8 - ADC Module

Summary

The Analog Devices AD573 10-bit, 20μsec analog-to-digital converter was selected for the data acquisition systems in this book. In the PDC plan, two of the IC's comprise the ADC module which interfaces to whatever application circuits need voltage measurements. Reasons for picking this particular device are in Chapter 4.

Obviously, more than two ADC's are necessary for conceivable applications. And cases may arise where faster and/or more accurate measurements are required. On the other hand, slower, less accurate and less expensive converters might be desirable when many voltages are measured. Because the devices are in a separate module, additional AD573's or some number of different ADC's can be substituted with minimal changes in the rest of the system.

ADC's such as the ADC0808 (Exercise 6 in Chapter 4) measure voltage from up to eight sources. Select controls determine which input is converted. However, the plan here is for two separate IC's to measure two voltages. This ensures simultaneous conversion of both channels, simplifies control requirements, and minimizes the few millivolts of cross-talk common in multiplexed chips. (Cross-talk means the voltage in one channel influences converted values in another.)

In addition to hosting conversion devices, the ADC module receives power supply voltages and sends +5 and ground to the rest of the PDC (as shown in Figure 7.1). The scheme allows the -12 volts required for AD573's to go only to the module where it's needed.

Circuit

The AD573 is a 20-pin IC. Ten connections are Data outputs (D0…D9). Three are control inputs: CV (ConVert), HE* (High byte Enable) and LE* (Low byte Enable).