Sr

A1

1

0

0

1

0

A0

R

A

0

0

0

0

D11 D10 D9

D8

A

D7

D6...D1

D0

N

P

S

A1

1

0

0

1

1

0

A0

W

A

SD

C2

C1

C0

PD0

X

X

A

S

A1

1

0

0

1

1

0

A0

W

A

X

X

X

X

X

X

X

A

P

Wait until the required

settling time is reached

Internal Reference Turn-On Sequence

Settled Internal Reference

Settled Internal Reference

Internal Reference

Turn-On

Settling Time

Typical Read

Sequence

in F/S Mode

(1)

From Master to Slave

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)

From Slave to Master

Command Byte

Write-Addressing Byte

Command Byte

Write-Addressing Byte

ADC Power-Down Mode

ADC Sampling Mode

ADC Converting Mode

ADC Power-Down Mode

(depending on power-down selection bits)

Read-Addressing Byte

2 x (8 bits + ack/nack)

See

Note (2)

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.

ADS7828-Q1

www.ti.com

SBAS456B – DECEMBER 2008 – REVISED JANUARY 2016

Figure 20. Internal Reference Turn-On Sequence and Typical Read Sequence (F/S Mode Shown)

When using an internal reference, there are three things that must be done:

1. To use the internal reference, the PD1 bit of Command Byte must always be set to logic 1 for each sample

conversion that is issued by the sequence, as shown in Figure 18.

2. 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 0 while using the ADS7828, then the settling time must be reconsidered after PD1 is

set to logic 1. 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 accuracy conversion.

3. When the internal reference is off, it is not turned on until both the first Command Byte with PD1 = 1 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 = 1 issued after the internal

reference is turned on serves only to keep the internal reference on. Otherwise, the internal reference would

be turned off by any Command Byte with PD1 = 0.

The example in Figure 20 can be generalized for an HS mode conversion cycle by changing the timing of the

conversion cycle. If using an external reference, PD1 must be set to 0, and the external reference must be

settled. The typical sequence in Figure 18 or Figure 19 can then be used.

Copyright © 2008–2016, Texas Instruments Incorporated

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