A good example of a ratiometric transducer is a potentiom-

eter used as a position sensor. The position of the wiper is

directly proportional to the output voltage which is a ratio of

the full-scale voltage across it. Since the data is represented

as a proportion of full-scale, reference requirements are

greatly reduced, eliminating a large source of error and cost

for many applications. A major advantage of the ADC0816,

ADC0817 is that the input voltage range is equal to the

supply range so the transducers can be connected directly

across the supply and their outputs connected directly into

the multiplexer inputs, (

Figure 9).

Ratiometric transducers such as potentiometers, strain

gauges, thermistor bridges, pressure transducers, etc., are

suitable for measuring proportional relationships; however,

many types of measurements must be referred to an abso-

lute standard such as voltage or current. This means a

system reference must be used which relates the full-scale

voltage to the standard volt. For example, if V

5.12V, then the full-scale range is divided into 256 standard

steps. The smallest standard step is 1 LSB which is then 20

mV.

2.0 RESISTOR LADDER LIMITATIONS

The voltages from the resistor ladder are compared to the

selected input 8 times in a conversion. These voltages are

coupled to the comparator via an analog switch tree which is

referenced to the supply. The voltages at the top, center and

bottom of the ladder must be controlled to maintain proper

operation.

The top of the ladder, Ref(+), should not be more positive

than the supply, and the bottom of the ladder, Ref(−), should

not be more negative than ground. The center of the ladder

voltage must also be near the center of the supply because

the analog switch tree changes from N-channel switches to

P-channel switches. These limitations are automaticaly sat-

isfied in ratiometric systems and can be easily met in ground

referenced systems.

Figure 10 shows a ground referenced system with a sepa-

rate supply and reference. In this system, the supply must be

trimmed to match the reference voltage. For instance, if a

5.12V reference is used, the supply should be adjusted to

the same voltage within 0.1V.

The ADC0816 needs less than a milliamp of supply current

so developing the supply from the reference is readily ac-

complished. In

Figure 11 a ground references system is

shown which generates the supply from the reference. The

buffer shown can be an op amp of sufficient drive to supply

the millliamp of supply current and the desired bus drive, or

if a capacitive bus is driven by the outputs a large capacitor

will supply the transient supply current as seen in

Figure 12.

The LM301 is overcompensated to insure stability when

loaded by the 10 µF output capacitor.

The top and bottom ladder voltages cannot exceed V

ground, respectively, but they can be symmetrically less than

V

voltage should always be near the center of the supply. The

sensitivity of the converter can be increased, (i.e., size of the

LSB steps decreased) by using a symmetrical reference

system. In

Figure 13, a 2.5V reference is symmetrically

centered about V

tical resistors. This system with a 2.5V reference allows the

LSB to be half the size of the LSB in a 5V reference system.

DS005277-11

FIGURE 9. Ratiometric Conversion System

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