ADA4522-4ARUZ-R7 datasheet(38/48 Pages) AD

38 / 48 page

Data Sheet

ADA4522-1/ADA4522-

2/ADA4522-4

analog.com

Rev I 38 of 48

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 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 Gauge 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

instrumentation or difference amplifier in an application.

Load Cell/Strain Gauge 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

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-∆ ADC. In addition, C3, C4, C5, R3, and R4 provide further common-mode and differential mode

filtering to reduce noise and unwanted signals.

ADA4522-4ARUZ-R7 Datasheet(PDF) 38 Page - Analog Devices