7.2 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 7-3. LMP7704-SP Configured for 25 × Gain With High Signal Source Impedance

7.2.1 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 the low offset voltage, offset voltage drift, and

ultra-low input bias current.

7.2.2 Detailed Design Procedure

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

often 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, shown in Figure 7-4, where VIN+

= VS – IBIAS × RS.

SENSOR

V

+

V

-

RS

+

-

IB

VIN+

VS

+

-

Figure 7-4. Offset Error Due to IBIAS

The last term, IBIAS * RS, shows the voltage drop across RS. To prevent errors introduced to the system due to

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 7-3 shows one channel of the LMP7704-SP configured for a gain of 25. 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). Figure 7-5 shows the output voltage vs input voltage of the

LMP7704-SP with a 10-MΩ source impedance. Figure 7-6 shows the output voltage vs input voltage for an

ideal amplifier with no input bias current. Comparing the two graphs shows that the LMP7704-SP maintains high

accuracy even with a large source impedance connected to an input.

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LMP7704-SP

SNOSDB6D – DECEMBER 2020 – REVISED OCTOBER 2024

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