Data-acquisition setup measures everything
Matt Smith, Analog Devices, Limerick, Ireland -- EDN, August 3, 2000
Using a product originally developed for PC-motherboard environmental monitoring, you can configure a low-cost, general-purpose DAS (data-acquisition system). The DAS in Figure 1 can directly monitor multiple voltage channels, as well as temperature and frequency. It can also monitor digital sensors. Using only a few additional components, the system can accommodate other sensor and transducer elements. Figure 1 shows one possible configuration. You can 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 IC was to monitor power supplies on PC motherboards, but it is flexible enough for general-purpose use. The maximum input-voltage ranges on the channels are 3.3, 3.6, 4.4, 6.64, and 16V. Figure 1 shows the DAS monitoring two power supplies: PS1 and PS2. You can measure voltages greater than the channel range by using a simple voltage divider, as illustrated with PS3.
You can use the onboard DAC in the DAS, originally intended as a fan-speed controller, as a programmable, precision voltage reference. This configuration, for example, would facilitate measuring resistance-type sensors with the voltage-sensing channels. You could also use it as a bridge-excitation voltage source for accurate bridge-sensing applications. You can determine an unknown resistance value by setting the DAC's output voltage to a known level and using a known fixed resistance (Figure 1). You can implement current sensing by placing an accurate series resistor, RSENSE, in the ground line and then monitoring the voltage drop across the resistor. An on-chip bandgap silicon sensor in the DAS provides temperature monitoring over the IC's –40 to +85°C operating range. You can use two frequency-monitoring channels in the DAS to monitor the pulsed digital output from a tachometer. The channels can also serve as general-purpose frequency counters.
The original intent of the five digital-input lines in the DAS was to monitor digital voltage-identification lines. You can use these lines as general-purpose input lines. They can sense high- or low-level status signals from digital sensors or from alarm channels. Figure 1 shows a thermostatic sensor. You control and read the DAS using a simple two-wire SMBus or I2C interface to a µC or µP. If a dedicated I2C controller is unavailable, 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 needs no additional communication lines, because multiple devices may reside on the same bus.
