ADC12H034 datasheet(28/41 Pages) NSC

28 / 41 page

Application Hints (Continued)

1.2 Changing Configuration

The configuration of the ADC12030/2/4/8 on power up de-

faults to 12-bit plus sign resolution, 12- or 13-bit MSB First,

10 CCLK acquisition time, user mode, no Auto Cal, no Auto

Zero, and power up mode. Changing the aquisition time and

turning the sign bit on and off requires an 8-bit instruction to

be issued to the ADC. This instruction will not start a con-

version. The instructions that select a multiplexer address

and format the output data do start a conversion. Figure 8

describes an example of changing the configuration of the

ADC12030/2/4/8.

During I/O sequence 1, the instruction on DI configures the

ADC12030/2/4/8 to do a conversion with 12-bit +sign reso-

lution. Notice that when the 6 CCLK Acquisition and Data

Out without Sign instructions are issued to the ADC, I/O

sequences 2 and 3, a new conversion is not started. The

data output during these instructions is from conversion N

which was started during I/O sequence 1. The Configuration

Modification timing diagram describes in detail the sequence

of events necessary for a Data Out without Sign, Data Out

with Sign, or 6/10/18/34 CCLK Acquisition time mode selec-

tion. Table 5 describes the actual data necessary to be input

to the ADC to accomplish this configuration modification. The

next instruction, shown in Figure 8, issued to the A/D starts

conversion N+1 with 8 bits of resolution formatted MSB first.

Again the data output during this I/O cycle is the data from

conversion N.

The number of SCLKs applied to the A/D during any conver-

sion I/O sequence should vary in accord with the data out

word format chosen during the previous conversion I/O se-

quence. The various formats and resolutions available are

shown in Table 1.In Figure 8, since 8-bit without sign MSB

first format was chosen during I/O sequence 4, the number

of SCLKs required during I/O sequence 5 is 8. In the follow-

ing I/O sequence the format changes to 12-bit without sign

MSB first; therefore the number of SCLKs required during

I/O sequence 6 changes accordingly to 12.

1.3 CS Low Continuously Considerations

When CS is continuously low, it is important to transmit the

exact number of SCLK pulses that the ADC expects. Not

doing so will desynchronize the serial communications to the

ADC. When the supply power is first applied to the ADC, it

will expect to see 13 SCLK pulses for each I/O transmission.

The number of SCLK pulses that the ADC expects to see is

the same as the digital output word length. The digital output

word length is controlled by the Data Out (DO) format. The

DO format maybe changed any time a conversion is started

or when the sign bit is turned on or off. The table below

details out the number of clock periods required for different

DO formats:

Number of

DO Format

SCLKs

Expected

8-Bit MSB or LSB First

SIGN OFF

SIGN ON

12-Bit MSB or LSB First

SIGN OFF

SIGN ON

16-Bit MSB or LSB first

SIGN OFF

SIGN ON

If erroneous SCLK pulses desynchronize the communica-

tions, the simplest way to recover is by cycling the power

supply to the device. Not being able to easily resynchronize

the device is a shortcoming of leaving CS low continuously.

The number of clock pulses required for an I/O exchange

may be different for the case when CS is left low continu-

ously vs the case when CS is cycled. Take the I/O sequence

detailed in Figure 7 (Typical Power Supply Sequence) as an

example. The table below lists the number of SCLK pulses

required for each instruction:

Instruction

CS Low

CS Strobed

Continuously

Auto Cal

13 SCLKs

8 SCLKs

Read Status

13 SCLKs

8 SCLKs

Read Status

13 SCLKs

8 SCLKs

12-Bit + Sign Conv 1

13 SCLKs

8 SCLKs

12-Bit + Sign Conv 2

13 SCLKs

1.4 Analog Input Channel Selection

The data input on DI also selects the channel configuration

for a particular A/D conversion (see Tables 2, 3, 4 and Table

5). In Figure 8 the only times when the channel configuration

could be modified would be during I/O sequences 1, 4, 5 and

6. Input channels are reselected before the start of each new

conversion. Shown below is the data bit stream required on

DI, during I/O sequence number 4 in Figure 8, to set CH1 as

the positive input and CH0 as the negative input for the

different versions of ADCs:

Part

DI Data

Number

DI0 DI1 DI2 DI3 DI4 DI5 DI6 DI7

ADC12H030

LHL

ADC12030

ADC12H032

LHL

ADC12032

ADC12H034

LLL

ADC12034

ADC12H038

LLLL

ADC12038

Where X can be a logic high (H) or low (L).

1.5 Power Up/Down

The ADC may be powered down at any time by taking the

PD pin HIGH or by the instruction input on DI (see Tables 5,

6, and the Power Up/Down timing diagrams). When the ADC

is powered down in this way, the circuitry necessary for an

A/D conversion is deactivated. The circuitry necessary for

digital I/O is kept active. Hardware power up/down is con-

trolled by the state of the PD pin. Software power-up/down is

controlled by the instruction issued to the ADC. If a software

power up instruction is issued to the ADC while a hardware

power down is in effect (PD pin high) the device will remain

in the power-down state. If a software power down instruc-

tion is issued to the ADC while a hardware power up is in

effect (PD pin low), the device will power down. When the

device is powered down by software, it may be powered up

by either issuing a software power up instruction or by taking

PD pin high and then low. If the power down command is

issued during an A/D conversion, that conversion is dis-

rupted. Therefore, the data output after power up cannot be

relied upon.

ADC12H034 Datasheet(PDF) 28 Page - National Semiconductor (TI)

ADC12H034 Datasheet(HTML) 28 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Datasheet Download

Link URL