AD9838 Datasheet, PDF (21/32 Pages) Analog Devices – 11 mW Power, 2.3 V to 5.5 V, Complete DDS Power-down option

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.
Table 18. Outputs from the SIGN BIT OUT Pin
OPBITEN MODE SIGN/PIB DIV2
Bit
Bit
Bit
Bit
SIGN BIT OUT Pin
0
X
X
X
High impedance
1
0
0
0
DAC data MSB/2
1
0
0
1
DAC data MSB
1
0
1
0
Reserved
1
0
1
1
Comparator output
1
1
X
X
Reserved
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.
Comparator Output
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
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.
AD9838
IOUT AND IOUTB PINS
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 19. Outputs from the IOUT and IOUTB Pins
OPBITEN Bit MODE Bit
IOUT and IOUTB Pin Output
0
0
Sinusoid
0
1
Triangle
1
0
Sinusoid
1
1
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.
Triangle 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.
V_{OUT MAX}
V_{OUT MIN}
3π/2
7π/2
Figure 23. Triangle Output
11π/2
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.
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