ADC0804LCWM datasheet(24/48 Pages) TI

24 / 48 page

ADC0801, ADC0802

ADC0803, ADC0804, ADC0805

SNOSBI1B – NOVEMBER 2009 – REVISED FEBRUARY 2013

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In general, the magnitude of the reference voltage will require an initial adjustment. Errors due to an improper

value of reference voltage appear as full-scale errors in the A/D transfer function. IC voltage regulators may be

used for references if the ambient temperature changes are not excessive. The LM336B 2.5V IC reference diode

(from National Semiconductor) has a temperature stability of 1.8 mV typ (6 mV max) over 0°C

≤TA≤+70°C. Other

temperature range parts are also available.

a) Analog Input Signal Example

*Add if VREF/2

≤ 1 VDC with LM358 to draw 3 mA to

ground.

b) Accommodating an Analog Input from 0.5V

(Digital Out = 00HEX) to 3.5 V (Digital Out=FFHEX)

Figure 51. Adapting the A/D Analog Input Voltages to Match an Arbitrary Input Signal Range

Errors and Reference Voltage Adjustments

Zero Error

The zero of the A/D does not require adjustment. If the minimum analog input voltage value, VIN(MIN), is not

ground, a zero offset can be done. The converter can be made to output 0000 0000 digital code for this minimum

input voltage by biasing the A/D VIN(−) input at this VIN(MIN) value (see Applications section). This utilizes the

differential mode operation of the A/D.

The zero error of the A/D converter relates to the location of the first riser of the transfer function and can be

measured by grounding the VIN(−) input and applying a small magnitude positive voltage to the VIN(+) input. Zero

error is the difference between the actual DC input voltage that is necessary to just cause an output digital code

transition from 0000 0000 to 0000 0001 and the ideal 1/2 LSB value (1/2 LSB = 9.8 mV for VREF/2=2.500 VDC).

Full-Scale

The full-scale adjustment can be made by applying a differential input voltage that is 11/2 LSB less than the

desired analog full-scale voltage range and then adjusting the magnitude of the VREF/2 input (pin 9 or the VCC

supply if pin 9 is not used) for a digital output code that is just changing from 1111 1110 to 1111 1111.

Adjusting for an Arbitrary Analog Input Voltage Range

If the analog zero voltage of the A/D is shifted away from ground (for example, to accommodate an analog input

signal that does not go to ground) this new zero reference should be properly adjusted first. A VIN(+) voltage that

equals this desired zero reference plus 1/2 LSB (where the LSB is calculated for the desired analog span, 1

LSB=analog span/256) is applied to pin 6 and the zero reference voltage at pin 7 should then be adjusted to just

obtain the 00HEX to 01HEX code transition.

The full-scale adjustment should then be made (with the proper VIN(−) voltage applied) by forcing a voltage to the

VIN(+) input which is given by:

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