–8–
Clock Generation
As distinct from the asynchronous VFCs that rely on the
stability of an external capacitor to set their full-scale frequency,
the AD7740 uses an external clock to define the full-scale output
frequency. The result is a more stable transfer function, which
allows the designer to determine the system stability and drift
based upon the selected external clock.
The AD7740 requires a master clock input, which may be an
external CMOS-compatible clock signal applied to the CLKIN
pin (CLKOUT not used). For a frequency of 1 MHz, a crystal
or resonator can be connected between CLKIN and CLKOUT
so that the clock circuit functions as a crystal controlled oscilla-
tor. Figure 7 shows a simple model of this.
CLKIN
CLKOUT
5M
C1
C2
ON-CHIP
CIRCUITRY
OFF-CHIP
CIRCUITRY
Figure 7. On-Chip Oscillator
Using the part with a crystal or ceramic resonator between the
CLKIN and CLKOUT pins generally causes more current to
be drawn from VDD than when the part is clocked from a driven
clock signal at the CLKIN pin. This is because the on-chip
oscillator is active in the case of the crystal or resonator. The
amount of additional current depends on a number of factors.
First, the larger the value of the capacitor on CLKIN and
CLKOUT pins, the larger the current consumption. Typical
values recommended by the crystal and resonator manufacturers
are in the range of 30 pF to 50 pF. Another factor that influ-
ences I
DD
is Effective Series Resistance of the crystal (ESR).
The lower the ESR value, the lower the current taken by the
oscillator circuit.
The on-chip oscillator also has a start-up time associated with it
before it oscillates at its correct frequency and voltage levels. The
typical start-up time is 10 ms with a V
DD
of 5 V and 15 ms with
a V
DD
of 3.3 V (both with a 1 MHz crystal).
The AD7740 master clock appears inverted on the CLKOUT
pin of the device. The maximum recommended load on this pin is
one CMOS load. When using a crystal to generate the AD7740’s
clock it may be desirable to then use this clock as the clock
source for the entire system. In this case, it is recommended that
the CLKOUT signal be buffered with a CMOS buffer before
being applied to the rest of the circuit (as shown in Figure 7).
Reference Input
The AD7740 performs conversions relative to the applied refer-
ence voltage. This reference may be taken from the internal 2.5 V
bandgap reference by leaving REFIN/OUT unconnected. Alterna-
tively an external precision reference may be used. This is
connected to the REFIN/OUT pin, overdriving the internal
reference. Drive capability, initial error, noise, and drift charac-
teristics should be considered when selecting an external refer-
ence. The AD780 and REF192 are suitable choices for external
references.
The internal reference is most suited to applications where
ratiometric operation of the signal source is possible. Using the
internal reference in systems where the signal source varies with
time, temperature, loading, etc., tends to cancel out errors.
Power-Down Mode
When CLKIN is inactive low for 1 ms (typ), the AD7740 auto-
matically enters a power-down mode. In this mode most of the
digital and analog circuitry is shut down and REFOUT floats.
FOUT goes high. This reduces the power consumption to 525 µW
max (5 V) and 360 µW (3.3 V).
APPLICATIONS
The basic connection diagram for the part is shown in Figure 8.
In the connection diagram shown, the AD7740 is configured in
unbuffered mode. The 5 V power supply is used as a reference to
the AD7740. A quartz crystal provides the master clock source
for the part. It may be necessary to connect capacitors (C1 and
C2 in the diagram) to the crystal to ensure that it does not oscil-
late at overtones of its fundamental operating frequency. The
values of capacitors will vary depending on the manufacturer’s
specifications.
REFIN
AD7740
CLKIN
CLKOUT
C1 C2
FOUT
GND
BUF
0.1F
10F
VIN
VDD
5V
Figure 8. Basic Connection Diagram
AD7740
REV. C
