Datasheet

Data Sheet AD5700/AD5700-1
Rev. F | Page 15 of 24
The internal filter configuration is shown in Figure 24. This
option is beneficial where cost or board space is a large concern
because it removes the need for multiple external components.
This configuration achieves an 8 kV ESD HBM rating but
requires extra external protection circuitry for EMC and surge
protection purposes if used in harsh industrial environments.
Figure 24. AD5700/AD5700-1 Using Internal Filter on HART_IN
CLOCK CONFIGURATION
The AD5700/AD5700-1 support numerous clocking configura-
tions to allow the optimal trade-off between cost and power:
• External crystal
• CMOS clock input
• Internal RC oscillator (AD5700-1 only)
The CLK_CFG0, CLK_CFG1, and
XTAL_EN
pins configure
the clock generation as shown in Table 7. The AD5700/AD5700-1
can also provide a clock output at CLKOUT (for more details,
see the CLKOUT section).
External Crystal
The typical connection for an external crystal (ABLS-3.6864MHZ-
L4Q-T) is shown in Figure 25. To ensure minimum current
consumption and to minimize stray capacitances, connections
between the crystal, capacitors, and ground should be made as
close to the AD5700/AD5700-1 as possible. Consult individual
crystal vendors for recommended load information and crystal
performance specifications.
Figure 25. Crystal Oscillator Connection
The ABLS-3.6864MHZ-L4Q-T crystal oscillator data sheet
recommended two 36 pF capacitors. Because the crystal current
consumption is dominated by the load capacitance, in an effort
to reduce the crystal current consumption, two 16 pF capacitors
were used on the XTAL1 and XTAL2 pins. The AD5700/AD5700-1
still functioned as expected, even with the resulting reduction in
frequency performance from the crystal due to the smaller
capacitance values. Crystals are available that support 16 pF
capacitors. It is recommended to consult the relevant crystal
manufacturers for this information.
CMOS Clock Input
A CMOS clock input can also be used to generate a clock for the
AD5700/AD5700-1. To use this mode, connect an external
clock source to the XTAL 1 pin, and leave XTAL2 open circuit
(see Figure 26).
Figure 26. CMOS Clock Connection
Internal Oscillator (AD5700-1 only)
Consuming typically 218 µA, the low power, internal, 0.5 %
precision RC oscillator, available only on theAD5700-1, has an
oscillation frequency of 1.2288 MHz. To use this mode, tie the
XTAL1 pin to ground and leave the XTAL2 pin open circuit
(see Figure 27).
Figure 27. Internal Oscillator Connection
CLKOUT
The AD5700/AD5700-1 can provide a clock output at CLKOUT
(see Table 7).
• If using the crystal oscillator, this clock output can be
configured as a 3.6864 MHz, 1.8432 MHz, or 1.2288 MHz
buffer clock.
• If using a CMOS clock, no clock output can be configured
at the CLKOUT pin.
• If using the internal RC oscillator, this clock output is only
available as a 1.2288 MHz buffer clock.
The amplitude of the clock output depends on the IOV
CC
level;
therefore, the clock output can be in the range of 1.71 V p-p to
5.5 V p-p. Enabling the clock output of the
AD5700/AD5700-1
increases the current consumption of the device. This increase
is due to the current required to drive any load at the CLKOUT
pin, which should not be more than 30 pF.
This capacitance should be minimized to reduce current
consumption and provide the clock with the cleanest edges.
The additional current drawn from the IOV
CC
supply can be
calculated using the following equation:
I = C × V × f
10435-021
HART
NETWORK
680pF
2.2nF
HART_OUT
HART_IN
ADC_IP
AD5700/
AD5700-1
10435-022
ABLS-3-6864MHZ-L4Q-T
36pF36pF
XTAL1 XTAL2
AD5700/AD5700-1
10435-027
XTAL1 XTAL2
AD5700/AD5700-1
10435-028
XTAL1 XTAL2
AD5700-1