ADC12H034 datasheet(33/41 Pages) NSC

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Application Hints (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 circuit in Figure

16 is an example of a very stable reference appropriate for

use with the device.

The ADC 12030/2/4/8 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.

Output

Temperature

Part Number

Voltage

Coefficient

Tolerance

LM4041CI-Adj

±0.5%

±100ppm/˚C

LM4040AI-4.1

±0.1%

±100ppm/˚C

LM4120AI-4.1

±0.2%

±50ppm/˚C

LM4121AI-4.1

±0.2%

±50ppm/˚C

LM4050AI-4.1

±0.1

±50ppm/˚C

LM4030AI-4.1

±0.05%

±10ppm/˚C

LM4031AI

±2.2

±26ppm/˚C

LM4031AC

±0.4

±46ppm/˚C

LM4140AC-4.1

±0.1%

±3.0ppm/˚C

Circuit of Figure 16

Adjustable

±2ppm/˚C

The reference voltage inputs are not fully differential. The

ADC12030/2/4/8 will not generate correct conversions or

comparisons if V

REF

+ is taken below V

REF

−. Correct conver-

sions result when V

REF

+ and V

REF

− differ by 1V and remain,

at all times, between ground and V

+. The V

REF common

mode range, (V

REF

+ +V

REF

−)/2 is restricted to (0.1 x V

+)to

(0.6 x V

+). Therefore, with V

+ = 5V the center of the

reference ladder should not go below 0.5V or above 3.0V.

Figure 17 is a graphic representation of the voltage restric-

tions on V

REF

+ and V

REF

−.

01135450

FIGURE 15. Fully Differential Biasing

01135442

*Tantalum

FIGURE 16. Low Drift Extremely

Stable Reference Circuit

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