The principal factors that affect data acquisition systems are: speed, accuracy, power dissipation, package size, and component cost, with varying factors becoming critical depending upon the application. This article shows how a single op amp can be used to drive the analog-to-digital converter (ADC) in an 8-channel data-acquisition system, reducing the cost and size of the overall system.
An example eight-channel, 12-bit plus sign, 1-MSPS ADC (Analog Devices’ AD7329) has true bipolar inputs with four independently programmable software-selectable input ranges: ±4×VREF, ±2×VREF, ±VREF, and 0-to-4×VREF. It can be configured to suit a wide variety of application requirements. As shown in Figure 1, the ADC comprises an 8-channel multiplexer followed by a track-and-hold and successive-approximation ADC, a channel sequencer, a 2.5-V reference, and an SPI-compatible interface.
The analog input channels are routed through the multiplexer to the MUXOUT+ and MUXOUT– pins. The ADCIN+ and ADCIN– pins connect to the track-and-hold input switch (R1) and sampling capacitor (C2), as shown in Figure 2. Note that the input source must provide the current required to drive the ADC input, settling to the required accuracy within the ADC’s 300-ns acquisition time. When the track-and-hold switch goes from hold to track, the transient kick back from the ADC can affect the input source. Applications operating at the maximum sampling rate may require an input buffer amplifier to drive the ADC, isolating the source from the track-and-hold switch.
The flexible design on the ADC allows an op amp to be placed between the MUXOUT+ and ADCIN+ pins. In Figure 3, the an ultralow-noise, ultralow-distortion op-amp, such as Analog Devices’ AD797 isolates the input source from the ADC’s input structure, increasing the input impedance and reducing the current required to drive the ADC. This configuration also allows a single op-amp to drive eight analog input channels at the maximum sampling frequency, leading to reduced component count, board area, and system cost.
Configuring the op-amp for gain, as shown in Figure 4, allows the ADC to accommodate signals in the millivolt range while maintaining high performance. Small signals are amplified by the AD797 with the amplified signal applied to ADCIN+. To maximize performance, the gain is chosen such that the full-scale input signal uses the full dynamic range of the ADC.
Some applications require the gain to be changed to accommodate input channels with different signal amplitudes. In these cases, a programmable-gain instrumentation amplifier (PGIA can be used in place of the op-amp.)
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