ADC12L034 datasheet(28/35 Pages) NSC

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Application Information (Continued)

3.0 REFERENCE VOLTAGE

The difference in the voltages applied to the V

REF

+ and

REF

− defines the analog input span (the difference between

the voltage applied between two multiplexer inputs or the

voltage applied to one of the multiplexer inputs and analog

ground), over which 4095 positive and 4096 negative codes

exist. The voltage sources driving V

REF

+ or V

REF

− must have

very low output impedance and noise.

The ADC12L030/2/4/8 can be used in either ratiometric or

absolute reference applications. In ratiometric systems, the

analog input voltage is proportional to the voltage used for

the ADC’s reference voltage. When this voltage is the sys-

tem power supply, the V

REF

+ pin is connected to V

+ and

REF

− is connected to ground. This technique relaxes the

system reference stability requirements because the analog

input voltage and the ADC reference voltage move together.

This maintains the same output code for given input condi-

tions. For absolute accuracy, where the analog input voltage

varies between very specific voltage limits, a time and tem-

perature stable voltage source can be connected to the

reference inputs. Typically, the reference voltage’s magni-

tude will require an initial adjustment to null reference volt-

age induced full-scale errors.

Below are recommended references along with some key

specifications.

Part Number

Output

Voltage

Tolerance

Temperature

Coefficient

(max)

LM4041CIM3-Adj

±0.5%

±100ppm/˚C

LM4040AIM3-2.5

±0.1%

±100ppm/˚C

LM4050AIM3-2.5

±0.1%

±50ppm/˚C

01183047

FIGURE 13. Pseudo-Differential Biasing with the Signal Source AC Coupled Directly into the ADC

01183050

FIGURE 14. Fully Differential Biasing

ADC12L034 Datasheet(PDF) 28 Page - National Semiconductor (TI)

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