ADS7828-Q1
SBAS456A –DECEMBER 2008–REVISED OCTOBER 2009.......................................................................................................................................... www.ti.com
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 2. 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 9 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.
Internal Reference
Turn-On
Settling Time
Internal ReferenceTurn-On Sequence
Wait until the required
settling time is reached
S
1
0
0
1
0
A1 A0
A
X
X
X
X
1
X
X
X
A
P
W
Write-Addressing Byte
Command Byte
Typical Read
Sequence(1)
in F/S Mode
Settled Internal Reference
ADC Power-Down Mode
ADC Sampling Mode
S
1
0
0
1
0
A1 A0
A
SD C2 C1 C0
1
PD0
X
X
A
W
Write-Addressing Byte
ADC Converting Mode
Command Byte
Settled Internal Reference
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
See
Note (2)
Read-Addressing Byte
2 x (8 bits + ack/nack)
A = Acknowledge (SDA low)
W = 0 (write)
R = 1 (read)
From Master to Slave
From Slave to Master
N = Not acknowledge (SDA high)
S = Start condition
P = Stop condition
Sr = Repeated Start condition
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 9. 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 7.
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.
18
Submit Documentation Feedback
Copyright © 2008–2009, Texas Instruments Incorporated
Product Folder Link(s): ADS7828-Q1
TI [ TEXAS INSTRUMENTS ]
