ADA4522-4ARUZ-R7 datasheet(28/33 Pages) AD

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Data Sheet

ADA4522-1/ADA4522-2/ADA4522-4

APPLICATIONS INFORMATION

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Rev. G | 28 of 33

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 us-

ing 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 com-

mon-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.

Figure 82. Discrete 3-Op-Amp Instrumentation Amplifier

To build a discrete instrumentation amplifier with external resistors

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 different

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 am-

plifier 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 Using the ADA4522-2 section and

the Precision Low-Side Current Shunt Sensor section show the

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

tation 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 applica-

tion with such requirements. Figure 83 shows a configuration 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.

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