LMC6484AIMSLASHNOPB datasheet(20/50 Pages) TI2

20 / 50 page

7.1.3 Instrumentation Circuits

The LMC648x have high input impedance, large common-mode range and high CMRR needed for designing

instrumentation circuits. Instrumentation circuits designed with the LMC648x can reject a larger range of

common-mode signals than most in-amps. This makes instrumentation circuits designed with the LMC648x an

excellent choice for noisy or industrial environments. Other applications that benefit from these features include

analytic medical instruments, magnetic field detectors, gas detectors, and silicon-based transducers.

A small valued potentiometer is used in series with RG to set the differential gain of the 3-op-amp

instrumentation circuit in Figure 7-2. This combination is used instead of one large valued potentiometer to

increase gain trim accuracy and reduce error due to vibration. An improved design that can help increase

accuracy, save cost, and reduce board space can be achieved by using the RES11A matched resistor pair

series.

–

LMC648x

VIN

VOUT

0.1%

50k

–

LMC648x

VIN+

50k

0.1%

50k

–

LMC648x

50k

48.7k

10pF

VREF

C4 = 3pF to 20pF

0.1%

Ri = 10k

ac CMR

adjust

dc CMR

adjust

Figure 7-2. Low Power, Three Op Amp Instrumentation Amplifier

The Figure 7-3 shows how a high precision, high CMRR, and low drift in-amp can be achieved using two

matched resistor pairs. Using a 1:4 ratio, a gain of 36V/V can be easily implemented. Other gain options are

possible by using the various ratios available. One downside to the original implementation in Figure 7-2 is

that very high performance, 0.01% resistors and a couple of potentiometers are needed to achieve very high

common-mode rejection and gain accuracy. High accuracy resistors can be very expensive and add to board

layout size and complexity. Another downside is that the temperature drift of the discrete resistors causes an

increase in gain error that is not easily calibrated out.

The RES11A matched resistor pairs provide high common-mode rejection and gain-error performance due to

excellent matching to less than 0.05%. The resistors are on the same substrate; therefore, the resistors drift in

the same direction, minimizing temperature-related errors such as gain error drift. For a more detailed analysis

of the benefits of the RES11A over discrete resistors, see the Optimizing CMRR in Differential Amplifier Circuits

With Precision Matched Resistor Divider Pairs application note.

RES11A40-Q1

Floating

–

IN+

LMC648x

RES11A40-Q1

REF

–

OUT

LMC648x

ac CMR

adjust

Figure 7-3. Improved Low Power, Three Op Amp Instrumentation Amplifier With RES11A

LMC6482

LMC6484

SNOS674J – OCTOBER 1997 – REVISED SEPTEMBER 2024

www.ti.com

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