AD9838 datasheet(21/32 Pages) AD

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AD9838

Rev. A | Page 21 of 32

IOUT AND IOUTB PINS

SIGN BIT OUT PIN

The AD9838 offers a variety of outputs from the chip. The

digital outputs are available from the SIGN BIT OUT pin. The

available outputs are the comparator output or the MSB of the

DAC data. The bits controlling the SIGN BIT OUT pin are

listed in Table 18.

The analog outputs from the AD9838 are available from the

IOUT and IOUTB pins. The available outputs are a sinusoidal

output or a triangular output (see Table 19).

Note that the SLEEP pin and the SLEEP12 bit must be set to 0

(the DAC is enabled) when using the IOUT and IOUTB pins.

Table 18. Outputs from the SIGN BIT OUT Pin

OPBITEN

Bit

MODE

Bit

SIGN/PIB

Bit

DIV2

Bit

SIGN BIT OUT Pin

High impedance

DAC data MSB/2

DAC data MSB

Reserved

Comparator output

Reserved

Table 19. Outputs from the IOUT and IOUTB Pins

OPBITEN Bit

MODE Bit

IOUT and IOUTB Pin Output

Sinusoid

Triangle

Sinusoid

Reserved

Sinusoidal Output

The SIN ROM converts the phase information from the

frequency and phase registers into amplitude information,

resulting in a sinusoidal signal at the output. To obtain a

sinusoidal output from the IOUT and IOUTB pins, set the

MODE bit (Bit D1) to 0.

The SIGN BIT OUT pin must be enabled before use. The

OPBITEN bit (Bit D5) in the control register enables and dis-

ables this pin. When OPBITEN = 1, the SIGN BIT OUT pin is

enabled. Note that if OPBITEN = 1, the MODE bit (Bit D1) in

the control register should be set to 0.

Triangle Output

Comparator Output

The SIN ROM can be bypassed so that the truncated digital output

from the NCO is sent to the DAC. In this case, the output is no

longer sinusoidal. The DAC produces a 10-bit linear triangular

function (see Figure 23). To obtain a triangle output from the

IOUT and IOUTB pins, set the MODE bit (Bit D1) to 1 and the

OPBITEN bit (Bit D5) to 0.

The AD9838 has an on-board comparator. To connect this com-

parator to the SIGN BIT OUT pin, the OPBITEN bit (Bit D5)

and the SIGN/PIB bit (Bit D4) must be set to 1. After filtering

the sinusoidal output from the DAC, the waveform can be

applied to the comparator to generate a square waveform.

MSB of the DAC Data

3π/2

7π/2

11π/2

VOUT MAX

VOUT MIN

The MSB of the DAC data can be output from the AD9838.

By setting the OPBITEN bit (Bit D5) to 1 and the SIGN/PIB bit

(Bit D4) to 0, the MSB of the DAC data is available at the SIGN

BIT OUT pin. This output is useful as a coarse clock source.

The square wave can also be divided by 2 before being output.

The DIV2 bit (Bit D3) in the control register controls the

frequency of this output from the SIGN BIT OUT pin.

Figure 23. Triangle Output

POWERING UP THE AD9838

The flowchart in Figure 24 shows the operating routine for the

AD9838. When the AD9838 is powered up, the part should be

reset. This resets the appropriate internal registers to 0 to provide

an analog output of midscale. To avoid spurious DAC outputs

during AD9838 initialization, the RESET pin or the RESET bit

should be set to 1 until the part is ready to begin generating

an output.

A reset does not reset the phase, frequency, or control registers.

These registers will contain invalid data and, therefore, should

be set to known values by the user. The RESET pin or bit should

then be set to 0 to begin generating an output. The data appears

on the DAC output eight or nine MCLK cycles after the RESET

pin or bit is set to 0.

AD9838 Datasheet(PDF) 21 Page - Analog Devices

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