DESCRIPCIÓN GENERAL

The AD8422 is a high precision, low power, low noise, rail-to-rail instrumentation amplifier that delivers the best performance per unit microampere in the industry. The AD8422 processes signals with ultralow distortion performance that is load independent over its full output range.

The AD8422 is the third generation development of the industrystandard AD620. The AD8422 employs new process technologies and design techniques to achieve higher dynamic range and lower errors than its predecessors, while consuming less than one-third of the power. The AD8422 uses the high performance pinout introduced by the AD8221.

Very low bias current makes the AD8422 error free with high source impedance, allowing multiple sensors to be multiplexed to the inputs. Low voltage noise and low current noise make the AD8422 an ideal choice for measuring a Wheatstone bridge.

The wide input range and rail-to-rail output of the AD8422 bring all of the benefits of a high performance in-amp to single-supply applications. Whether using high or low supply voltages, the power savings make the AD8422 an excellent choice for high channel count or power sensitive applications.

The AD8422 uses robust input protection that ensures reliability without sacrificing noise performance. The AD8422 has high ESD immunity, and the inputs are protected from continuous voltages up to 40 V from the opposite supply rail.

A single resistor sets the gain from 1 to 1000. The reference pin can be used to apply a precise offset to the output voltage.

The AD8422 is specified from −40°C to +85°C for the 8-lead MSOP and the 8-lead SOIC_N and from −40°C to +125°C for the 8-lead LFCSP.

 

CARACTERÍSTICAS

Low power: 368 μA maximum quiescent current

Rail-to-rail output

Low noise and distortion

8 nV/√Hz maximum input voltage noise at 1 kHz

0.15 μV p-p RTI noise (G = 100)

0.5 ppm nonlinearity with 2 kΩ load (G = 1)

Excellent ac specifications

80 dB minimum CMRR at 10 kHz (G = 1)

2.2 MHz bandwidth (G = 1)

High precision dc performance (AD8422BRZ)

150 dB minimum CMRR (G = 1000)

0.04% maximum gain error (G = 1000)

0.3 μV/°C maximum input offset drift

0.5 nA maximum input bias current

Wide supply range

4.6 V to 36 V single supply

±2.3 V to ±18 V dual supply

Input overvoltage protection: 40 V from opposite supply

Gain range: 1 to 1000

Available in 8-lead LFCSP, 8-lead MSOP, and 8-lead SOIC_N

 

APLICACIONES

Instrumentación médica

Industrial process controls

Strain gages

Transducer interfaces

Precision data acquisition systems

Channel-isolated systems

Portable instrumentation

 

TEORÍA DE FUNCIONAMIENTO

ARCHITECTURE The AD8422 is based on the classic 3-op-amp instrumentation amplifier topology. This topology has two stages: a preamplifier to provide differential amplification followed by a difference amplifier that removes the common-mode voltage.

Topologically, Q1, A1, R1 and Q2, A2, R2 can be viewed as precision current feedback amplifiers that maintain a fixed current in the emitters of Q1 and Q2. Any change in the input signal forces the output voltages of A1 and A2 to change accordingly and maintain the Q1 and Q2 current at the correct value. This causes a precise diode drop from –IN and +IN to Node 3 and Node 4, respectively, so that the differential signal applied to the inputs is replicated across the RG pins. Any current through RG must also flow through R1 and R2, creating the gained differential voltage between Node 1 and Node 2.

The amplified differential signal and the common-mode signal are applied to a difference amplifier that rejects the common-mode voltage but preserves the amplified differential voltage.

Laser-trimmed resistors allow for a highly accurate in-amp with a gain error of less than 0.01% and a CMRR that exceeds 94 dB (G = 1). The supply current is precisely trimmed to reduce uncertainties due to part-to-part variations in power dissipation and noise. The high performance pinout and special attention to design and layout allow for high CMRR across a wide frequency and temperature range. Using superbeta input transistors and bias current compensation, the AD8422 offers extremely high input impedance and low bias current, as well as very low voltage noise while using only 300 μA supply current. The overvoltage protection scheme allows the input to go 40 V from the opposite rail at all gains without compromising the noise performance.