8.2 Typical Application

+

±

240 k

10 k

+

±

GND

GND

-5 V

5 V

10 M

GND

Vin = ±0.1 V

GND

Vout

Signal Source with

high output impedance

Figure 8-3. LMP7704-SP Configured for 25x Gain With High Signal Source Impedance

8.2.1 Design Requirements

Many precision, analog sensors such as temperature or pressure (bridge) sensors, require a high precision

amplifier with low input bias to condition the signal before the analog-to-digital converter. The LMP7704-SP is an

excellent amplifier choice for a voltage gain stage thanks to its low offset voltage, offset voltage drift and ultra-

low input bias current.

8.2.2 Detailed Design Procedure

Many sensors have high source impedances that may range up to 10 MΩ. The output signal of sensors often

needs to be amplified or otherwise conditioned by means of an amplifier. The input bias current of this amplifier

can load the sensor output and cause a voltage drop across the source resistance, as shown in Figure 8-4,

SENSOR

V

+

V

-

+

-

+

-

this voltage, an op amp with very low input bias current must be used with high impedance sensors. An amplifier

with low input bias also has low input current noise, further improving the accuracy of systems with high source

resistance.

Figure 8-3 shows one channel of the LMP7704-SP configured for a gain of 10. A high source impedance is

placed between the input signal and the noninverting input of the amplifier to represent the output impedance of

the sensor.

With the ultra-low input bias current of the LMP7704-SP, even with a signal source that has high output

impedance the system output maintains very good linearity to the ideal output voltage (that is, the output of an

ideal amplifier in the same configuration) as shown in Figure 8-6.

LMP7704-SP

SNOSDB6A – DECEMBER 2020 – REVISED JANUARY 2021

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