If reference voltages are desired that are more than a few

tens of millivolts from the self-bias values, the circuit of

Fig-

ure 5 will allow forcing the reference voltages to whatever

levels are desired. This circuit provides the best performance

because of the low source impedance of the transistors.

Note that the V

V

supply voltage, and V

and 1.0V below V

accurate conversions, the total reference voltage range (V

RT

-V

about V

and V

3.0 Power Supply Considerations

Many A/D converters draw sufficient transient current to cor-

rupt their own power supplies if not adequately bypassed. A

10µF tantalum or aluminum electrolytic capacitor should be

placed within an of inch (2.5 centimeters) of the A/D power

pins, with a 0.1 µF ceramic chip capacitor placed as close as

possible to the converter’s power supply pins. Leadless chip

capacitors are preferred because they have low lead induc-

tance.

While a single voltage source should be used for the analog

and digital supplies of the ADC1173, these supply pins

should be well isolated from each other to prevent any digital

noise from being coupled to the analog power pins. A 47

Ohm resistor is recommend between the analog and digital

supply lines, with a ceramic capacitor close to the analog

supply pin. Avoid inductive components in the analog supply

line.

The converter digital supply should not be the supply that is

used for other digital circuitry on the board. It should be the

same supply used for the A/D analog supply.

As is the case with all high speed converters, the ADC1173

should be assumed to have little power supply rejection, es-

pecially when self-biasing is used by connecting V

V

No pin should ever have a voltage on it that is in excess of

the supply voltages or below ground, not even on a trasient

DS100890-15

FIGURE 5. Driving the reference to force desired values requires driving with a low impedance source, provided by

the transistors. Note that pins 16 and 22 are not connected.

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