Rainbow-electronics ADC12041 Bedienungsanleitung PDF herunterladen (Seite 25)

Analog Application Information

REFERENCE VOLTAGE

The ADC12041 has two reference inputs, V

REF

and

REF

. They define the zero to full-scale range of the ana-

log input signals over which 4095 positive and 4096 nega-

tive codes exist. The reference inputs can be connected to

span the entire supply voltage range (V

REF

AGND,

REF

) or they can be connected to different volt-

ages when other input spans are required. The reference

inputs of the ADC12041 have transient capacitive switching

currents. The voltage sources driving V

REF

and V

REF

must have very low output impedence and noise and must

be adequately bypassed. The circuit in

Figure 8

is an exam-

ple of a very stable reference source.

The ADC12041 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. This technique relaxes the system

reference requirements because the analog input voltage

moves with the ADC’s reference. The system power supply

can be used as the reference voltage by connecting the

REF

pin to V

and the V

REF

pin to AGND. For abso-

lute accuracy, where the analog input voltage varies be-

tween very specific voltage limits, a time and temperature

stable voltage source can be connected to the reference

inputs. Typically, the reference voltage’s magnitude will re-

quire an initial adjustment to null reference voltage induced

full-scale errors.

The reference voltage inputs are not fully differential. The

ADC12041 will not generate correct conversions if V

REF

–

REF

is below 1V.

Figure 9

shows the allowable relation-

ship between V

REF

and V

REF

TL/H/12441–43

FIGURE 9. V

REF

Operating Range

OUTPUT DIGITAL CODE VERSUS ANALOG INPUT

VOLTAGE

The ADC12041’s fully differential 12-bit

sign ADC gener-

ates a two’s complement output that is found by using the

equation shown below:

Output code

) (4096)

REF

)

Round off the result to the nearest integer value between -

*Tantalum

**Ceramic

TL/H/12441–44

FIGURE 8. Low Drift Extremely Stable Reference Circuit

Part Number

Output Voltage Temperature

Tolerance Coefficient

LM4041CI-Adj

0.5%

100ppm/

LM4040AI-4.1

0.1%

100ppm/

LM9140BYZ-4.1

0.5%

25ppm/

LM368Y-5.0

0.1%

20ppm/

Circuit of

Figure 8

Adjustable

2ppm/

1 2 ... 20 21 22 23 24 25 26 27 28 29 30

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