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INA338 Datasheet(PDF) 11 Page - Texas Instruments |
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INA338 Datasheet(HTML) 11 Page - Texas Instruments |
11 / 14 page INA337, INA338 11 SBOS222A www.ti.com INPUT AND OUTPUT VOLTAGE The INA337 and INA338 feature nearly rail-to-rail input behavior, with the linear input voltage range extending from 0.25V above the negative rail to 0.1V above the positive rail. The output is able to swing to within 0.25V of the negative rail and 0.075V of the positive rail. See Typical Characteristics Curve “Output Swing to the Negative Rail” for additional detail. The INA337 uses a new, unique internal circuit topology that provides near rail-to-rail input. Unlike other instru- mentation amplifiers, it can linearly process inputs from 0.25V above the negative rail to 0.1V beyond the positive rail. Conventional instrumentation amplifier circuits cannot deliver such performance, even if rail-to-rail op amps are used. The ability to reject common-mode signals is derived in most instrumentation amplifiers through a combination of amplifier CMR and accurately matched resistor ratios. The INA337 converts the input voltage to a current. Current- mode signal processing provides rejection of common- mode input voltage and power-supply variation without accurately matched resistors. The topology of the INA337 avoids aliasing issues that appear in instrumentation amplifiers that use sampled data techniques. A simplified diagram shows the basic circuit function. The differential input voltage, (VIN+) – (VIN–) is applied across R1. The signal-generated current through R1 comes from A1 and A2’s output stages. A2 combines the current in R1 with a mirrored replica of the current from A1. The result- ing current in A2’s output and associated current mirror is two times the current in R1. This current flows in (or out) of pin 5 into R2. The resulting gain equation is: G R R = 2 2 1 Amplifiers A1, A2 and their associated mirrors are pow- ered from internal charge-pumps that provide voltage supplies that are beyond the positive negative supply. As a result, the voltage developed on R2 can actually swing 100mV above the positive power-supply rail. A3 provides a buffered output of the voltage on R2. A3’s input stage is also operated from the charge-pumped power supplies for true rail-to-rail operation. FIGURE 4. Simplified Circuit Diagram. INSIDE THE INA337 INPUT PROTECTION The inputs of the INA337 are protected with internal diodes connected to the power-supply rails. These diodes will clamp the applied signal to prevent it from damaging the input circuitry. If the input signal voltage can exceed the power supplies by more than 0.5V, the input signal current should be limited to less than 10mA to protect the internal clamp diodes. This can generally be done with a series input resistor. Some signal sources are inherently current-limited and do not require limiting resistors. A1 V+ V– Current Mirror Current Mirror I R1 I R1 I R1 R 1 R 2 C 2 V O V IN– V IN+ I R1 2I R1 2I R1 2I R1 2I R1 2I R1 A3 A2 IA COMMON 0.1 µF Current Mirror Current Mirror |
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