–10–

Temperature Sensor Application

The AD7740 can be used with an AD22100S temperature

sensor to give a digital measure of ambient temperature. The

output voltage of the AD22100S is proportional to the tempera-

ture times the supply voltage. It uses a single 5 V supply, and its

output swings from 0.25 V at –50

°C to 4.75 V at +150°C. By

feeding its output through the AD7740, the value of ambient

temperature is converted into a digital pulse train. See Figure 12.

REFIN

AD7740

AD22100S

V+

CLKIN

CLKOUT

C1

C2

FOUT

GND

BUF

0.1 F

10 F

0.1 F

VIN

VDD

5V

10 F

Figure 12. Using the AD7740 with a Temperature Sensor

Due to its ratiometric nature this application provides an

extremely cost-effective solution. The need for an external pre-

cision reference is eliminated since the 5 V power-supply is used

as a reference to both the VFC and the AD22100S.

32 kHz Operation

The AD7740 oscillator circuit will not operate at 32 kHz. If

the user wishes to use a 32 kHz watch crystal, some additional

external circuitry is required. The circuit in Figure 13 is for a

crystal with a required drive of 1

µW. Resistors R1 and R2

reduce the power to this level.

R3

1M

CLKIN

R2

100k

R1

220k

32kHz

40106

40106

Figure 13. 32 kHz Watch Crystal Circuit

Power Supply Bypassing and Grounding

In any circuit where accuracy is important, careful consideration

of the power supply and ground return layout helps to ensure

the rated performance. The printed circuit board housing the

AD7740 should be designed such that the analog and digital

sections are separated and confined to certain areas of the board.

To minimize capacitive coupling between them, digital and

analog ground planes should only be joined in one place, close

to the AD7740, and should not overlap.

Avoid running digital lines under the device, as these will couple

noise onto the die. The analog ground plane should be allowed

to run under the AD7740 to avoid noise coupling. The power

supply lines to the AD7740 should use as large a trace as pos-

sible to provide low impedance paths and reduce the effects of

glitches on the power supply line. Fast switching signals like

clocks should be shielded with digital ground to avoid radiating

noise to other parts of the board, and clock signals should never

be run near analog inputs. Avoid crossover of digital and analog

signals. Traces on opposite sides of the board should run at right

angles to each other. This reduces the effect of feedthrough

through the board. A microstrip technique is by far the best but

is not always possible with a double-sided board. In this technique,

the component side of the board is dedicated to the ground plane

while the signal traces are placed on the solder side.

Good decoupling is also important. All analog supplies should

be decoupled to GND with surface mount capacitors, 10

µF in

parallel with 0.1

µF located as close to the package as possible,

ideally right up against the device. The lead lengths on the by-

pass capacitor should be as short as possible. It is essential that

these capacitors be placed physically close to the AD7740 to

minimize the inductance of the PCB trace between the capacitor

and the supply pin. The 10

µF are the tantalum bead type and

are located in the vicinity of the VFC to reduce low-frequency

ripple. The 0.1

µF capacitors should have low Effective Series

Resistance (ESR) and Effective Series Inductance (ESI), such

as the common ceramic types, which provide a low imped-

ance path to ground at high frequencies to handle transient

currents due to internal logic switching. Additionally, it is ben-

eficial to have large capacitors (> 47

µF) located at the point

where the power connects to the PCB.

AD7740

REV.

C