ADA4522-4ARZ-R7 datasheet(27/32 Pages) AD

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Data Sheet

ADA4522-1/ADA4522-2/ADA4522-4

Rev. E | Page 27 of 32

APPLICATIONS INFORMATION

SINGLE-SUPPLY INSTRUMENTATION AMPLIFIER

The extremely low offset voltage and drift, high open-loop gain,

high common-mode rejection, and high power supply rejection of

the ADA4522-1/ADA4522-2/ADA4522-4 make them excellent op

amp choices as discrete, single-supply instrumentation amplifiers.

Figure 82 shows the classic 3-op-amp instrumentation amplifier

using the ADA4522-1/ADA4522-2/ADA4522-4. The key to high

CMRR for the instrumentation amplifier are resistors that are well

matched for both the resistive ratio and relative drift. For true

difference amplification, matching of the resistor ratio is very

important, where R5/R2 = R6/R4. The resistors are important

in determining the performance over manufacturing tolerances,

time, and temperature. Assuming a perfect unity-gain difference

amplifier with infinite common-mode rejection, a 1% tolerance

resistor matching results in only 34 dB of common-mode rejection.

Therefore, at least 0.01% or better resistors are recommended.

VIN1

VIN2

RG1

RG2

VOUT

RG1 = RG2, R1 = R3, R2 = R4, R5 = R6

VOUT = (VIN2 – VIN1) (1 + R1/RG1) (R5/R2)

Figure 82. Discrete 3-Op-Amp Instrumentation Amplifier

To build a discrete instrumentation amplifier with external resis-

tors without compromising on noise, pay close attention to the

resistor values chosen. RG1 and RG2 each have thermal noise that

is amplified by the total noise gain of the instrumentation amplifier

and, therefore, a sufficiently low value must be chosen to reduce

thermal noise contribution at the output while still providing an

accurate measurement. Table 10 shows the external resistors noise

contribution referred to the output (RTO).

Table 10. Thermal Noise Contribution Example

Resistor

Value

(kΩ)

Resistor Thermal

Noise (nV/√Hz)

Thermal Noise

RTO (nV/√Hz)

RG1

0.4

2.57

128.30

RG2

0.4

2.57

128.30

12.83

25.66

12.83

25.66

12.83

25.66

12.83

25.66

18.14

Note that A1 and A2 have a high gain of 1 + R1/RG1. Therefore,

use a high precision, low offset voltage and low noise amplifier

for A1 and A2, such as the ADA4522-1/ADA4522-2/ADA4522-4.

Conversely, A3 operates at a much lower gain and has a differ-

ent set of op amp requirements. Its input noise, referred to the

overall instrumentation amplifier input, is divided by the first

stage gain and is not as important. Note that the input offset

voltage and the input voltage noise of the amplifiers are also

amplified by the overall noise gain.

Any unused channel of the ADA4522-1/ADA4522-2/

ADA4522-4 must be configured in unity gain with the input

common-mode voltage tied to the midpoint of the power

supplies.

Understanding how noise impacts a discrete instrumentation

amplifier or a difference amplifier (the second stage of a 3-op-

amp instrumentation amplifier) is important, because they are

commonly used in many different applications. The Load

Cell/Strain Gage Sensor Signal Conditioning section and the

Precision Low-Side Current Shunt Sensor section show the

ADA4522-1/ADA4522-2/ADA4522-4 used as a discrete

instrumentation or difference amplifier in an application.

LOAD CELL/STRAIN GAGE SENSOR SIGNAL

CONDITIONING USING THE ADA4522-2

The ADA4522-2, with its ultralow offset, drift, and noise, is well

suited to signal condition a low level sensor output with high gain

and accuracy. A weigh scale/load cell is an example of an

application with such requirements. Figure 83 shows a configura-

tion for a single-supply, precision, weigh scale measurement

system. The ADA4522-2 is used at the front end for amplification

of the low level signal from the load cell.

Current flowing through a PCB trace produces an IR voltage

drop; with longer traces, this voltage drop can be several

millivolts or more, introducing a considerable error. A 1 inch

long, 0.005 inch wide trace of 1 oz copper has a resistance of

approximately 100 mΩ at room temperature. With a load

current of 10 mA, the resistance can introduce a 1 mV error.

Therefore, a 6-wire load cell is used in the circuit. The load cell

has two sense pins, in addition to excitation, ground, and two

output connections. The sense pins are connected to the high

side (excitation pin) and low side (ground pin) of the

Wheatstone bridge. The voltage across the bridge can then be

accurately measured regardless of voltage drop due to wire

resistance. The two sense pins are also connected to the analog-

to-digital converter (ADC) reference inputs for a ratiometric

configuration that is immune to low frequency changes in the

power supply excitation voltage.

The ADA4522-2 is configured as the first stage of a 3-op-amp

instrumentation amplifier to amplify the low level amplitude

signal from the load cell by a factor of 1 + 2R1/RG. Capacitors

C1 and C2 are placed in the feedback loops of the amplifiers

and interact with R1 and R2 to perform low-pass filtering. This

filtering limits the amount of noise entering the Σ-Δ ADC. In

addition, C3, C4, C5, R3, and R4 provide further common-mode

and differential mode filtering to reduce noise and unwanted

signals.

ADA4522-4ARZ-R7 Datasheet(PDF) 27 Page - Analog Devices

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