March 14, 1997

Simple control shuts down data-acquisition system

Kevin Hoskins, Linear Technology Corp, Milpitas, CA

One of the most effective ways to reduce circuit power is to shut down unused components. Some circuits with power-shutdown capability reduce supply current 1000-fold compared with normal operation. Placing data-conversion and support components in shutdown between conversions or when conversions are unnecessary significantly reduces supply current and average dissipation.

The circuit in Figure 1 shuts down a high-speed data-acquisition system using one control signal. The circuit uses a 3-MHz gain-bandwidth op amp, IC₁, and a 12-bit, 100k- sample/sec ADC, IC₂, each featuring shutdown capability. IC₁ conditions analog input signals and can buffer a high source impedance so that a low-impedance source drives the converter. Using infinite ohms for R₂ configures the op amp as a buffer for a ±2.048V input-signal magnitude. You can change the values of gain-setting resistors R₁ and R₂ to amplify input signals with magnitudes less than ±2.048V to match the converter's full-scale magnitude.

Applying 0V to its shutdown pin (Pin 5) activates the op amp, and applying ­5V to this pin places the amp in shutdown. Conversely, applying a logic high to its SLEEP pin activates the ADC, and a logic low places it in shutdown. Q₁ level-shifts the shutdown-control signal, allowing just one signal to simultaneously control the op amp's and the ADC's shutdown operations. When shutdown is necessary, the shutdown-control signal changes to a logic low, IC₂ goes into its power-saving shutdown mode, and the REFRDY signal changes to a logic low, turning off Q₁. Q₁'s collector is now pulled to ­V, placing IC₁ in shutdown.

Applying a logic-high signal to IC₂'s SLEEP pin resumes normal operation. The converter's bias currents return to their nominal operating points, and the reference begins charging its bypass capacitance. The REFRDY signal, which remains low until the reference output is stable, forces the op amp to remain shut down. When the converter's reference returns to its nominal operating point, the REFRDY signal changes to a logic high, turning on Q₁, and the op amp returns to full operation, ready for a new conversion. You can use the positive-going REFRDY signal to indicate to a µP that the ADC and op amp are ready to generate a conversion.

During full, 100k-sample/sec operation with no shutdown between conversions, the system draws 3.2 mA. When the shutdown signal is active, the total power-supply current is just 19 µA. (DI #2001)

| EDN Access | Feedback | Table of Contents |