ADC0804LCWM datasheet(24/56 Pages) TI1

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ADC0801, ADC0802, ADC0803, ADC0804, ADC0805

SNOSBI1C – NOVEMBER 2009 – REVISED JUNE 2015

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9.2.1.2.2.2

Input Source Resistance

Large values of source resistance where an input bypass capacitor is not used, will not cause errors as the input

currents settle out prior to the comparison time. If a low pass filter is required in the system, use a low valued

series resistor (

≤ 1 kΩ) for a passive RC section or add an op amp RC active low pass filter. For low source

resistance applications, (

≤ 1 kΩ), a 0.1 μF bypass capacitor at the inputs will prevent noise pickup due to series

lead inductance of a long wire. A 100

Ω series resistor can be used to isolate this capacitor — both the R and C

are placed outside the feedback loop — from the output of an op amp, if used.

9.2.1.2.2.3

Noise

The leads to the analog inputs (pins 6 and 7) should be kept as short as possible to minimize input noise

coupling. Both noise and undesired digital clock coupling to these inputs can cause system errors. The source

resistance for these inputs should, in general, be kept below 5 k

Ω. Larger values of source resistance can cause

undesired system noise pickup. Input bypass capacitors, placed from the analog inputs to ground, will eliminate

system noise pickup but can create analog scale errors as these capacitors will average the transient input

switching currents of the ADC (see Analog Inputs — Input Current). This scale error depends on both a large

source resistance and the use of an input bypass capacitor. This error can be eliminated by doing a full-scale

adjustment of the ADC (adjust VREF/2 for a proper full-scale reading — see Full-Scale) with the source resistance

and input bypass capacitor in place.

Noise spikes on the VCC supply line can cause conversion errors as the comparator will respond to this noise. A

low inductance tantalum filter capacitor should be used close to the converter VCC pin and values of 1 µF or

greater are recommended. If an unregulated voltage is available in the system, a separate LM340LAZ-5.0, TO-

92, 5-V voltage regulator for the converter (and other analog circuitry) will greatly reduce digital noise on the VCC

supply.

9.2.1.2.3

Reference Voltage

9.2.1.2.3.1

Span Adjust

For maximum applications flexibility, these ADCs have been designed to accommodate a 5 VDC, 2.5 VDC or an

adjusted voltage reference. This has been achieved in the design of the IC as shown in Figure 32.

Figure 32. The VREFERENCE Design on the IC

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