THE MAGAZINE FOR COMPUTER APPLICATIONS
Circuit Cellar Online offers articles illustrating creative solutions
and unique applications through complete projects, practical
tutorials, and useful design techniques.

ANALOG SYSTEM DESIGN

by George Martin

Start ý Considerations ý Transducer Interfaces ý Thermocouples ý ADCs ý Almost Thereý ý Sources and PDF

CONSIDERATIONS

There are two areas you must consider before you get into transducer selectionýoperating voltages and power supply.

Operating voltages are the voltages available for the circuitry. In the ý70s and ý80s, typical voltages were 15 and ý15 V for the analog bias voltage and 5 V for the logic. The op-amps were powered with the ý15 V, and the signal range was typically 10 to ý10 V. And, the ADC had ý10 V as negative full scale and 10 V as positive full scale. The ADC was usually a separate IC and ranged from 10 to 12 bits.

As devices got better, analog systems had RS-232 devices for I/O. The ý15-V supply became a ý12-V supply, and the signals were still scaled at ý10 V. Amplifiers had better performance specification. They could linearly handle signals going closer to the positive and negative rails. The ý12 V were used for the RS-232 drivers as well as the analog bias, with proper decoupling! Recently, the ADCs have moved onto the CPU and become more accurate and flexible as to the voltage inputs. Also, there are the RS-232 drivers that make their own ý12 V.

There is another approach to supply voltage selection, single-sided supply. A common design you will see in application notes is the 0- to 5-V design. The amplifiers are biased with 0 and 5 V. This is good because it reduces the number of supply voltages required in the system. However, we all know thereýs no such thing as a free lunch. The amplifiers cannot fully swing to 0 and 5 V. They get close but with little drive capability. And as the drive requirements increase (sinking or sourcing current), their ability to drive to the rails is decreased. So, one solution in a 5-V system would be to scale the analog voltage from 0.5 to 4.5 V. That would be a 4-V swing and the amplifiers could sink and source at those voltages. If cost is a factor (and it usually is), this might be a wise approach.

Another consideration is power supply. My specific design is a vehicle with a 12-V battery. If you look at DC-to-DC conversion, youýll find that there are buck, boost, and buck-boost converters. Buck converters produce a smaller output voltage than the input voltage, boost converters produce a larger output voltage, and buck-boost converters can produce an output voltage that is either smaller or larger than the input. Both buck and boost converters can be over 90% efficient, but buck-boost converters do not reach the same level of efficiency.

How important is efficiency in your design? In a car, itýs probably not too important. In a battery-operated device, it becomes very important. Also consider the office product that has a wall-mounted transformer. Because the wall power 115 VAC can range from 90 to 130 VAC, the input to the power supply can vary in direct proportion. Then, an inefficient power supply that will run with a 90-VAC input will produce a lot of heat at the 130-VAC condition. And remember, heat kills!

There is no one correct answer to the best bias voltages. You need to consider all the voltages that are required for your system along with the power supply requirements. Some of these voltage requirements come from the ICs you are going to use and others come from transducer interfacing, which Iýll look at next.

PREVIOUS • NEXT