Data Sheet

AD5700/AD5700-1

Rev. A | Page 15 of 20

The internal filter configuration is shown in Figure 22. 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 22. 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 23. 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 23. Crystal Oscillator Connection

The ABLS-3.6864MHZ-L4Q-T crystal oscillator data sheet

recommended two 18 pF capacitors. Because the crystal current

consumption is dominated by the load capacitance, in an effort

to reduce the crystal current consumption, two 8 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 8 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 24).

Figure 24. CMOS Clock Connection

Internal Oscillator (AD5700-1 only)

Consuming typically 218 μA, the low power, internal, 0.5 %

precision RC oscillator, available only on the AD5700-1, 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 25).

Figure 25. 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.

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.

calculated using the following equation:

I = C × V × f

HART

NETWORK

680pF

2.2nF

HART_OUT

HART_IN

ADC_IP

AD5700/

AD5700-1

ABLS-3-6864MHZ-L4Q-T

18pF

18pF

AD5700/AD5700-1

AD5700/AD5700-1

AD5700-1