AD620 Low Cost, Low Power Instrumentation Amplifier

The AD620 is a low cost, high accuracy instrumentation amplifier that requires only one external resistor to set gains of 1 to 1000. Furthermore, the AD620 features 8-lead SOIC and DIP packaging that is smaller than discrete designs, and offers lower power (only 1.3 mA max supply current), making it a good fit for battery powered, portable (or remote) applications.The AD620, with its high accuracy of 40 ppm maximum nonlinearity, low offset voltage of 50 mV max and offset drift of 0.6 mV/°C max, is ideal for use in precision data acquisition systems, such as weigh scales and transducer interfaces. Furthermore, the low noise, low input bias current, and low power of the AD620 make it well suited for medical applications such as ECG and noninvasive blood pressure monitors.The low input bias current of 1.0 nA max is made possible with the use of Superbeta processing in the input stage. The AD620 works well as a preamplifier due to its low input voltage noise of 9 nV/ÖHz at 1 kHz, 0.28 mV p-p in the 0.1 Hz to 10 Hz band, 0.1 pA/ÖHz input current noise. Also, the AD620 is well suited for multiplexed applications with its settling time of 15 ms to 0.01% and its cost is low enough to enable designs with one inamp per channel. 

The log circuit consists of an instrumentation amp and an Op amp together with a diode-connectedtransistor that produces a voltage proportional to the logarithm of the current A circuit consisting of avoltage reference, an instrumentation amp,and an op amp,togetherwith a diode-connected transistor,acts as a reference circuit. A thermometer IC,a fixed-gain instrumentation amp,and a divider circuitprovide the necessary temperature compensation and scaling for a transfer function

V_out51.985 log10 (V_in/1V)

V_ref must be set to 1.000 V and, with V_in 5V_ref, the gain adjust has to be set so that V_O 50 V. Calirelbration low input voltage is done by changing buffer A4's offset voltage.