
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which acts as a guard driver. The output of the guard driver
is connected to the guard of the triax via a 100Ω isolation re-
sistor. Ideally, the inverting and the non-inverting inputs of the
amplifier are kept at the same potential through the operation
of the amplifier. By connecting the signal conductor to the
non-inverting input and letting the inverting input drive the
guard, the signal conductor and the guard are kept at the
same potential which prevents leakage from the signal
source.
LAYOUT AND ASSEMBLY CONSIDERATIONS
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 par-
asitic 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 po-
tential is raised to the input line’s voltage. The input pins
should be fully guarded as shown in Figure 6. 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.
Solder mask should not cover the input and the guard area
including guard traces on either side of the PCB.
Sockets are not recommended as they can be a significant
leakage source. After assembly, a thorough cleaning using
commercial solvent is necessary.
FIGURE 6. Circuit Board Guard Layout
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