ADC 2 Manual do Utilizador descarregar pdf (Página 14)

REV. A–14–

AD7866

Digital Inputs

The digital inputs applied to the AD7866 are not limited by the

maximum ratings that limit the analog inputs. Instead, the digital

inputs applied can go to 7 V and are not restricted by the V

0.3 V limit as on the analog inputs. See maximum ratings.

Another advantage of SCLK, RANGE, REF SELECT, A0, and

CS not being restricted by the V

+ 0.3 V limit is that power

supply sequencing issues are avoided. If one of these digital inputs

is applied before V

, there is no risk of latch-up, as there

would be on the analog inputs if a signal greater than 0.3 V were

applied prior to V

DRIVE

The AD7866 also has the V

DRIVE

feature, which controls the

voltage at which the serial interface operates. V

DRIVE

allows the

ADC to easily interface to both 3 V and 5 V processors. For

example, if the AD7866 was operated with a V

of 5 V, the

DRIVE

pin could be powered from a 3 V supply, allowing a large

dynamic range with low voltage digital processors. For example,

the AD7866 could be used with the 2  V

REF

input range, with a

of 5 V while still being able to interface to 3 V digital parts.

REFERENCE CONFIGURATION OPTIONS

The AD7866 has various reference configuration options. The

REF SELECT pin allows the choice of using an internal 2.5 V

reference or applying an external reference, or even an individual

external reference for each on-chip ADC if desired. If the REF

SELECT pin is tied to AGND, then the on-chip 2.5 V reference

is used as the reference source for both ADC A and ADC B. In

addition, pins V

REF

, D

CAP

A, and D

CAP

B must be tied to decoupling

capacitors (100 nF, 470 nF, and 470 nF recommended,

respectively). If the REF SELECT pin is tied to a logic high, an

external reference can be supplied to the AD7866 through the

REF

pin to overdrive the on-chip reference, in which case

decoupling capacitors are required on D

CAP

A and D

CAP

B again.

However, if the V

REF

pin is tied to AGND while REF SELECT

is tied to a logic low, an individual external reference can be

applied to both ADC A and ADC B through pins D

CAP

A and

CAP

B, respectively. Table II summarizes these reference options.

For specified performance, the last configuration was used with

the same reference voltage applied to both D

CAP

A and D

CAP

The connections for the relevant reference pins are shown in the

typical connection diagrams. If the internal reference is being

used, the V

REF

pin should have a 100 nF capacitor connected to

AGND very close to the V

REF

pin. These connections are shown

in Figure 12.

CAP

REF

470nF

AD7866

470nF

100nF

Figure 12. Relevant Connections when Using an

Internal Reference

CAP

REF

AD7866

REF

REF SELECT

Figure 13. Relevant Connections when Applying

an External Reference at D

CAP

A and/or D

CAP

REF

470nF

AD7866

470nF

REF

REF SELECT

DRIVE

Figure 14. Relevant Connections when Applying

an External Reference at V

REF

Figure 13 shows the connections required when an external

reference is applied to D

CAP

A and D

CAP

B. In this example, the

same reference voltage is applied at each pin; however, a different

voltage may be applied at each of these pins for each on-chip

ADC. An external reference applied at these pins may have a

range from 2 V to 3 V, but for specified performance it must be

within ± 1% of 2.5 V. Figure 14 shows the third option, which is

to overdrive the internal reference through the V

REF

pin. This is

possible due to the series resistance from the V

REF

pin to the

internal reference. This external reference can have a range from

2 V to 3 V; but again, to get as close as possible to the specified

performance, a 2.5 V reference is desirable. D

CAP

A and D

CAP

decouple each on-chip reference buffer, as shown in Figure 15.

Table II. Reference Selection

Reference Option REF SELECT V

REF

CAP

A and D

CAP

Internal Low Decoupling Capacitor Decoupling Capacitor

Externally through V

REF

High External Reference Decoupling Capacitor

Externally through

CAP

A and/or D

CAP

B Low AGND External Reference A and/or

Reference B

NOTES

Recommended value of decoupling capacitor = 100 nF.

Recommended value of decoupling capacitor = 470 nF.

1 2 ... 9 10 11 12 13 14 15 16 17 18 19 ... 23 24

Comentários a estes Manuais

Sem comentários

ADC 2 Manual do Utilizador Página 14

Comentários a estes Manuais

Manuais e produtos relacionados com Redes ADC 2