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Revision 1.03

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AS1538/AS1540

Data Sheet - A p p l i c a t i o n I n f o r m a t i o n

Reading with Internal Reference On/Off

The internal reference defaults to off when the AS1538/AS1540 power is on. If the reference (internal or external) is

continuously 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 reference capacitor. For example for a

reference capacitor of 4.7µF and considering the output impedance of the internal reference of 30

Ω and the amount of

time to fully charge the capacitor will be 1.4ms. If the reference capacitor is not fully discharged this time can be

reduced greatly.

Figure 29 shows the correct internal reference enable sequence before issuing the typical read sequences required for

the mode when an internal reference is used.

Note: Typical read sequences can be re-used once the internal reference has settled.

Figure 29. Internal Reference Enable Sequence and Typical Read Sequence

Where:

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)

X: Dont Care

S

1

0

0

1

0

A1

A0

W

X

X

X

X

1

X

X

X

A

A

Write-Addressing Byte

Command Byte

Internal-Reference Enable Sequence

P

Wait until required settling time reached

From Master to Slave

From Slave to Master

Internal-Reference

Enable Settling Time

Sr

1

0

0

1

0

A1

A0

W

SD

C2

C1

C0

1

PD0

X

X

A

A

Write-Addressing Byte

Command Byte

ADC Powerdown Mode

ADC Sampling Mode

0

0

0

0

D11 D10 D9

D8

D7

A

D0

N

P

D6 ... D1

Sr

1

0

0

1

0

A1

A0

R

A

ADC Conversion Mode

ADC Powerdown Mode *

Read-Addressing Byte

2 x (8-bits +ACK/NACK

Settled Internal Reference

Typical Mode Read Sequence

**

* Dependant on powerdown selection

bits PD0 and PD1.

** To remain in HS mode, use repeated

STARTs instead of STOPs

Settled Internal Reference