Data-acquisition circuit measures almost everything

Matt Smith, Analog Devices, Limerick, Ireland -- EDN, 11/9/2000

Using a product developed for PC-motherboard environmental monitoring, you can configure a low-cost, general-purpose DAS (data-acquisition system) (Figure 1). The DAS can directly monitor multiple voltage channels as well as temperature and frequency. It can also directly monitor digital sensors. Using only a few additional components, the system can accommodate other sensor and transducer elements. The flexibility exists to expand the scheme to cover additional input channels if necessary. For voltage sensing, the ADM9240 contains a multichannel ADC that can directly monitor as many as six input channels. The original intent of the ADC was to monitor power supplies on PC motherboards, but the converter is flexible enough for general-purpose use. The maximum input-voltage ranges for the channels are 3.3, 3.6, 4.4, 6.64, and 16V. Figure 1 shows the system monitoring two power supplies: PS1 and PS2. The DAS can monitor voltages greater than the channel range by using a simple voltage divider at the front end, as illustrated with PS3.

The ADM9240's on-board DAC (originally intended as a fan-speed controller) can serve as a programmable, precision reference source. This function, for example, would facilitate measuring resistance-type sensors on the voltage-sensing channels. You could also use it as a bridge-excitation voltage source for accurate bridge-sensing elements. You can determine an unknown resistance value, such as a thermistor, by setting the DAC's output voltage to a known level with a known fixed resistance (Figure 1). You can implement current sensing by placing an accurate series resistor (R_SENSE) in the ground line and monitoring the voltage drop across the resistor. The DAS also provides temperature sensing by using an on-chip bandgap silicon sensor. The system can directly monitor temperatures over a –40° to +85°C range.

The DAS provides two frequency-monitoring channels. You can use them to monitor pulsed digital output from a tachometer or as general-purpose frequency counters. Five digital-input lines were originally intended to monitor digital voltage-identification lines. You can use them for general-purpose input lines, whose inputs can sense high- or low-level status signals from digital sensors or from alarm channels. In Figure 1, the DAS monitors a thermostatic sensor. The DAS handles control and reading functions via a simple two-wire SMBus or an I²C interface to a µP or µC. If a dedicated I²C controller is not available, then you can use a port "bit-banging" technique. Easy expansion is also possible by selecting a different device address. Using a different device-address bus entails no additional communication lines, because multiple devices can reside on the same bus.