ADS7828E Datasheet, PDF(14/22 Page) Texas Instruments – 12-Bit, 8-Channel Sampling ANALOG-TO-DIGITAL CONVERTER with I2C Interface

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ADS7828E Datasheet, PDF (14/22 Pages) Texas Instruments – 12-Bit, 8-Channel Sampling ANALOG-TO-DIGITAL CONVERTER with I2C Interface

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READING WITH REFERENCE ON/OFF

The internal reference defaults to off when the ADS7828

power is on. To turn the internal reference on or off, see

Table I. If the reference (internal or external) is constantly

turned on and off, a proper amount of settling time must be

added before a normal conversion cycle can be started. The

exact amount of settling time needed varies depending on

the configuration.

See Figure 5 for an example of the proper internal reference

turn-on sequence before issuing the typical read sequences

required for the F/S mode when an internal reference is

used.

When using an internal reference, there are three things that

must be done:

1) In order to use the internal reference, the PD1 bit of

Command Byte must always be set to logic â1â for each

sample conversion that is issued by the sequence, as

shown in Figure 3.

2) In order to achieve 12-bit accuracy conversion when

using the internal reference, the internal reference

settling time must be considered, as shown in the

Internal VREF vs Turn-On Time Typical Characteristic plot.

If the PD1 bit has been set to logic â0â while using the

ADS7828, then the settling time must be reconsidered

after PD1 is set to logic â1â. In other words, whenever the

internal reference is turned on after it has been turned off,

the settling time must be long enough to get 12-bit accu-

racy conversion.

3) When the internal reference is off, it is not turned on until

both the first Command Byte with PD1 = â1â is sent and

then a STOP condition or repeated START condition is

issued. (The actual turn-on time occurs once the STOP or

repeated START condition is issued.) Any Command Byte

with PD1 = â1â issued after the internal reference is turned

on serves only to keep the internal reference on. Other-

wise, the internal reference would be turned off by any

Command Byte with PD1 = â0â.

The example in Figure 5 can be generalized for a HS mode

conversion cycle by simply swapping the timing of the con-

version cycle.

If using an external reference, PD1 must be set to â0â, and the

external reference must be settled. The typical sequence in

Figure 3 or Figure 4 can then be used.

Internal Reference Turn-On Sequence

Internal Reference

Turn-On

Settling Time

0 A1 A0 W A

Wait until the required

settling time is reached

Write-Addressing Byte

Command Byte

Settled Internal Reference

ADC Power-Down Mode

ADC Sampling Mode

Typical Read

Sequence(1)

in F/S Mode

S 1 0 0 1 0 A1 A0 W A SD C2 C1 C0 1 PD0 X X A

Write-Addressing Byte

Command Byte

Settled Internal Reference

ADC Converting Mode

ADC Power-Down Mode

(depending on power-down selection bits)

Sr 1 0 0 1 0 A1 A0 R A 0 0 0 0 D11 D10 D9 D8 A D7 D6 . . .D1 D0 N P

Read-Addressing Byte

2 x (8 Bits + ack/not-ack)

see

note

(2)

From Master to Slave

From Slave to Master

A = acknowledge (SDA LOW)

N = not acknowledge (SDA HIGH)

S = START Condition

P = STOP Condition

Sr = repeated START condition

W = '0' (WRITE)

R = '1' (READ)

NOTES: (1) Typical read sequences can be reused after the internal reference is settled.

(2) To secure bus operation and loop back to the stage of write-addressing for next conversion, use repeated START.

FIGURE 5. Internal Reference Turn-On Sequence and Typical Read Sequence (F/S mode shown).

ADS7828

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