AD5755-1_11 AD [Analog Devices], AD5755-1_11 Datasheet - Page 41

• Manufacturer Part Number: AD5755-1_11
• Description: Quad Channel, 16-Bit,Serial Input, 4 mA to 20 mA and Voltage Output DAC,Dynamic Power Control
• Manufacturer: AD [Analog Devices]

HART
The AD5755-1 has four CHART pins, one corresponding to
each output channels. A HART signal can be coupled into these
pins. The HART signal appears on the corresponding current
output, if the output is enabled. Table 31 shows the recommended
input voltages for the HART signal at the CHART pin. If these
voltages are used, the current output should meet the HART
amplitude specifications. Figure 79 shows the recommended
circuit for attenuating and coupling in the HART signal.
Table 31. CHART Input Voltage to HART Output Current
R
Internal R
External R
A minimum capacitance of C1 + C2 is required to ensure that
the 1.2 kHz and 2.2 kHz HART frequencies are not significantly
attenuated at the output. The recommended values are C1 =
22 nF, C2 = 47 nF.
Digitally controlling the slew rate of the output is necessary to
meet the analog rate of change requirements for HART.
DIGITAL SLEW RATE CONTROL
The slew rate control feature of the AD5755-1 allows the user to
control the rate at which the output value changes. This feature
is available on both the current and voltage outputs. With the
slew rate control feature disabled, the output value changes at a
rate limited by the output drive circuitry and the attached load.
To reduce the slew rate, this can be achieved by enabling the
slew rate control feature. With the feature enabled via the SREN
bit of the slew rate control register (see Table 26), the output,
instead of slewing directly between two values, steps digitally at
a rate defined by two parameters accessible via the slew rate
control register, as shown in Table 26.
The parameters are SR_CLOCK and SR_STEP. SR_CLOCK
defines the rate at which the digital slew is updated, for
example, if the selected update rate is 8 kHz, the output updates
every 125 μs. In conjunction with this, SR_STEP defines by how
much the output value changes at each update. Together, both
parameters define the rate of change of the output value. Table 32
and Table 33 outline the range of values for both the
SR_CLOCK and SR_STEP parameters.
SET
SET
SET
CHART Input
Voltage
150 mV p-p
170 mV p-p
HART MODEM
OUTPUT
Figure 79. Coupling HART Signal
C1
C2
CHARTx
Current Output
(HART)
1 mA p-p
1 mA p-p
Rev. A | Page 41 of 48
Table 32. Slew Rate Update Clock Options
SR_CLOCK
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
1
Table 33. Slew Rate Step Size Options
SR_STEP
000
001
010
011
100
101
110
111
The following equation describes the slew rate as a function of
the step size, the update clock frequency, and the LSB size:
where:
Slew Time is expressed in seconds.
Output Change is expressed in amps for I
When the slew rate control feature is enabled, all output changes
occur at the programmed slew rate (see the DC-to-DC Converter
Settling Time section for additional information). For example,
if the CLEAR pin is asserted, the output slews to the clear value
at the programmed slew rate (assuming that the clear channel is
enabled to be cleared). If a number of channels are enabled for
slew, care must be taken when asserting the CLEAR pin. If one
of the channels is slewing when CLEAR is asserted, other chan-
nels may change directly to their clear values not under slew
rate control. The update clock frequency for any given value is
the same for all output ranges. The step size, however, varies
across output ranges for a given value of step size because the
LSB size is different for each output range.
These clock frequencies are divided down from the 13 MHz internal
oscillator. See Table 1, Figure 69, and Figure 70.
Slew
Step
Time
Size
×
=
Update
Update Clock Frequency (Hz)
64 k
32 k
16 k
8 k
4 k
2 k
1 k
500
250
125
64
32
16
8
4
0.5
Output
Clock
Step Size (LSBs)
1
2
4
16
32
64
128
256
Change
Frequency
OUT_x
×
LSB
or volts for V
Size
AD5755-1
1
OUT_x
.