AD623ARZ-RL

The AD623 is an integrated single-supply instrumentation amplifier from Analog Devices that delivers rail-to-rail output swing on a 3 V to 12 V supply. It follows the industry-standard 8-pin in-amp pinout (same as AD620, INA126), making it a drop-in upgrade in many existing designs. The AD623ARZ-RL is the A-grade version in SOIC-8 tape-and-reel packaging with 200-µV max input offset voltage.

The gain is set by a single external resistor according to G = 1 + (100 kΩ/RG). With no external resistor, the device operates at unity gain (G = 1), simplifying PCB layout for applications that do not require amplification. The gain range of 1 to 1000 covers the vast majority of instrumentation applications, from bridge signal conditioning to thermocouple amplification.

The AD623’s key advantage over competing in-amps is its ability to operate from a single 3-V supply while maintaining high CMRR. The CMRR remains stable up to 200 Hz, effectively rejecting power-line harmonics in industrial environments. At G = 10, CMRR is 90 dB minimum at 60 Hz with 1 kΩ source imbalance — sufficient for most sensor applications.

The input common-mode voltage range extends 150 mV below the negative supply rail, enabling the AD623 to amplify signals at or slightly below ground in single-supply systems. This is critical for current-sense applications where the shunt voltage may be near ground.

The rail-to-rail output stage swings to within 10 mV of the supply rails under light load, maximizing dynamic range in low-voltage systems. The 550-µA max supply current is low enough for battery-powered medical instruments and portable data acquisition systems.

The B-grade (AD623BRZ) offers 100-µV max Vos and 1-µV/°C max drift for higher-precision applications at additional cost.

**Three-Op-Amp Instrumentation Architecture:** The AD623 uses the classic three-op-amp instrumentation amplifier topology: two non-inverting input buffers (A1, A2) and a difference amplifier (A3). The input buffers present high impedance (2 GΩ) to the signal sources and provide gain. The difference amplifier rejects common-mode voltage and extracts the differential signal. The gain resistor RG connects between the RG pins and programs the gain of the input stage.

**Gain Setting Mechanism:** Without RG (RG pins open), the input buffers operate as unity-gain followers, and the overall gain is 1. When RG is connected, the feedback loops of A1 and A2 include RG in their gain paths: G = 1 + (100 kΩ/RG). The internal 100 kΩ resistors are laser-trimmed to 0.01% accuracy, ensuring precise gain.

**Common-Mode Rejection:** The difference amplifier A3 is laser-trimmed for resistor matching, achieving high CMRR. The CMRR increases with gain because the differential signal is amplified by the input stage before reaching A3, while the common-mode voltage is not. At G = 1000, CMRR exceeds 105 dB.

**Rail-to-Rail Output Stage:** The output stage uses a complementary push-pull architecture that can swing to within millivolts of both supply rails. This is essential in 3-V and 5-V single-supply systems where output headroom is limited. The output can drive loads as low as 2 kΩ to the rails.